Perioperative hypovolemia is a common cause of acute circulatory failure. The purpose of administering fluid therapy to hemodynamically unstable patients is to increase the intravascular volume, thereby improving tissue perfusion and oxygenation. However, overzealous fluid therapy can cause pulmonary and interstitial edema.1 In human medicine, goal-directed fluid therapy is associated with improved patient outcomes, a decrease in morbidity, and shorter duration of hospitalization.2,3 For hemodynamically unstable patients, the cardiac preload status and the probability of whether the patient will respond to a fluid challenge should be thoroughly assessed before fluid therapy is initiated. According to the Frank-Starling law of the heart, administration of a fluid bolus will increase the ventricular preload and CO in fluid responders but have minimal effect on the CO of fluid nonresponders. Results of studies1,4 involving human patients indicate that < 50% of patients admitted to critical care units are fluid responsive, which is concerning because fluid nonresponders can easily become volume overloaded.
The PVI is derived from pulse oximetry and provides an automated measurement of the percentage variation in the amplitude of the plethysmographic waveform. It is calculated as the percentage variation in the PI of mechanically ventilated individuals.5 Pulse pressure variation is derived from the direct arterial pressure signal and is the percentage variation in pulse pressure recorded during the respiratory cycle of mechanically ventilated individuals.3,4 Both the PVI and PPV reflect beat-to-beat oscillations in stroke volume caused by changes in intrathoracic pressure during mechanical ventilation. Measurement of ventilation-induced variation in pulse pressure and plethysmographic waveform allows clinicians to predict a patient's cardiovascular response to changes in intravascular volume.3–5 In fact, PVI and PPV have proven beneficial for assessment of fluid responsiveness during periods of hypovolemia in both human patients3–6 and dogs.7
In human medicine, a PLRM is commonly used as a simple bedside test to identify fluid responders among septic and critically ill patients.8–10 This maneuver involves passively raising a patient's legs to angles of 30° to 60° from the horizontal plane of the hospital bed, which transfers venous blood within the capacitance veins of the legs toward the heart and increases cardiac preload. This abrupt increase in preload generally results in a > 10% increase in CO (measured with arterial pulse contour analysis or echocardiography) in fluid responders.11,12 The primary advantage of the PLRM is that it is readily reversible and can be repeatedly used to assess a patient's response to an increase in cardiac preload without the risks associated with fluid therapy (eg, pulmonary edema), particularly for fluid nonresponders.8,10 The PLRM can be easily and reliably performed in spontaneously breathing patients, even those with low tidal volume ventilation and low lung compliance in which dynamic indices such as PPV and PVI cannot be reliably measured.1,13–16 Although PVI is not as accurate in spontaneously breathing patients as it is in mechanically ventilated patients, its measurement following the PLRM has been successfully used to assess changes in left ventricular stroke volume in spontaneously breathing human patients.17–19
The standard PLRM used for human patients cannot be used in veterinary species because of the wide variability in pelvic limb conformation, size, and blood volume across species. In the field of biomedical research, there has been a recent trend to use pigs instead of dogs in terminal studies. Pigs are an excellent alternative for human and canine research, especially with regard to cardiovascular function, because of the numerous physiologic, anatomic, and pathological similarities among those 3 species.20 Results of 1 study21 indicate that use of a PLRM during CPR significantly improves coronary perfusion and neurologic scores in pigs with experimentally induced prolonged ventricular fibrillation. The investigators of that study21 concluded that use of the PLRM during CPR resulted in return of spontaneous circulation and 24-hour survival rates that were comparable to those achieved with standard patient positioning during CPR. Although the PLRM is commonly used to predict fluid responsiveness in human patients during anesthesia and in critical care settings, to our knowledge, its hemodynamic effects in clinical settings have not been evaluated for veterinary species.
The purpose of the study reported here was to describe the use of a modified PLRM to assess fluid responsiveness during experimental induction and correction of hypovolemia in isoflurane-anesthetized pigs. Our hypotheses were that the PLRM would increase CO and accurately predict fluid responsiveness during hypovolemia in mechanically ventilated pigs, dynamic cardiac preload indices such as PVI and PPV could be used to accurately track variations in peripheral perfusion caused by the PLRM, and the PLRM would not affect CO, PVI, and PPV but would accurately predict fluid nonresponsiveness during volume replacement.
Materials and Methods
Animals
All study procedures were reviewed and approved by the University of Florida Institutional Animal Care and Use Committee (protocol No. 201609273). Six purpose-bred male Landrace pigs were used for the study. Prior to study initiation, all pigs were 8 to 10 months old, weighed approximately 20 kg, and were determined to be healthy on the basis of results of a physical examination, CBC, and serum biochemical evaluation. The pigs were housed and cared for in accordance with Association for Assessment and Accreditation of Laboratory Animal Care International guidelines.
Anesthetic induction and instrumentation
For each pig, food but not water was withheld for 15 hours before anesthesia induction. Pigs were premedicated with ketaminea (10 mg/kg, IM). Anesthesia was induced with 5% isofluraneb in oxygen delivered via a face mask and circle system by an anesthesia machinec until the pig could be orotracheally intubated. After successful intubation was achieved, the endotracheal tube was secured, and anesthesia was maintained with isoflurane.
A 22-gauge catheterd was aseptically placed in an auricular vein. The pig was then positioned in dorsal recumbency, after which the end-tidal partial pressure of CO2 and etISO were continuously monitored by an infrared gas analyzer,e which was attached to a multiparameter patient monitor.f Isoflurane administration was adjusted as necessary to maintain the etISO at approximately 1.6%, which is approximately 1.2 times the minimum alveolar concentration of isoflurane reported for pigs.22 A spirometry sensor was placed between the endotracheal tube and y-piece of the circle system and connected to a monitorg to measure inspiratory and expiratory flow, tidal volume, minute volume, peak inspiratory pressure, plateau pressure, mean airway pressure, and dynamic compliance of the respiratory system. Dynamic compliance of the respiratory system was calculated as the expired tidal volume/(peak inspiratory pressure – positive end-expiratory pressure). The multiparameter monitor also continuously displayed the heart rate, lead II ECG tracing, noninvasive blood pressure readings, and esophageal temperature, which was maintained between 37° and 39°C by use of a forced-air warming device.h A clip-on pulse oximeter probei was positioned on the thinnest portion of the tongue and connected to a pulse oximeter monitor that continuously displayed the PVI, which was automatically calculated from the minimum and maximum PI over each respiratory cycle. To enhance the PI signal and increase the reliability of PVI values, the tongue was gently massaged and the probe was repositioned approximately 5 minutes before each designated data acquisition time.
The left and right femoral arteries were dissected, and a 5F catheterj was placed in each artery and secured. The catheter in the right femoral artery was used for collection of arterial blood samples for blood gas analyses,k measurement of PCV and TPP concentration, and recording of direct systolic and diastolic blood pressures and MAP. A saline (0.9% NaCl) solution–filled pressure transducer systeml was calibrated and used for direct blood pressure recordings. The transducer was zeroed and placed at the level of the manubrium before each predetermined recording. The catheter in the left femoral artery was used for withdrawal of blood during controlled hemorrhage (ie, induction of hypovolemia).
The left and right external jugular veins were carefully dissected, and an 8F catheter introducerm was inserted in each vein and secured. A 7F thermodilution pulmonary artery cathetern was advanced through the introducer in the right jugular vein until its distal pressure sensing lumen was located in the pulmonary artery. Correct placement of the thermodilution catheter was determined on the basis of observation of characteristic pressure waveforms and pressure values after it was connected to the multiparameter monitor. The proximal port of the pulmonary artery catheter was connected to a saline solution–filled pressure transducer,l which was used to measure CVP. Cardiac output was measured with a thermodilution technique, whereby a 10-mL bolus of chilled (3° to 5°C) 5% dextrose solution was periodically injected into the central venous port of the thermodilution catheter following the recording of direct blood pressure values. The temperature of the injected dextrose solution was measured by an in-line thermistor that was located between the syringe and injection port of the pulmonary artery catheter. At each data acquisition time, the CO recorded represented the mean of 3 consecutive measurements. The catheter in the left jugular vein was used for administration of blood and colloids during volume replacement. All hemodynamic data were transferred to a bioamplifier, which sent continuous real-time data to a laptop computer equipped with softwareo to record it.
Experimental design
Following completion of instrumentation, a neuromuscular blockade was induced with a bolus of atracuriump (1 mg/kg, IV). The efficiency of the blockade was monitored by a supramaximal train-of-four electrical stimulusq of a common peroneal nerve. Atracurium dosing was repeated as necessary on the basis of a return of twitches during the train-of-four function on the nerve stimulator. Each pig underwent volume-controlled ventilation throughout the duration of the neuromuscular blockade. Tidal volume was held constant at 12 mL/kg, and the respiratory frequency was adjusted between 12 and 20 breaths/min to maintain Paco2 between 35 and 45 mm Hg.
While under the neuromuscular blockade, each pig underwent 5 sequential nonrandom experimental stages. During each stage, cardiopulmonary and hemodynamic variables were recorded immediately before and 3 minutes after a modified PLRM was performed and 1 minute after the pelvic limbs were returned to their original position. The modified PLRM consisted of placement of a wooden plank between the caudal portion of the pig's body and the table surface in a craniocaudal orientation, with 1 end of the plank located at the level of the xiphoid process (Figure 1). At each designated time, the free end of the plank was elevated until a 15° angle was achieved between the horizontal plane of the table surface and the pelvic limbs and abdomen. A wooden block was placed between the table and plank to maintain the 15° angle for 5 minutes, after which the block was removed and the pelvic limbs and abdomen were returned to their original positions. The head and thorax of the pig remained in the same horizontal plane as the table surface throughout the PLRM. There was a 10-minute interval between stages and a 10-minute interval between experimental manipulation of the circulation and the PLRM to allow hemodynamic variables to stabilize.
Illustrations depicting the positioning of a wooden plank before (A) and during (B) a modified PLRM that was used to assess fluid responsiveness in 6 healthy male Landrace pigs during states of normovolemia, hypovolemia, and hypervolemia. The wooden plank was placed between the caudal portion of the pig's body and the table surface in a craniocaudal orientation with 1 end of the plank located at the level of the xiphoid process. At each designated time, the free end of the plank was elevated until a 15° angle was achieved between the horizontal plane of the table surface and the pelvic limbs and abdomen. A wooden block was placed between the table and plank to maintain the 15° angle for 5 minutes, after which the block was removed and the pelvic limbs and abdomen were returned to their original positions. The head and thorax of the pig remained in the same horizontal plane as the table surface throughout the PLRM.
Citation: American Journal of Veterinary Research 80, 1; 10.2460/ajvr.80.1.24
Baseline measurements were obtained during stage 1. After baseline measurements were recorded, controlled hemorrhage was initiated. Blood was withdrawn through the catheter in the left femoral artery over a period of 15 minutes until 20% (stage 2) and 40% (stage 3) of the estimated total circulating blood volume was removed, considering the total circulating blood volume for pigs is approximately 70 mL/kg of body weight.23 Prior to anesthesia induction, each pig was individually weighed so that the exact amount of blood to be withdrawn during stages 2 and 3 could be calculated. To ensure that blood was removed at a fairly constant rate during stages 2 and 3, two 60-mL syringes, which were flushed with an anticoagulant solution containing sodium citrate, citric acid, dextrose, and adenine, were used to transfer blood from the femoral artery catheter to blood collection bags that contained the same anticoagulant solution. The bags were placed on scales and continuously weighed as they were being filled to ensure that only the calculated amount of blood was removed during each stage.
During stage 4, the total volume of blood removed during stages 2 and 3 was reinfused via the catheter in the left jugular vein over a period of 15 minutes to return the pig to a state of normovolemia. During stage 5, a 500-mL bolus of a colloidr was administered through the catheter in the left jugular vein over a period of 15 minutes to achieve a state of hypervolemia. After the last data for stage 5 were acquired, each pig was euthanized by IV administration of a pentobarbitals overdose.
During all experimental stages, fluid responsiveness was defined as a > 15% increase in CO and > 15% decrease in PVI between recordings obtained immediately before and 3 minutes after the PLRM was performed. Fluid nonresponsiveness was defined as a ≤ 15% change in CO and PVI in response to the PLRM.
Statistical analysis
The distribution of each variable at each data acquisition time (before and after PLRM and after the pelvic limbs and abdomen were returned to their original positions during each experimental stage) was assessed for normality by means of the Shapiro-Wilk test. All variables were found to be normally distributed; therefore, the results for all variables were summarized as the mean ± SD. A 1-way ANOVA for repeated measures was used to compare variables among the 3 data acquisition times within each experimental stage and among the 5 experimental stages. A post hoc Tukey adjustment was used when multiple pairwise comparisons were performed. Values of P < 0.05 were considered significant. All analyses were performed with commercially available statistical software.t
Results
Pigs
The 6 male study pigs had a median age of 9 months (range, 8 to 10 months) and mean ± SD body weight of 20 ± 0.47 kg. Throughout the duration of anesthesia, the mean ± SD etISO was 1.59 ± 0.31%. The mean ± SD duration was 160.83 ± 17.3 minutes between premedication with ketamine and completion of instrumentation, 177.45 ± 23.54 minutes for the 5 experimental stages, and 344.88 ± 13.45 minutes between premedication and euthanasia. Those durations did not differ significantly (P = 0.14) among the 6 pigs. The mean ± SD core (esophageal) body temperature was 37.9 ± 0.41 °C throughout the duration of the experiment and did not differ significantly (P = 0.56) among pigs. The mean ± SD volume of blood withdrawn from each pig during stage 2 was 280 ± 12 mL, and the same volume was withdrawn from each pig during stage 3. Thus, for each pig, approximately 560 mL of blood was withdrawn to induce hypovolemic shock. Following the PLRM, all pigs were classified as fluid responders when in a state of hypovolemia (stages 2 and 3) and fluid nonresponders when in a state of normovolemia (stages 1 and 4) or hypervolemia (stage 5).
Response of hemodynamic variables to changes in blood volume and the PLRM
The mean ± SD values for hemodynamic variables before and after the PLRM and after the pelvic limbs and abdomen were returned to their original positions during each experimental stage were summarized (Table 1). Compared with baseline (stage 1) values, experimental induction of hypovolemia (stages 2 and 3) was associated with a significant decrease in CO, PI, MAP, CVP, and PAOP and a significant increase in PVI, PPV, and heart rate before the PLRM was performed. The magnitude of those differences from baseline was greater during stage 3 than during stage 2. In fact, all of those variables differed significantly between stages 2 and 3 at each of the 3 data acquisition times. All variables returned to their baseline values when normovolemia was restored (stage 4). Experimental induction of hypervolemia caused a significant increase in CO and MAP, compared with baseline values.
Mean ± SD values for hemodynamic variables for 6 healthy male isoflurane-anesthetized and mechanically ventilated Landrace pigs immediately before and 3 minutes after a PLRM was performed and 1 minute after the pelvic limbs and abdomen were returned to their original positions during each of 5 experimental stages.
| Stage 1 | Stage 2 | Stage 3 | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Variable | Before PLRM | After PLRM | After RTOP | Before PLRM | After PLRM | After RTOP | Before PLRM | After PLRM | After RTOP |
| CO (L/min) | 3.4 ± 0.9 | 3.8 ± 0.6 | 3.6 ± 0.7 | 2.4 ± 0.5*‡a | 3.4 ± 0.6‡b | 2.5 ± 0.4*‡a | 1.6 ± 0.6*†‡a | 2.4 ± 0.7*†‡b | 1.7 ± 0.6*†‡a |
| PVI (%) | 10 ± 2 | 9 ± 4 | 10 ± 3 | 29 ± 4*‡a | 17 ± 2*‡b | 19 ± 2*‡b | 37 ± 5*†‡a | 23 ± 4*†‡b | 21 ± 2*†‡b |
| PPV (%) | 7 ± 2 | 8 ± 2 | 8 ± 3 | 18 ± 2*‡a | 9 ± 3‡b | 17 ± 1*‡a | 25 ± 3*†‡a | 15 ± 1*†‡b | 22 ± 2*†‡a |
| PI (%) | 2.8 ± 1.3 | 2.7 ± 1.1 | 2.6 ± 0.3 | 1.5 ± 0.9* | 1.6 ± 0.4* | 1.5 ± 0.4* | 0.7 ± 0.1*†‡ | 0.9 ± 0.1*†‡ | 0.8 ± 0.1*†‡ |
| Spo2 (%) | 99 ± 1 | 99 ± 1 | 99 ± 1 | 97 ± 2 | 97 ± 2 | 96 ± 3 | 95 ± 3 | 95 ± 3 | 95 ± 2 |
| Heart rate (beats/min) | 104 ± 12 | 101 ± 14 | 107 ± 15 | 127 ± 10*‡ | 119 ± 13*‡ | 123 ± 12*‡ | 147 ± 14*†‡ | 137 ± 15*†‡ | 137 ± 15*†‡ |
| MAP (mm Hg) | 77 ± 8 | 82 ± 5 | 82 ± 6 | 54 ± 10*‡a | 67 ± 5*‡b | 57 ± 8*‡a | 40 ± 5*†‡a | 50 ± 10*†‡b | 43 ± 10*†‡a |
| CVP (mm Hg) | 6 ± 2 | 7 ± 2 | 6 ± 2 | 3 ± 1*‡a | 8 ± 1‡b | 4 ± 1*‡a | 1 ± 1*†‡a | 5 ± 1*†‡b | 2 ± 1*†‡a |
| PAOP (mm Hg) | 9 ± 2 | 9 ± 2 | 9 ± 2 | 5 ± 1*‡a | 9 ± 2‡b | 4 ± 2*‡a | 2 ± 1*†‡a | 8 ± 2†‡b | 3 ± 1*†‡a |
| Respiratory rate (breaths/min) | 11 ± 1 | 11 ± 1 | 11 ± 1 | 10 ± 1 | 10 ± 1 | 10 ± 1 | 9 ± 3 | 9 ± 3 | 9 ± 3 |
| Petco2 (mm Hg) | 40 ± 5 | 40 ± 5 | 40 ± 5 | 34 ± 2 | 34 ± 2 | 34 ± 2 | 29 ± 2 | 28 ± 2 | 27 ± 2 |
| PIP (cm H2O) | 10 ± 2 | 10 ± 2 | 10 ± 2 | 10 ± 2 | 10 ± 2 | 10 ± 2 | 10 ± 1 | 10 ± 1 | 10 ± 1 |
| Dynamic compliance (mL/cm H2O) | 23 ± 2 | 23 ± 2 | 23 ± 2 | 23 ± 2 | 23 ± 2 | 23 ± 2 | 22 ± 2 | 22 ± 2 | 22 ± 2 |
| Stage 4 | Stage 5 | |||||
|---|---|---|---|---|---|---|
| Variable | Before PLRM | After PLRM | After RTOP | Before PLRM | After PLRM | After RTOP |
| CO (L/min) | 3.6 ± 0.8 | 3.9 ± 0.6 | 3.8 ± 0.7 | 3.9 ± 0.9* | 3.8 ± 0.8 | 3.6 ± 0.7 |
| PVI (%) | 12 ± 4 | 10 ± 1 | 9 ± 2 | 9 ± 4 | 9 ± 3 | 9 ± 3 |
| PPV (%) | 8 ± 2 | 9 ± 1 | 8 ± 1 | 8 ± 1 | 9 ± 1 | 8 ± 1 |
| PI (%) | 1.3 ± 0.4 | 1.3 ± 0.3 | 1.4 ± 0.4 | 1.8 ± 0.6 | 1.7 ± 0.5 | 1.7 ± 0.6 |
| Spo2 (%) | 97 ± 2 | 98 ± 1 | 98 ± 1 | 99 ± 1 | 99 ± 1 | 99 ± 1 |
| Heart rate (beats/min) | 108 ± 8 | 105 ± 7 | 105 ± 10 | 109 ± 12 | 111 ± 12 | 110 ± 12 |
| MAP (mm Hg) | 80 ± 1 | 82 ± 14 | 79 ± 15 | 84 ± 16* | 85 ± 15 | 83 ± 15 |
| CVP (mm Hg) | 9 ± 1 | 10 ± 1 | 9 ± 1 | 10 ± 1 | 9 ± 1 | 10 ± 1 |
| PAOP (mm Hg) | 11 ± 2 | 12 ± 1 | 11 ± 1 | 12 ± 1 | 11 ± 1 | 11 ± 1 |
| Respiratory rate (breaths/min) | 11 ± 1 | 11 ± 1 | 11 ± 1 | 11 ± 1 | 11 ± 1 | 11 ± 1 |
| Petco2 (mm Hg) | 37 ± 2 | 35 ± 1 | 36 ± 1 | 38 ± 1 | 37 ± 1 | 37 ± 1 |
| PIP (cm H2O) | 11 ± 1 | 11 ± 1 | 11 ± 1 | 10 ± 2 | 10 ± 2 | 10 ± 2 |
| Dynamic compliance (mL/cm H2O) | 21 ± 3 | 21 ± 3 | 21 ± 3 | 21 ± 3 | 21 ± 3 | 21 ± 3 |
Pigs were positioned in dorsal recumbency throughout all 5 experimental stages. The PLRM consisted of raising the pelvic limbs and caudal portion of the abdomen until they were at a 15° angle relative to the horizontal plane of the table surface. Each pig was in a state of normovolemia during stage 1 (baseline). During stage 2, a volume of blood equal to 20% of the estimated total circulating blood volume was withdrawn over a period of 15 minutes. During stage 3, additional blood was withdrawn over a period of 15 minutes until approximately 40% of the estimated total circulating blood volume was removed, and the pig was in a state of hypovolemic shock. During stage 4, the total volume of blood removed during stages 2 and 3 was reinfused over a period of 15 minutes to restore the pig to a state of normovolemia. During stage 5, each pig received a 500-mL IV bolus of a colloid over 15 minutes to achieve a hypervolemic state. There was a 10-minute interval between stages and a 10-minute interval between experimental manipulation of the circulation and the PLRM to allow hemodynamic variables to stabilize. None of the mean values differed significantly between stages 4 and 5.
Value differs significantly (P < 0.05) from the corresponding value at stage 1.
Value differs significantly (P < 0.05) from the corresponding value at stage 2.
Value differs significantly (P < 0.05) from the corresponding values at stages 4 and 5.
Petco2 = End-tidal partial pressure of CO2. PIP = Peak inspiratory pressure. RTOP = Return to original position. Spo2 = Oxygen saturation as measured by pulse oximetry.
Within a stage, values with different superscript letters differ significantly (P < 0.05).
The PLRM had no significant effect on any of the hemodynamic variables evaluated when pigs were in a state of normovolemia (stages 1 and 4) or hypervolemia (stage 5). When pigs were hypovolemic, the PLRM resulted in a significant increase in CO and a significant decrease in PVI (Figure 2). During both stages 2 and 3, the CO increased by > 30% following the PLRM for all pigs. The PLRM also caused a significant increase in MAP, CVP, and PAOP and a significant decrease in PPV. After the pelvic limbs were returned to their original position, all of those variables except PVI returned to values similar to those before the PLRM. The PVI remained unchanged from that at 3 minutes after the PLRM. The PLRM had no effect on the PI or dynamic compliance of the respiratory system.
Mean ± SD CO (A), PVI (B), and PPV (C) immediately before (dotted line with circles) and 3 minutes after (solid line with triangles) the modified PLRM described in Figure 1 was performed and 1 minute after the pelvic limbs and abdomen were returned to their original positions (dashed line with squares) during each of 5 sequential experimental stages for 6 healthy isoflurane-anesthetized Landrace pigs that were mechanically ventilated following induction of a neuromuscular blockade (1 mg of atracurium/kg, IV). Each pig was in a state of normovolemia during stage 1 (baseline). During stage 2, a volume of blood equal to 20% of the estimated total circulating blood volume was withdrawn over a period of 15 minutes. During stage 3, additional blood was withdrawn over a period of 15 minutes until approximately 40% of the estimated total circulating blood volume was removed and the pig was in a state of hypovolemic shock. During stage 4, the total volume of blood removed during stages 2 and 3 was reinfused over a period of 15 minutes to restore the pig to a state of normovolemia. During stage 5, each pig received a 500-mL IV bolus of a colloid over 15 minutes to achieve a hypervolemic state. There was a 10-minute interval between stages and a 10-minute interval between experimental manipulation of the circulation and the PLRM to allow hemodynamic variables to stabilize. None of the mean values differed significantly between stages 4 and 5. *Value differs significantly (P < 0.05) from the corresponding value at stage 1. †Value differs significantly (P < 0.05) from the corresponding value at stage 2. ‡Value differs significantly (P < 0.05) from the corresponding values at stages 4 and 5.a,b Values with different lowercase letters differ significantly (P < 0.05).
Citation: American Journal of Veterinary Research 80, 1; 10.2460/ajvr.80.1.24
Hematologic variables
The mean ± SD values for hematologic variables during each experimental stage were summarized (Table 2). The effect of the PLRM on those variables was not evaluated. The mean PCV did not differ significantly (P = 0.16) among the 5 stages. The mean TPP concentration, Paco2, and arterial pH decreased significantly and the mean lactate concentration increased significantly from baseline values following experimental induction of hypovolemia. When normovolemia was restored (stage 4), all of those variables returned to values similar to those at baseline, except for the mean lactate concentration, which remained significantly increased from baseline but was significantly decreased from that at stage 3. The mean lactate concentration differed significantly (P = 0.03) between stages 2 and 3.
Mean ± SD hematologic variables during each of the 5 experimental stages for the pigs of Table 1.
| Variable | Stage 1 | Stage 2 | Stage 3 | Stage 4 | Stage 5 |
|---|---|---|---|---|---|
| PCV (%) | 34.0 ± 3.9 | 31.8 ± 4.0 | 29.6 ± 3.3 | 30.0 ± 2.4 | 32.0 ± 1.7 |
| TPP (g/dL) | 5.3 ± 0.2 | 4.5 ± 0.3* | 3.4 ± 0.3*‡ | 5.0 ± 0.4 | 4.9 ± 0.4 |
| Lactate (mmol/L) | 0.2 ± 0.4 | 2.4 ± 0.8*‡ | 3.8 ± 0.9*†‡ | 1.4 ± 1.5* | 0.5 ± 1.2 |
| Paco2 (mm Hg) | 43.1 ± 1.2 | 36.2 ± 2.2*‡ | 31.9 ± 1.1*†‡ | 42.1 ± 3.1 | 42.9 ± 1.9 |
| Pao2 (mm Hg) | 358.6 ± 88.9 | 361.3 ± 91.2 | 351.0 ± 84.7 | 352.5 ± 94.0 | 343.1 ± 90.4 |
| Arterial pH | 7.27 ± 0.01 | 7.22 ± 0.01*‡ | 7.17 ± 0.01*†‡ | 7.27 ± 0.01 | 7.28 ± 0.0 |
Hematologic variables were recorded only once during each experimental stage.
See Table 1 for remainder of key.
Discussion
Results of the present study indicated that a modified PLRM was useful for assessing fluid responsiveness in hypovolemic pigs. When the PLRM was applied to pigs that had undergone controlled hemorrhage such that the total circulating blood volume was depleted by 20% and 40%, the CO increased by approximately 30% and 33%, respectively. The PLRM also caused a significant decrease in PVI and PPV when applied to hypovolemic pigs; however, when the PLRM was discontinued and the pelvic limbs were returned to their original position, the PPV immediately returned to its pre-PLRM value, whereas the PVI was slower to respond. Interestingly, the PCV did not vary significantly as pigs transitioned from a state of normovolemia to severe hypovolemia back to normovolemia and eventually to hypervolemia. We attributed that to splenic contraction induced by the physiologic stress associated with the experimental procedures, which resulted in the release of RBCs into the circulation, and the fact that blood withdrawal and reinfusion occurred over a fairly short period of time, which precluded those changes from being reflected in the PCV. Conversely, lactate concentration appeared to be acutely sensitive to changes in the circulating blood volume.
In human patients, both the PLRM and Trendelenburg maneuver are routinely used to assess fluid responsiveness or as therapeutic maneuvers for individuals awaiting fluid resuscitation.24,25 The primary difference between the modified PLRM described in the present study and the Trendelenburg maneuver is that, during the modified PLRM, the head and thorax remain in a horizontal plane while the pelvic limbs and caudal portion of the abdomen are raised. During the Trendelenburg maneuver, the whole body is tilted such that the head is in a dependent position. The Trendelenburg maneuver is associated with several undesirable effects that are not observed during the PLRM, such as an increase in intraocular and intracranial pressures, cerebral edema secondary to venous congestion, a decrease in respiratory expansion and vital lung capacity resulting in atelectasis and altered ventilation-perfusion ratios, and a high risk for regurgitation and aspiration of gastric contents.24,25 Furthermore, Trendelenburg positioning will increase blood flow in the carotid arteries, which can cause physiologic changes that may mask fluid resuscitation–induced changes to the Frank-Starling curve, thereby making differentiation between hypoperfused and hyperperfused states more challenging.26
Regardless of whether patients undergo the PLRM or Trendelenburg maneuver, the magnitude of the physiologic effect is dependent on the tilt angle and duration of the maneuver. For the present study, we modified the PLRM commonly used in human patients on the basis of our understanding of blood volume differences between the legs of humans and pelvic limbs of quadrupeds. We chose to raise the pelvic limbs and caudal portion of the abdomen to a 15° angle relative to the horizontal plane of the table surface on the basis of results of a pilot study in which we used a similar PLRM in dogs. That was a substantially smaller angle than the angle (45°) used by investigators of another study21 in which a PLRM was applied to pigs. Nevertheless, the 15° angle was sufficient to cause a > 30% increase in the CO of moderately to severely hypovolemic pigs. Although the pigs of the present study were healthy, we do not see any reason why this inexpensive and easy-to-perform modified PLRM could not be used for clinical patients, particularly those in which the Trendelenburg maneuver is contraindicated. Potential indications for use of the modified PLRM described in this study included determination of the fluid responsiveness of a patient or as a therapeutic maneuver to aid in the temporary resuscitation of patients in acute circulatory shock.
The pigs of the present study were premedicated with ketamine (10 mg/kg, IM) to provide sedation and restraint during application of a face mask for delivery of isoflurane during anesthesia induction. Ketamine (5 to 20 mg/kg, IM) is commonly used in combination with other drugs to provide sedation and injectable anesthesia for short procedures in pigs.27 The mean ± SD duration between premedication with ketamine and completion of instrumentation was 160.83 ± 17.3 minutes. Therefore, the systemic effects of ketamine should have dissipated before initiation of the 5 experimental stages and measurement of hemodynamic variables.
Cyclic oscillations in stroke volume induced by mechanical ventilation correspond to beat-to-beat changes in the amplitude of pulse pressure measured with an intra-arterial catheter (PPV) and in the amplitude of the plethysmographic waveform displayed by pulse oximetry (PVI). Changes in intrathoracic pressure during an entire respiratory cycle of the ventilator are used to predict the magnitude of cyclic oscillations in stroke volume, which further determines whether an individual is likely to be responsive (ie, is at a steep part of the Frank-Starling curve) or nonresponsive (ie, is at a flat portion of the Frank-Starling curve) to fluid therapy.4,5 Hence, PVI and PPV are commonly referred to as dynamic preload variables and need to be measured under strict ventilatory conditions (ie, controlled ventilation with a tidal volume > 8 mL/kg)28,29 because spontaneous respiration makes their measurement less reliable.1 The pigs of the present study were administered atracurium to induce a neuromuscular blockade and were mechanically ventilated throughout the duration of the experiment to prevent patient-ventilator dyssynchrony. Complete control of the ventilation of the study pigs was critical to assess the cyclic oscillations in the plethysmographic waveform and percentage variation in PI and calculate the PVI and PPV. Compliance of the chest wall relative to compliance of the lungs is an important determinant for PVI.1,4 In the present study, the PLRM did not significantly affect the dynamic compliance of the lungs and chest wall, which increased the sensitivity of the PVI for detection of PLRM-induced hemodynamic changes.
Traditionally, variables associated with cardiac filling, such as CVP and PAOP, are used to assess fluid responsiveness of patients. For the pigs of the present study, the changes observed in the CVP and PAOP in response to the controlled hemorrhage, PLRM, and volume replacement were similar to those observed for dogs of another study.7 The relationship between cardiac filling pressures, such as CVP and PAOP, and fluid responsiveness of individuals is poor in various clinical settings,30–32 most likely because cardiac end-diastolic volume is not solely dependent on cardiac filling pressure; it is also affected by compliance of the cardiac chambers, venous tone, and intrathoracic pressures.
The authors of a systematic review and meta-analysis33 concluded that PVI, compared with CO, stroke volume, and PPV, was reasonably accurate for identification of individuals likely to be responsive to fluid therapy when measured under controlled ventilation. Results of a study7 involving isoflurane-anesthetized dogs that underwent controlled hemorrhage to induce hypotension followed by volume replacement indicate that PPV and PVI might be useful for identification of fluid-responsive dogs. Interestingly, when hypovolemia was induced in the pigs of the present study, the PVI, CO, and PPV readily responded to the PLRM, but when the pelvic limbs were returned to their original position, the PVI was slow to respond, whereas the CO and PPV rapidly returned to pre-PLRM values. The PVI is measured noninvasively by the pulse oximeter and is therefore dependent on peripheral blood flow (ie, the pulse oximeter was placed on the tongue). We postulated that the apparent lack of responsiveness in the PVI after the pelvic limbs were returned to their original position was caused by interference in light absorption induced by sudden modifications in blood flow as the position of the pelvic limbs was changed. Also, the pulse oximeter used in the present study has a built-in algorithm for calculation of the PVI, and there is a lag between peripheral blood flow measurement and generation of the PVI readout. It is possible that recording the PVI 1 minute after the pelvic limbs were returned to their original position was an insufficient amount of time for the pulse oximeter to detect and register changes in PVI associated with repositioning of the pelvic limbs.
Perfusion index is an indicator of the pulse strength at the site where the pulse oximeter probe is placed. To optimize measurement of PI, it is important that the peripheral tissue where the probe is placed has good blood perfusion and oxygenation. Consequently, factors that affect perfusion and oxygenation of peripheral tissues, such as low CO, hypotension, hypothermia, vasoactive drug administration, vasoconstriction, and peripheral vascular disease, can adversely affect measurement of PI.34 The pigs of the present study were not administered any drugs (eg, vasopressors or α2-adrenergic receptor agonists) that might cause peripheral vasoconstriction and were maintained in a state of normothermia throughout the experiment. Therefore, we believe that the decrease in PI observed during stages 2 and 3 (hypovolemia) was the result of hypoperfusion of the tongue secondary to substantial blood loss and compensatory vasoconstriction. It is also important to note that the pulse oximeter cannot distinguish whether a change in PI was caused by a change in intrathoracic pressure or peripheral tissue perfusion.33 Thus, anything that affects peripheral tissue perfusion (and by extension PI) will affect the accuracy of PVI for prediction of fluid responsiveness.34 In the present study, the PI values varied by < 1% throughout the duration of stage 3 (severe hypovolemia), and PVI values should be interpreted cautiously. The effects of hypovolemia and poor peripheral tissue perfusion on PVI warrant further investigation in veterinary species.
The present study had multiple limitations. The sample size was small owing to the terminal nature of the study and other ethical considerations. The sequence of the experimental stages could not be randomized because the transition of pigs from normovolemia to hypovolemia back to normovolemia and eventually to hypervolemia in that order was critical to evaluate whether the PLRM could be used to predict fluid responsiveness and nonresponsiveness among the study subjects. The data acquisition times (immediately before and 3 minutes after the PLRM and 1 minute after the pelvic limbs and abdomen were returned to their original positions) for hemodynamic variables during each experimental stage were selected arbitrarily because we did not have any pilot data and could not find any information in the scientific literature to help guide that decision. However, cardiovascular variables such as heart rate, invasive blood pressure, CVP, PPV, and PVI were measured continuously. On the basis of evaluation of the cardiovascular data collected during this study, we believe that the PLRM induced a steady hemodynamic state for approximately 5 minutes, after which the heart slowly adjusted to the new volume status. Finally, experimental blood volume depletion was set at 20% and 40% during stages 2 and 3, respectively; therefore, receiver operating characteristic curve analysis could not be conducted to determine the optimal blood volume loss necessary to accurately identify fluid responsiveness on the basis of PLRM results.
In the present study, a modified PLRM in which the pelvic limbs and caudal portion of the abdomen were raised at a 15° angle to the horizontal plane was used to assess fluid responsiveness during experimental induction and correction of severe hypovolemia in isoflurane-anesthetized and mechanically ventilated pigs. Results indicated that, when pigs were in a state of moderate (20% blood volume depletion) or severe (40% blood volume depletion) hypovolemia, use of the PLRM resulted in a significant increase in CO and decrease in PVI and PPV. When the pelvic limbs and abdomen were returned to their original positions, the CO and PPV rapidly returned to their pre-PLRM values; however, the PVI, which was noninvasively measured by means of pulse oximetry, was slower to respond. Nevertheless, the findings suggested that the modified PLRM might be useful for identification of hemodynamically unstable animals that are likely to respond to fluid therapy. Further research is necessary to validate the modified PLRM for prediction of fluid responsiveness in clinically ill veterinary patients.
Acknowledgments
Supported by University of Florida research funds. The funding source did not have any involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript.
The authors declare that there were no conflicts of interest.
The authors thank Dr. Luisito Pablo for creating the schematics for Figure 1.
ABBREVIATIONS
| CO | Cardiac output |
| CVP | Central venous pressure |
| etISO | End-tidal isoflurane concentration |
| MAP | Mean arterial pressure |
| PAOP | Pulmonary artery occlusion pressure |
| PI | Perfusion index |
| PLRM | Passive leg-raising maneuver |
| PPV | Pulse pressure variation |
| PVI | Plethysmographic variability index |
| TPP | Total plasma protein |
Footnotes
Zetamine, MWI Animal Health, Boise, Idaho.
Isoflo, Abbott Laboratories, North Chicago, Ill.
Narkomed GS, Drager Inc, Telford, Pa.
Surflo Etfe IV catheters, Terumo Medical Corp, Somerset, NJ.
IntelliVue Gas Module G1, Philips Healthcare, Cambridge, Mass.
Intellivue MP50 Patient Monitor, Philips Healthcare, Cambridge, Mass.
Datex Ohmeda Cardiocap AS-5 with spirometry, GE Healthcare, Little Chalfont, England.
Bair Hugger Animal Health Blankets, 3M, Minneapolis, Minn.
Masimo Corp, Irvine, Calif.
ARROW Femoral Arterial Line Catheterization Kit, Teleflex Inc, Morrisville, NC.
i-STAT handheld blood gas analyzer, Abbott Laboratories, East Windsor, NJ.
Deltran Pressure Transducer System DPT 324, Utah Medical Products Inc, Midvale, Utah.
ARROW Sheath Introducers, Teleflex Inc, Morrisville, NC.
Swan-Ganz Catheter Model 131HF7, Edwards Lifesciences Corp, Irvine, Calif.
Chart Pro, version 7.3, ADInstruments Pty Ltd, Bella Vista, NSW, Australia.
Atracurium besylate injection, Sagent Pharmaceuticals Inc, Schaumburg, Ill.
Innervator NS 252 Nerve Stimulator, Fisher and Paykel Healthcare, Irvine, Calif.
VetStarch, Zoetis Inc, Parsippany, NJ.
VetCo, Albuquerque, NM.
SAS, version 9.4, SAS Institute Inc, Cary, NC.
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