Horses positioned in dorsal recumbency often develop hypoxemia because of ventilation-perfusion mismatching and substantial right-to-left shunting.1–4 Although the cause is multifactorial, one of the main mechanisms for this hypoxemia is the formation of widespread alveolar atelectasis secondary to lung compression and absorption of alveolar oxygen. Studies5,6 of horses have indicated that including an inert gas such as nitrogen or helium in the inspired gas mixture may delay alveolar collapse, thus preserving lung function and improving oxygenation. However, this hypothesis remains controversial.4
Several ventilation strategies have been described for the recruitment of collapsed alveoli and improvement of oxygenation in horses.7–11 One strategy is the open-lung concept, which involves an ARM to reopen terminal bronchioles and alveoli in nonventilated lung fields, which is followed by application of PEEP to prevent recollapse of those lung areas.12,13 A recent study11 conducted to investigate the efficacy of a PEEP-titration ARM in horses revealed that differences in outcome depend on the composition of the inspired gas mixture. Although improvements in gas exchange were achieved in horses breathing pure oxygen in that study,11 the ARM in horses breathing heliox (a mixture of 30% oxygen and 70% helium) resulted in greater improvements in pulmonary oxygen exchange and a better Cdyn associated with a lower PIP. Helium reduces turbulent flow and thus airway resistance, which thereby facilitates lung ventilation and gas exchange. Helium is less dense than air and oxygen; thus, it is thought that helium better maintains laminar gas flow through narrow bronchioles and results in more homogenous gas distribution throughout the lungs.14 In addition, reducing the Fio2 by adding helium as an inert gas to inspired gas mixtures might reduce the risk for formation of reabsorption atelectasis.15 The extent to which use of helium versus other inert gases (eg, nitrogen) offers clinical benefits for improving pulmonary gas exchange and respiratory mechanics is controversial.16
The objective of the study reported here was to evaluate whether the inert gas (nitrogen vs helium) would impact the efficacy of an ARM for improving lung compliance and Pao2 in isoflurane-anesthetized horses. We hypothesized that because of its physical properties, helium would be a more beneficial inert gas, allow application of lower peak airway pressures during an ARM, and require a lower PEEP to maintain Pao2.
Materials and Methods Animals
Six healthy university-owned horses (4 Standard-breds and 2 Thoroughbreds; 2 mares and 4 geldings) with a mean ± SD body weight of 587 ± 37 kg and mean age of 7 ± 4 years were used in a prospective crossover study. Horses were considered healthy on the basis of results of preanesthetic physical examination. Horses were housed in stalls for 12 to 24 hours before each experiment and were allowed ad libitum access to hay and water. The study was approved by the Institutional Animal Care and Use Committee of the University of Pennsylvania (trial registry No. 806212-aadaiai).
Anesthesia
Food, but not water, was withheld for 8 hours before each experiment. Tissues over the left jugular vein were infiltrated with lidocaine, and a 14-gauge catheter was placed aseptically. Each horse was pre-medicated with xylazine hydrochloridea (0.8 mg/kg, IV), and anesthesia was induced by IV administration of midazolamb (0.05 mg/kg) and ketamine hydrochloridec (2.2 mg/kg). After anesthesia was induced, orotracheal intubation was performed with a 26-mm (internal diameter) cuffed Murphy tube. Horses were lifted with a hoist and positioned in dorsal recumbency on a thick foam mattress.
Anesthesia was maintained with isofluraned in 30% oxygen and 70% nitrogen (treatment N) or heliox (treatment H). The ETISO was targeted at 1.3% and adjusted as necessary. A crystalloid solutione was administered IV at a rate of 5 mL/kg/h, and dobutaminef was administered as needed to maintain an MAP > 70 mm Hg throughout anesthesia. Dobutamine infusion was stopped when the MAP increased to > 85 mm Hg.
The transverse facial artery was cannulated with a 20-gauge catheter for invasive monitoring of blood pressure and collection of arterial blood samples. The catheter was connected to a calibrated pressure transducerg via fluid-filled (heparinized saline [0.9% NaCl] solution) rigid extension lines and calibrated to zero (atmospheric pressure) at the level of the shoulder joint.
After the experiment concluded, horses were lifted with a hoist and moved to a recovery stall. When spontaneous ventilation commenced, the orotracheal tube was removed and the horses were nasotracheally intubated to facilitate insufflation of oxygen at a rate of 15 L/min. Each horse received xylazine (0.2 mg/kg, IV) as a sedative and was allowed to recover unassisted.
Procedures were repeated after a minimum washout period of 1 week. At that time, horses received the other carrier gas.
Ventilation strategy
After anesthesia was induced and endotracheal intubation was completed, each horse was connected to the large-animal breathing circuit of an anesthesia workstationh equipped with a piston-driven ventilator. Horses were ventilated by use of a volume-controlled, pressure-limited ventilation mode with an inspiratory-to-expiratory ratio of 1:2 and a fixed VT of 14 mL/kg. Respiratory rate was adjusted to maintain PETCO2 between 35 and 45 mm Hg. A Pitot flowmeteri and anesthesia monitorj were used for respirometry. Before each experiment, the Pitot flowmeter was calibrated with a 3-L calibration syringek by use of the respective carrier gas mixture. The PIP was measured by the integral multiparameter monitor unit of the anesthesia workstation, and Cdyn was calculated as VT/(PIP - PEEP).17
After horses were anesthetized for a 60-minute equilibration period, ventilation with a PEEP of 0 cm H2O (baseline) was started, and measurements were obtained. A PEEP-titration ARM was performed by increasing the PEEP in increments of 5 cm H2O every 5 minutes to a maximum of 30 cm H2O while maintaining a constant Vt, which was followed by decreasing the PEEP in increments of 5 cm H2O every 5 minutes until each horse's optimal PEEP was identified. Optimal PEEP was defined as the lowest PEEP attainable that did not cause a substantial (> 10%) decrease in Pao2.18 Horses were ventilated at the optimal PEEP for 120 minutes.
Immediately before changing the PEEP during each titration step and at 5, 30, 60, 90, and 120 minutes during the period when horses were maintained at optimal PEEP, the MAP, heart rate, Petco2, PIP, and Cdyn were recorded. In addition, an arterial blood sample was collected anaerobically and analyzed immediately with a blood gas analyzerl to measure Pao2 and Paco2. The Fio2 was monitored continuously with a paramagnetic analyzer that was part of the integral multiparameter monitor unit of the anesthesia workstation; the paramagnetic analyzer recalibrated itself on a regularly scheduled basis. The arterial component (Pao2) of Pao2 - Pao2 was calculated by use of the following equation:
where PATM is the atmospheric pressure measured the morning of each experiment, PH2O is the vapor pressure of water at body temperature, and R is the respiratory exchange ratio (value = 0.8). The ratio of Paco2 to Petco2 was calculated as follows: (Paco2 - Petco2)/Paco2. Values for ΔPinsp were calculated as PIP - PEEP.
Statistical analysis
An a priori power analysis (type I error = 0.05; type II error = 0.2) revealed that 6 animals would be necessary to detect significant differences in Cdyn and Pao2 on the basis of results for a previous study.11
Data analysis was performed with statistical software.m,n Visual assessment of Q-Q plots and the Shapiro-Wilk test were used to confirm normal distribution of model residuals of dependent variables. All variables were compared with baseline values and between treatments.
Data were reported as mean ± SD. Statistical analysis was undertaken in terms of a 2-factor repeated-measures ANOVA and mixed-effects linear regressions. The χ2 test was used to determine significant differences between treatments H and N for the optimal titrated PEEP. For all analyses, significance was at values of P 0.05.
Results
All horses successfully received both treatments. All horses recovered uneventfully from anesthesia.
No significant differences in dobutamine administration were detected between the 2 treatments; mean ± SD infusion rate was 1.03 ± 0.17 μg/kg/min for treatment N and 0.86 ± 0.02 μg/kg/min for treatment H. Similarly, MAP and heart rate did not change significantly with increasing or decreasing PEEP, compared with baseline values (Table 1).
Mean ± SD values for cardiorespiratory variables of 6 isoflurane-anesthetized horses ventilated with 30% oxygen and 70% nitrogen (treatment N) or heliox (30% oxygen and 70% helium; treatment H) during increasing and decreasing PEEP titration and during a 120-minute period at optimal PEEP.
| MAP (mm Hg) | Heart rate (beats/min) | Respiratory rate (breaths/min) | ||||
|---|---|---|---|---|---|---|
| PEEP (cm H2O) | N | H | N | H | N | H |
| 0 | 77 ± 6 | 76 ± 11 | 41 ± 6 | 39 ± 5 | 5 ± 1 | 5 ± 1 |
| 5 | 82 ± 13 | 78 ± 10 | 43 ± 7 | 41 ± 5 | 5 ± 1 | 5 ± 1 |
| 10 | 84 ± 13 | 84 ± 10 | 44 ± 7 | 41 ± 8 | 5 ± 1 | 5 ± 1 |
| 15 | 82 ± 11 | 84 ± 10 | 45 ± 8 | 42 ± 4 | 5 ± 1 | 5 ± 1 |
| 20 | 79 ± 10 | 83 ± 6 | 46 ± 7 | 47 ± 4 | 5 ± 1 | 5 ± 1 |
| 25 | 75 ± 20 | 79 ± 10 | 50 ± 4 | 48 ± 6 | 5 ± 1 | 5 ± 1 |
| 30 | 68 ± 23 | 79 ± 21 | 48 ± 5 | 50 ± 8 | 5 ± 1 | 5 ± 1 |
| 25 | 75 ± 20 | 79 ± 17 | 45 ± 6 | 47 ± 6 | 5 ± 1 | 5 ± 1 |
| 20 | 80 ± 11 | 85 ± 10 | 41 ± 6 | 42 ± 4 | 5 ± 1 | 5 ± 1 |
| 15 | 83 ± 11 | 84 ± 12 | 41 ± 5 | 41 ± 1 | 5 ± 1 | 5 ± 1 |
| 10 | 87 ± 9 | 82 ± 9 | 41 ± 6 | 42 ± 3 | 5 ± 1 | 5 ± 1 |
| 5 | ND | 76 ± 8 | ND | 42 ± 3 | ND | 5 ± 1 |
| Optimal | ||||||
| T5 | 86 ± 12 | 80 ± 5 | 41 ± 3 | 41 ± 3 | 5 ± 1 | 5 ± 1 |
| T30 | 80 ± 12 | 82 ± 19 | 41 ± 4 | 42 ± 5 | 5 ± 1 | 5 ± 1 |
| T60 | 89 ± 15 | 92 ± 10 | 41 ± 4 | 40 ± 6 | 5 ± 1 | 5 ± 1 |
| T90 | 88 ± 14 | 90 ± 13 | 40 ± 5 | 40 ± 5 | 5 ± 1 | 5 ± 1 |
| T120 | 93 ± 15 | 92 ± 9 | 40 ± 5 | 40 ± 5 | 5 ± 1 | 5 ± 1 |
Optimal PEEPs of T5, T30, T60, T90, and T120 represent times that data were collected during the 120-minute period when horses were maintained at optimal PEEP.
ND = Not determined; data were not available because none of the horses maintained Pao2 at a PEEP of 5 cm H2O.
Measured and calculated respiratory variables were determined (Table 2). At baseline, there were no significant differences in Pao2, Pao2 - Pao2, PIP, ΔPinsp, Cdyn, and the ratio of Paco2 to Petco2 between treatments N and H. The ratio of Paco2 to Petco2 did not differ significantly between treatments or change over time during the experiments. The PIP steadily increased with increasing PEEP for both treatments. At a PEEP of 30 cm H2O, PIP was significantly (P = 0.009) higher for treatment N than treatment H, which resulted in a significantly higher ΔPinsp at this PEEP for treatment N.
Mean ± SD values for respiratory variables of 6 isoflurane-anesthetized horses ventilated with 30% oxygen and 70% nitrogen (treatment N) and heliox (30% oxygen and 70% helium; treatment H) during increasing and decreasing PEEP titration and during a 120-minute maintenance phase at optimal PEEP.
| Pao2 (mm Hg) | Pao2 - Pao2 (mm Hg) | PIP (cm H2O) | ΔPinsp (cm H2O) | Cdyn (mL/cm H2O) | Ratio of Paco2 to Petco2 (%) | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PEEP (cm H2O) | N | H | N | H | N | H | N | H | N | H | N | H |
| 0 | 577 ± 13 | 77 ± 10 | 79 ± 16 | 71 ± 16 | 26 ± 6 | 25 ± 3 | 26 ± 6 | 25 ± 3 | 305 ± 66 | 334 ± 67 | 13 ± 6 | 12 ± 4 |
| 5 | 79 ± 9 | 82 ± 6 | 81 ± 12 | 71 ± 11 | 30 ± 6 | 30 ± 5 | 25 ± 6 | 25 ± 4 | 317 ± 58 | 352 ± 72 | 12 ± 6 | 12 ± 5 |
| 10 | 82 ± 8 | 83 ± 7 | 76 ± 11 | 68 ± 11 | 34 ± 5* | 31 ± 3 | 24 ± 5 | 21 ± 3 | 324 ± 61 | 361 ± 58 | 9 ± 5 | 10 ± 7 |
| 15 | 85 ± 9 | 87 ± 8 | 72 ± 10 | 63 ± 14 | 39 ± 3* | 37 ± 2* | 24 ± 3 | 22 ± 2 | 322 ± 46 | 349 ± 67 | 15 ± 4 | 16 ± 7 |
| 20 | 87 ± 11 | 92 ± 12 | 69 ± 13 | 57 ± 17 | 44 ± 3* | 41 ± 2* | 24 ± 3 | 21 ± 3* | 319 ± 38 | 363 ± 38† | 17 ± 8 | 16 ± 7 |
| 25 | 91 ± 10 | 95 ± 11* | 58 ± 11 | 47 ± 15 | 49 ± 3* | 46 ± 3* | 24 ± 3 | 21 ± 2* | 308 ± 31 | 366 ± 37† | 18 ± 7 | 21 ± 5 |
| 30 | 99 ± 12* | 99 ± 14* | 61 ± 15 | 55 ± 18 | 54 ± 2* | 49 ± 2*† | 24 ± 2 | 19 ± 2† | 317 ± 42 | 350 ± 39† | 15 ± 9 | 17 ± 8 |
| 25 | 106 ± 11* | 116 ± 5* | 47 ± 12* | 36 ± 8* | 45 ± 3* | 42 ± 2* | 20 ± 3* | 17 ± 2* | 370 ± 38* | 417 ± 49*† | 14 ± 8 | 16 ± 7 |
| 20 | 109 ± 8* | 122 ± 5* | 50 ± 10* | 39 ± 11* | 38 ± 3* | 35 ± 2* | 18 ± 3* | 15 ± 4* | 446 ± 68* | 465 ± 55* | 14 ± 8 | 14 ± 6 |
| 15 | 111 ± 8* | 126 ± 4*† | 49 ± 10* | 28 ± 9*† | 33 ± 3* | 30 ± 3 | 18 ± 3* | 15 ± 2* | 460 ± 83* | 497 ± 64* | 15 ± 5 | 17 ± 5 |
| 10 | 101 ± 7* | 123 ± 6*† | 52 ± 9* | 31 ± 15*† | 28 ± 3 | 26 ± 1 | 18 ± 3* | 16 ± 2* | 434 ± 73* | 495 ± 69*† | 15 ± 8 | 18 ± 6 |
| 5 | ND | 113 ± 4* | ND | 34 ± 9* | ND | 22 ± 1 | ND | 17 ± 1* | ND | 442 ± 70 | ND | 13 ± 10 |
| Optimal | ||||||||||||
| T5 | 101 ± 8* | 113 ± 2*† | 57 ± 10* | 38 ± 9*† | 34 ± 4* | 28 ± 4 | 19 ± 5* | 17 ± 2* | 413 ± 77* | 466 ± 72*† | 14 ± 3 | 12 ± 2 |
| T30 | 102 ± 7* | 115 ± 3*† | 56 ± 9* | 38 ± 7*† | 35 ± 4* | 29 ± 4 | 20 ± 4* | 17 ± 2* | 409 ± 67* | 447 ± 72*† | 11 ± 3 | 10 ± 2 |
| T60 | 99 ± 8* | 115 ± 4*† | 58 ± 8* | 40 ± 9*† | 35 ± 4* | 30 ± 5 | 20 ± 4* | 19 ± 3* | 389 ± 63* | 453 ± 62*† | 18 ± 2 | 14 ± 4 |
| T90 | 100 ± 7* | 114 ± 3*† | 56 ± 10* | 40 ± 6*† | 36 ± 4* | 30 ± 6 | 21 ± 4* | 18 ± 4* | 372 ± 50* | 419 ± 62*† | 14 ± 6 | 13 ± 6 |
| T120 | 99 ± 8* | 115 ± 4*† | 57 ± 10* | 41 ± 7*† | 36 ± 3* | 30 ± 5 | 21 ± 3* | 18 ± 3* | 375 ± 62* | 434 ± 75*† | 17 ± 2 | 13 ± 4 |
The APinsp represents the pressure difference between PIP and PEEP. The ratio of Paco2 to Petco2 represents the alveolar dead space.
Within a column, value differs significantly (P < 0.05) from the value at 0 cm H2O (baseline PEEP).
Within a variable, value differs significantly (P < 0.05) from the value for treatment N.
See Table 1 for remainder of key.
The optimal PEEP was consistently 15 cm H2O for treatment N, whereas for treatment H, it was 10 cm H2O for 4 horses and 15 cm H2O for 2 horses. There was a significant difference in optimal PEEP between treatments.
At the highest PEEP (30 cm H2O), the ARM resulted in a significant increase in Pao2 and a significant decrease in Pao2 - Pao2 for both treatments; however, when receiving treatment N, horses had a significantly lower Pao2 at PEEPs of 10 and 15 cm H2O during the decreasing phase of the ARM as well as during the subsequent maintenance period with the optimal PEEP. For treatment N, Pao2 - Pao2 was higher at PEEPs of 10 and 15 cm H2O during the decreasing phase of the ARM and during the 120-minute maintenance period, compared with the Pao2 - Pao2 for treatment H (Table 2). The Cdyn started to improve with increases in PEEP for both treatments, but a significant difference from the baseline value was obvious only during the decreasing phase of the ARM; this significant difference was then maintained throughout the subsequent 120-minute maintenance period. The Cdyn did not improve as rapidly for treatment N as for treatment H during the incremental PEEP-titration phase of the ARM. At PEEPs of 20, 25, and 30 cm H2O, Cdyn was consistently lower for treatment N than for treatment H until the highest PEEP of 30 cm H2O was reached. This difference recurred at a PEEP of 10 cm H2O during the decreasing phase of the ARM and during optimal PEEP in the 120-minute maintenance period. The PIP and ΔPinsp were significantly different between the 2 treatments at the highest PEEP of 30 cm H2O.
Discussion
Findings for the study reported here supported our hypothesis that an ARM with increasing and decreasing PEEP titration would be more efficacious in horses breathing a gas mixture containing helium than one containing nitrogen as the inert gas component. Addition of helium to the fresh gas has been found to improve pulmonary oxygen exchange and respiratory mechanics in horses.6,11 However, investigators of those studies compared heliox with oxygen as the inspired gas (ie, a gas mixture with a low FIO2 versus one with a high Fio2). Therefore, it was unclear whether those findings were primarily attributable to the inertia of the second gas component (which was helium in the present study) keeping alveoli open by minimizing formation of atelectasis, as has been reported for other species (eg, dogs breathing fresh gas with a low Fio28,19), or to the fact that helium is a gas that is much less dense than nitrogen or oxygen.
Comparing baseline values of the study reported here with those of a previous study11 revealed that Cdyn was consistently higher in horses breathing a gas mixture with a low Fio2 than in horses breathing pure oxygen. This difference was also maintained throughout most phases of the ARM. The higher Cdyn might indicate overall better lung aeration in animals breathing gas mixtures with a low Fio2.20 However, these data do not allow us to determine whether improved aeration was primarily attributable to prevention of lung compression as a result of dorsal recumbency or to less absorption atelectasis. A study21 of anesthetized horses provided evidence that a low Fio2 is not associated with a decrease in pulmonary shunting and does not change the ratio of Paco2 to Petco2 during ventilation; therefore, some authors have concluded that compression rather than absorption atelectasis is primarily responsible for low oxygen exchange in anesthetized horses.4
In the present study, Pao2 and Pao2 - Pao2 improved significantly during and following the decreasing phase of the ARM for both treatments; however, Pao2 - Pao2 was significantly lower for treatment H during the decreasing phase of the ARM and during the subsequent 120-minute maintenance period in which horses were ventilated at an optimal PEEP. In addition, from the beginning of exposure to a high PEEP (≥ 20 cm H2O) until the end of the subsequent 120-minute maintenance period, Cdyn was consistently higher for treatment H than treatment N. Together, these findings indicated that lung recruitment in horses breathing heliox was more efficacious for treatment N, even though the overall difference would be considered small in a clinical setting. Indeed, the lower density of helium as compared with that of nitrogen or oxygen promotes a more laminar gas flow, which causes less turbulences and hence less flow resistance in the smaller airways and thereby facilitates better overall lung aeration during and after an ARM.22 However, the lower Cdyn observed for treatment H did not translate into a significantly lower PIP or ΔPinsp, compared with values for treatment N, except at a PEEP of 30 cm H2O.
From the perspective of lung-protective ventilation, optimal PEEP has been defined as the lowest pressure required to maximize oxygenation, limit formation of end-expiratory atelectasis, and minimize end-inspiratory overdistension.23 Inappropriately high PEEP will lead to high PIPs that can cause overdistension of alveoli and result in barotrauma, whereas inadequate PEEP can result in atelectrauma attributable to repeated opening and closing of alveoli.23 Additionally, optimal PEEP results in the lowest ratio of Paco2 to Petco2 and greatest compliance.24 Several methods for determining the optimal PEEP have been described.18,23–26 During the decreasing phase of the ARM, the PEEP at which a decrease in both Pao2 and Cdyn can be detected indicates that the airway pressure is not adequate to prevent recollapse of alveoli. The previous PEEP is then identified as optimal for that individual animal.10,18,24,25 In the study reported here, benefits of the ARM were preserved well during the 120-minute maintenance period for both treatments. This supported the contention that the inclusion of an inert gas in the inspired gas and the application of an optimal PEEP after an ARM would result in optimized recovery of respiratory mechanics, thus reducing the likelihood of having to repeat the ARM during the same anesthetic episode.
The ARM led to greater improvement in Cdyn and Pao2 when horses were ventilated with heliox. Because Vt values are used for calculation of Cdyn, it could be argued that inaccuracies in ventilator-delivered Vt determinations attributable to differences in inspired gas compositions for treatments N and H could have accounted for the differences in Cdyn between the treatments. However, a recent study27 in which ventilator-delivered and reference volumes were compared revealed no significant differences for volume or gas composition, yielding a bias of −0.046 L (limits of agreement, −0.175 to 0.082 L), −0.007 L (limits of agreement, −0.115 to 0.099 L), −0.002 L (limits of agreement, −0.033 to 0.028 L), and 0.031 L (limits of agreement, −0.079 to 0.142 L) for air, > 95% oxygen, a mixture of 30% oxygen and 70% nitrogen, and heliox, respectively.
Overall differences in efficacy of the ARM for improving arterial oxygenation and Cdyn were subtle between the 2 treatments in the present study. Therefore, in clinical settings, the additional expense associated with the use of heliox must be considered. In the United States, heliox currently is approximately 2.5 times as expensive as oxygen and approximately 8 times as expensive as medical air. Considering that a modern anesthesia workstation similar to the one used in the study reported here would contain an air pump or medical airflow meter to allow blending of pure oxygen with air to produce the desired Fio2, the lower expense associated with the use of air would be substantial.
The study had some limitations. First, the use of relatively large incremental PEEP steps of 5 cm H2O limited the ability to precisely determine the optimal PEEP for the 2 treatments. Use of a smaller PEEP step could have enabled us to more accurately determine the pressure at which a decrease in Pao2 and Cdyn began to recur, which potentially could have led to more consistent determination of a lower optimal PEEP for the maintenance period for treatment H. Second, in contrast to clinical settings, we did not perform a second ARM after PEEP titration to reopen the lungs before returning to the optimal PEEP. Third, we did not include a treatment of pure oxygen in the present study to allow for a direct comparison with data for treatments N and H. However, when comparing data of the present study with findings from a previous study11 conducted under similar experimental conditions to determine the efficacy of an ARM in dorsally recumbent horses breathing heliox or pure oxygen, it may be concluded that the presence of an inert gas is beneficial for obtaining open airways and improving gas exchange with an ARM. Finally, the study cohorts were rather small (6 horses/treatment), even though the power analysis indicated that this number of animals would be sufficient to detect significant differences between treatments.
In the study reported here, a PEEP-titration ARM performed in dorsally recumbent isoflurane-anes-thetized horses was more efficacious for improving Pao2 and Cdyn when animals were ventilated with a gas mixture containing helium instead of nitrogen as an inert gas component. However, these differences were of limited clinical importance.
Acknowledgments
Funded by the Raymond Firestone Trust and Tamworth Research Grant.
The authors declare that there were no conflicts of interest. Presented in abstract form at the Association of Veterinary Anaesthetists’ Fall Meeting, Venice, Italy, September 2018.
ABBREVIATIONS
| ARM | Alveolar recruitment maneuver |
| Cdyn | Dynamic lung compliance |
| Δinsp | Difference between peak inspiratory pressure and positive end-expiratory pressure |
| ETISO | End-tidal isoflurane concentration |
| Fio2 | Fraction of inspired oxygen |
| MAP | Mean arterial blood pressure |
| Pao2 - Pao2 | Alveolar-arterial gradient in partial pressure of oxygen |
| PEEP | Positive end-expiratory pressure |
| Petco2 | End-tidal partial pressure of carbon dioxide |
| PIP | Peak inspiratory pressure |
| Vt | Tidal volume |
Footnotes
X-ject, Henry Schein Inc, Dublin, Ohio.
Akorn Medical, Vernon Hills, Ill.
Ketathesia, Henry Schein Inc, Dublin, Ohio.
Isothesia, Henry Schein Inc, Dublin, Ohio.
Normosol, Abbott Laboratories, Abbott Park, Ill.
Hospira Inc, Lake Forest, Ill.
PressureTrans, Henry Schein Inc, Dublin, Ohio.
Tafonius, Hallowell EMC, Pittsfield, Mass.
H-Lite, Morpheus Engineering, Wenum Wiesel, Netherlands.
Cardiocap/5, Datex-Ohmeda Inc, Madison, Wis.
Ohio Cal-check 3-liter calibration syringe, Ohio Medical Products, Gurnee, Ill.
Opti CCA-TS2, blood gas analyzer, Opti Medical Systems Inc, Roswell, Ga.
SAS, version 9.3, SAS Institute Inc, Cary, NC.
GraphPad Prism, version 7, GraphPad Software Inc, San Diego, Calif.
References
1. Hall LW, Gillespie JR, Tyler WS. Alveolar-arterial oxygen tension differences in anaesthetized horses. Br J Anaesth 1968;40:560–568.
2. Hall LW. Disturbances of cardiopulmonary function in anaesthetised horses. Equine Vet J 1971;3:95–98.
3. Nyman G, Funkquist B, Kvart C, et al. Atelectasis causes gas exchange impairment in the anaesthetised horse. Equine Vet J 1990;22:317–324.
4. Hubbell JA, Muir WW. Oxygenation, oxygen delivery and anesthesia in the horse. Equine Vet J 2015;47:25–35.
5. Joyce CJ, Baker AB, Kennedy RR. Gas uptake from an unventilated area of lung: computer model of absorption atelectasis. Physiol Rep 1993;74:1107–1116.
6. Staffieri F, Bauquier S, Moate P, et al. Pulmonary gas exchange in anaesthetized horses mechanically ventilated with oxygen or helium/oxygen mixture. Equine Vet J 2009;41:747–752.
7. Rothen HU, Neumann P, Berglund JE, et al. Dynamics of re-expansion of atelectasis during general anesthesia. Br J Anaesth 1999;82:551–556.
8. Celebi S, Koner O, Menda F, et al. The pulmonary and hemodynamic effects of two different recruitment maneuvers after cardiac surgery. Anesth Analg 2007;104:384–390.
9. Cakmakkaya OS, Kaya G, Altintas F, et al. Restoration of pulmonary compliance after laparoscopic surgery using a simple alveolar recruitment maneuver. J Clin Anesth 2009;21:422–426.
10. Hopster K, Wogatzki A, Geburek F, et al. Effects of positive end-expiratory pressure titration on intestinal oxygenation and perfusion in isoflurane anaesthetised horses. Equine Vet J 2017;49:250–256.
11. Hopster K, Duffee LR, Hopster-Iversen CCS, et al. Efficacy of an alveolar recruitment maneuver in improving gas exchange and pulmonary mechanics in anesthetized horses ventilated with oxygen or a helium-oxygen mixutre. Am J Vet Res 2018;79:1021–1027.
12. Lachmann B. Open up the lung and keep the lung open. Intensive Care Med 1992;18:319–321.
13. Barbas CS, de Matos GF, Pincelli MP, et al. Mechanical ventilation in acute respiratory failure: recruitment and high positive end-expiratory pressure are necessary. Curr Opin Crit Care 2005;11:18–28.
14. Papamoschou D. Theoretical validation of the respiratory benefits of helium-oxygen mixtures. Respir Physiol 1995;99:183–190.
15. Dale WA, Rahn H. Rate of gas absorption during atelectasis. Am J Physiol 1952;170:606–613.
16. Kleiman AM, Huffmeyer JL. Helium: is the sky the limit? Respir Care 2018;63:488–490.
17. Moens YP, Gootjes P, Ionita JC, et al. In vitro validation of a Pitot-based flow meter for the measurement of respiratory volume and flow in large animal anaesthesia. Vet Anaesth Analg 2009;36:209–219.
18. Suh GY, Kwon OJ, Yoon JW, et al. A practical protocol for titrating “optimal” PEEP in acute lung injury: recruitment maneuver and PEEP decrement. J Korean Med Sci 2003;18:349–354.
19. Staffieri F, Franchini D, Carella GL, et al. Computed tomo-graphic analysis of the effects of two inspired oxygen concentrations on pulmonary areation in anesthetized and mechanically ventilated dogs. Am J Vet Res 2007;68:925–931.
20. Ambrisko TD, Schramel J, Hopster K, et al. Assessment of distribution of ventilation and regional lung compliance by electrical impedance tomography in anaesthetized horses undergoing alveolar recruitment manoeuvres. Vet Anaesth Analg 2017;44:264–272.
21. Hubbell JA, Aarnes TK, Bednarski RM, et al. Effect of 50% and maximal inspired oxygen concentrations on respiratory variables in isoflurane-anesthetized horses. BMC Vet Res 2011;7:23.
22. Hurford WE, Cheifetz IM. Repiratory controversies in the critical care setting. Should heliox be used for mechanically ventilated patients? Respir Care 2007;52:582–591.
23. Baikunje N, Sehgal IS, Dhooria S, et al. Titration of ideal positive end-expiratory pressure in acute respiratory distress syndrome: comparison between lower inflection point and esophageal presure method using volumetric capnography. Indian J Crit Care Med 2017;21:322–325.
24. Suter PM, Fairley B, Isenberg MD Optimum end-expiratory airway pressure in patients with acute pulmonary failure. N Engl J Med 1975;292:284–289.
25. Suarez-Sipmann F, Bohm SH, Tusman G, et al. Use of dynamic compliance for open lung positive end-expiratory titration in an experimental study. Crit Care Med 2007;35:214–221.
26. Moens Y, Schramel JP, Tusman G, et al. Variety of noninvasive continuous monitoring methodologies including electrical impedance tomography provides novel insights into the physiology of lung collapse and recruitment—case report on an anesthetized horse. Vet Anaesth Analg 2014;41:196–204.
27. Hopster K, Bertone C, Driessen B. Evaluation of the effects of gas volume and composition on accuracy of volume measurement by two flow seonsors and delivery by a piston-driven large-animal ventilator. Am J Vet Res 2019;80:135–143.
