Effects of pneumoperitoneum induced at various pressures on cardiorespiratory function and working space during laparoscopy in cats

Philipp D. Mayhew Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Peter J. Pascoe Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Philip H. Kass Department of Population Health and Reproduction, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Yael Shilo-Benjamini Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Abstract

Objective—To evaluate the effect of pneumoperitoneum on cardiorespiratory variables and working space during experimental induction of 3 intra-abdominal pressures (IAPs) in cats.

Animals—6 healthy young adult neutered male domestic shorthair cats.

Procedures—All cats were anesthetized through use of a standardized protocol. A catheter was placed in the right femoral artery for blood pressure and blood gas monitoring. A thermodilution catheter was placed in the right jugular vein via fluoroscopic guidance. Cardiopulmonary variables were measured before (baseline) and 2 and 30 minutes after initiation of pneumoperitoneum at IAPs of 4, 8, and 15 mm Hg; these were created through the use of a mechanical insufflator. At each IAP, abdominal dimensions (height, width, and circumference) were measured at a standardized location.

Results—At 4 mm Hg and 8 mm Hg IAP, no clinically important changes were identified in cardiorespiratory values. Heart rate, cardiac index, and stroke volume index remained unchanged throughout the study at all IAPs. Mean arterial blood pressure began to increase at 8 mm Hg and was significantly higher, compared with baseline, at both time points at 15 mm Hg. At 15 mm Hg, Paco2 was significantly higher and cats were more acidotic than at baseline. Working space was subjectively greater at 8 mm Hg than at 4 mm Hg IAP; however, at 15 mm Hg, no clinically important enlargement of the working space was identified, compared with at 8 mm Hg.

Conclusions and Clinical Relevance—Values of cardiopulmonary variables were largely unchanged by induction of pneumoperitoneum in healthy cats up to an IAP of 8 mm Hg, and no clinically important increases in working space were evident at an IAP of 15 versus 8 mm Hg. These findings provide little justification for use of IAPs > 8 mm Hg in healthy cats undergoing laparoscopic procedures; however, whether the situation is similar in diseased or elderly cats remains to be determined.

Abstract

Objective—To evaluate the effect of pneumoperitoneum on cardiorespiratory variables and working space during experimental induction of 3 intra-abdominal pressures (IAPs) in cats.

Animals—6 healthy young adult neutered male domestic shorthair cats.

Procedures—All cats were anesthetized through use of a standardized protocol. A catheter was placed in the right femoral artery for blood pressure and blood gas monitoring. A thermodilution catheter was placed in the right jugular vein via fluoroscopic guidance. Cardiopulmonary variables were measured before (baseline) and 2 and 30 minutes after initiation of pneumoperitoneum at IAPs of 4, 8, and 15 mm Hg; these were created through the use of a mechanical insufflator. At each IAP, abdominal dimensions (height, width, and circumference) were measured at a standardized location.

Results—At 4 mm Hg and 8 mm Hg IAP, no clinically important changes were identified in cardiorespiratory values. Heart rate, cardiac index, and stroke volume index remained unchanged throughout the study at all IAPs. Mean arterial blood pressure began to increase at 8 mm Hg and was significantly higher, compared with baseline, at both time points at 15 mm Hg. At 15 mm Hg, Paco2 was significantly higher and cats were more acidotic than at baseline. Working space was subjectively greater at 8 mm Hg than at 4 mm Hg IAP; however, at 15 mm Hg, no clinically important enlargement of the working space was identified, compared with at 8 mm Hg.

Conclusions and Clinical Relevance—Values of cardiopulmonary variables were largely unchanged by induction of pneumoperitoneum in healthy cats up to an IAP of 8 mm Hg, and no clinically important increases in working space were evident at an IAP of 15 versus 8 mm Hg. These findings provide little justification for use of IAPs > 8 mm Hg in healthy cats undergoing laparoscopic procedures; however, whether the situation is similar in diseased or elderly cats remains to be determined.

Laparoscopy is an emerging modality in the growing field of veterinary minimally invasive surgery Although laparoscopic procedures have been greeted enthusiastically by some surgeons as a way to reduce postoperative pain and hasten return to normal activity postoperatively, few studies1–5 have been performed to evaluate the physiologic response to pneumoperitoneum in small animals.

Use of laparoscopy in cats was first reported in 1977,6 yet few reports7–11 of laparoscopic surgery in cats have been published since that time. Intentional induction of pneumoperitoneum is integral to most laparoscopic procedures, and the physiologic effects of pneumoperitoneum have been described in dogs.1–5 As IAP increases, an initial increase in cardiac output can occur as blood from splanchnic circulation is pushed into systemic venous circulation and returned to the heart.2 However, an overall decrease in cardiac output is reported to develop during pneumoperitoneum as pressure on thin-walled vascular structures in the abdomen compromises venous return and a general increase in systemic vascular resistance occurs.5 This is particularly true at high IAPs.1,2 A study5 in dogs also revealed that at pressures up to 15 mm Hg, no clinically relevant reduction in cardiac output occurs.5 From consideration of these data has come the general recommendation that IAPs up to 15 mm Hg are considered safe in dogs. No such studies have been performed in cats, and species-specific guidelines are needed for clinicians wishing to perform laparoscopic procedures in feline patients.

The volume of working space created by intentionally induced pneumoperitoneum is largely dependent on the degree of abdominal insufflation. To the authors’ knowledge, the relationship between working space and IAP in dogs or cats has not been scientifically established, although anecdote suggests that in cats, even at relatively low IAPs (approx 4 mm Hg), adequate working space can be created to complete some procedures.8 The reason for this phenomenon is unknown, but a hypothesis is that the considerable distensibility of the abdominal wall in cats might allow low-pressure pneumoperitoneum to produce a greater volume of working space for any given IAP relative to some other species.8 Elucidation of the actual relationship between working space and IAP in cats would allow guidelines to be created for surgeons wishing to incorporate laparoscopy into their practices. Indeed, if low-pressure pneumoperitoneum can be routinely used, then its use may decrease the incidence of cardiorespiratory compromise during laparoscopic procedures. Because pneumoperitoneum has been suggested to be one of the principal causes of surgical stress during laparoscopy, high-pressure pneumoperitoneum may be particularly contraindicated in geriatric cats or those with preexisting cardiorespiratory disease that may not tolerate high IAP as well as healthy cats.12

The purpose of the study reported here was to evaluate the cardiorespiratory response in healthy adult cats during surgical induction of pneumoperitoneum at various IAPs and to quantify the working space provided by pneumoperitoneum at those IAPs. Our hypotheses were that pneumoperitoneum would be well tolerated by the cats at all IAPs and that no significant difference in volume of working space would exist between low and high pressure pneumoperitoneum.

Materials and Methods

Animals—Six healthy neutered male domestic shorthair research cats (median body weight, 6.1 kg [range, 5.6 to 6.6 kg]; approx age, 3 years) were used in the study. Prior to undergoing anesthesia, all cats underwent a CBC, serum biochemical analysis, and thoracic radiography to rule out preexisting disease. The study protocol was approved by the Institutional Animal Care and Use Committee of the University of California-Davis.

Anesthesia—Food was withheld from the cats for 12 hours in preparation for anesthesia. Atropine sulfatea (0.02 mg/kg) and buprenorphine hydrochloride13 (0.02 mg/kg) were administered SC in each cat. A 22-gauge catheter was then placed in a cephalic vein, and lactated Ringer's solution was administered IV at a rate of 5 mL/kg/h. Anesthesia was induced by slow IV administration of propofolc to effect (median dose, 4.05 mg/kg; range, 3.6 to 5.5 mg/kg). A cuffed endotracheal tube was placed in the trachea, and cats were connected to a nonrebreathing anesthetic circuit delivering isofluraned to effect. Cats were allowed to breathe spontaneously unless the end-tidal CO2 concentration exceeded 65 mm Hg, in which case positive pressure ventilation was instituted with minute ventilation adjusted to maintain eucapnia. The end-tidal concentration of isoflurane was adjusted to 1.8%.

Instrumentation—A surgical cutdown to the right femoral artery was performed in each anesthetized cat to allow placement of a 9-cm, 24-gauge arterial cathetere for collection of blood gas samples as well as continuous monitoring of arterial blood pressure. A 7.5-cm, 5F introducer sheathf was simultaneously placed percutaneously into the right jugular vein. A 75-cm, 4F thermodilution catheterg was passed through the introducer and advanced into the pulmonary artery with fluoroscopic guidance. The catheter was manipulated into position in the pulmonary artery, with its final position confirmed by fluoroscopic evaluation and detection of characteristic waveforms and pressures.

Surgical approach—After instrumentation, cats were positioned in dorsal recumbency and an aseptic surgical scrub of the ventral aspect of the abdomen was performed. A camera portal was established by use of the Hasson technique 1 cm caudal to the umbilicus for placement of a 6-mm cannulah into the peritoneal cavity. In all cats, a purse-string suture line of 2–0 nylon was placed into the linea alba surrounding the cannula in an attempt to minimize the likelihood of CO2 leakage. Pneumoperitoneum was induced with CO2 administered through a mechanical insufflator.i A 5-mm, 0° laparoscopej was briefly inserted into the subumbilical portal at the start of the procedure to ensure that the tip of the cannula was within the peritoneal cavity and not encroaching on any abdominal organs.

Experimental procedure—In each cat, baseline measurements were made with the subumbilical portal in place, prior to induction of pneumoperitoneum. Intra-abdominal pressures of 4, 8, and 15 mm Hg were then created with a mechanical insufflator in a randomized sequence, which was established for each cat with the aid of a randomization program.k At each IAP, pneumoperitoneum was maintained for 30 minutes. The IAP was cross-referenced to mechanical insufflator pressure readings by placement of a 20-gauge needle percutaneously into the peritoneal cavity. The needle was connected to a water manometer zeroed at the midabdominal level. At each IAP, a reading from the water manometer was obtained and compared with that recorded on the mechanical insufflator.

Data collection—A multiparameter monitorl was used to record cardiorespiratory variables, including HR, respiratory rate, end-tidal CO2 tension, end-tidal isoflurane concentration, oxygen saturation as measured via pulse oximetry, MAP, central venous pressure, PAP, and PAOP. The monitor used was assessed for accurate calibration at 20 and 200 mm Hg against a mercury manometer at the start of each trial. Cardiac output was measured with another monitor.m Core body temperature was measured through the thermodilution catheter, which was attached to the cardiac output monitor and was maintained from 35.8° to 38.6°C.

All variables were recorded at baseline and 2 and 30 minutes after induction of each IAP (4, 8, and 15 mm Hg). For cardiac output, at each time point 3 mL of ice-cold 5% dextrose solution was injected into the proximal port of the thermodilution catheter. Three thermodilution measurements that were ≤ 10% higher or lower than each other were recorded, and the mean of these values was used. Blood samples for arterial and mixed venous blood gas and arterial hemoglobin and mixed venous hemoglobin content were collected from each cat at baseline and 2 and 30 minutes after initiation of pneumoperitoneum, and measurements were carried out with a blood gas analyzer.n Variables recorded included pH, PaO2, Paco2, bicarbonate (HCO3), BE, and lactate. The recorded data and the following variables were calculated by use of standard formulas13–15: CI, SVI, SVVRI, pulmonary vascular resistance index, Cao2, Do2, o2, and oxygen extraction ratio.

Abdominal dimensions were measured in 3 ways at baseline as well as once after initiation of pneumoperitoneum at each IAP. Circumferential diameter of the abdominal cavity was measured with a flexible cloth tape measureo placed at a level 2 cm cranial to the location of the subumbilical cannula. Abdominal height and width were measured at the same level with a radiographic caliper. For the height measurement, one end of the caliper was placed under the cat's spinal column on the operating table, with the other end of the caliper lowered until it was flush with the ventral midline of the body wall at a location 2 cm cranial to the subumbilical portal. For width measurements, the 2 ends of the caliper were placed flush to the left and right lateral aspects of the body wall at the same level. At each IAP, the laparoscope was briefly inserted into the abdomen, and still images and short video segments were recorded for subjective assessment of the degree of working space created.

Postoperative care—All cats were recovered from anesthesia. Buprenorphine hydrochloride (0.01 mg/kg, IV) was administered when surgery concluded and every 6 hours for 24 hours afterward. Meloxicam was also administered in 1 dose (0.3 mg/kg, SC) when surgery concluded.

Statistical analysis—Values of cardiorespiratory variables at the various measurement points and IAPs are reported as mean ± SD. Mixed-effects linear regression was performed to evaluate the fixed effects of IAP, time (when applicable), and their interaction (when applicable) on cardiorespiratory variables, while controlling for the potentially confounding effect of sequence; individual cat was treated as a random effect. Mixed-effects linear regression was also performed to evaluate the fixed effects of manometry measurements of IAP. Post hoc comparisons were made with a Bonferroni correction. Statistical softwarep was used for all analyses. Values of P < 0.05 were considered significant for all analyses.

Results

Animals—All 6 cats underwent the surgical procedures to induce pneumoperitoneum at various IAPs without intraoperative complications. One cat had postoperative signs of conscious proprioceptive deficits in the right pelvic limb consistent with peroneal nerve neuropraxia, most likely related to the limb ties used during anesthesia. Deficits resolved without intervention in the 4 weeks after surgery. While anesthetized, all of the cats breathed spontaneously and did not require positive pressure ventilation.

Cardiorespiratory variables—At an IAP of 4 mm Hg, no changes from baseline values (before pneumoperitoneum induction) were identified in any cardiorespiratory variables at any measurement point (Table 1). At an IAP of 8 mm Hg, few changes were evident, although BE was significantly (P < 0.05) higher than the baseline value 30 minutes after pneumoperitoneum was induced. Mean arterial blood pressure was significantly higher than baseline 2 minutes after pneumoperitoneum induction at 8 mm Hg.

Table 1—

Mean ± SD values of cardiorespiratory variables in 6 healthy adult cats before (baseline) and 2 and 30 minutes after pneumoperitoneum was induced at various IAPs.

 4 mm Hg8 mm Hg15 mm Hg
VariableBaseline2 minutes30 minutesBaseline2 minutes30 minutesBaseline2 minutes30 minutes
Body temperature (°C)37.5 ± 0.937.6 ± 0.737.9 ± 0.537.5 ± 0.837.4 ± 0.937.7 ± 0.937.6 ± 0.737.7 ± 0.738 ± 0.5
HR (beats/min)181 ± 12183 ± 13185 ± 12186 ± 12182 ± 15182 ± 17183 ± 13188 ± 21186 ± 14
RR (breaths/min)21 ± 423 ± 619 ± 621 ± 2026 ± 2227 ± 1914 ± 520 ± 720 ± 12
MAP (mm Hg)79 ± 1293 ± 1884 ± 1479 ± 14100 ± 17a91 ± 2375 ± 13112 ± 19a,b106 ± 22a,b
Central venous7.5 ± 2.37.2 ± 3.18.2 ± 4.47 ± 3.28.8 ± 5.68.2 ± 4.46.3 ± 2.79.8 ± 5.28.7 ± 5.4
pressure (mm Hg)         
Paco2 (mm Hg)40.1 ± 441.4 ± 3.440.6 ± 6.742.6 ± 8.541.4 ± 7.441.4 ± 9.739.8 ± 8.947.6 ± 7.650.6 ± 9a,b,c
Pao2 (mm Hg)439 ± 53457 ± 40441 ± 33422 ± 89441 ± 63467 ± 61405 ± 12418 ± 114438 ± 73.8
Spo2 (%)98 ± 198 ± 1.298 ± 1.498 ± 1.297 ± 1.598 ± 1.597 ± 1.497 ± 1.897 ± 1.4
Cao2 (mL/L)14.8 ± 1.513.7 ± 1.113.6 ± 1.214.4 ± 1.414.1 ± 1.714.8 ± 214.4 ± 1.915.4 ± 2a,b16.3 ± 2.4a,b
Blood pH7.29 ± 0.057.29 ± 0.047.29 ± 0.067.29 ± 0.067.29 ± 0.067.29 ± 0.077.31 ± 0.117.22 ± 0.05a,b,c7.21 ± 0.06a,b,c
Blood HCO3 (mmol/L)18.9 ± 1.519.4 ± 1.918.9 ± 1.319.7 ± 2.118.9 ± 1.518.7 ± 1.519.2 ± 0.718.8 ± 119.1 ± 1.4
Blood lactate (mmol/L)1.3 ± 0.51.3 ± 0.61.3 ± 0.51.5 ± 0.71.4 ± 0.71.5 ± 0.61.2 ± 0.61.3 ± 0.51.4 ± 0.7
BE (mmol/L)-6.4 ± 1.9-5.9 ± 2.1-6.3 ± 1.5-5.6 ± 1.8-6.5 ± 1.3-6.7 ± 1a-5.8 ± 1.7-7.4 ± 0.8a,b-7.2 ± 1.4b
Hemoglobin (mg/dL)10.5 ± 1.29.7 ± 0.99.6 ± 1.110.3 ± 1.210.2 ± 1.310.4 ± 1.710.3 ± 1.611.1 ± 1.6b11.8 ± 2b,c,c
PAP (mm Hg)14.3 ± 1.816.3 ± 3.216.3 ± 2.716.2 ± 3.917.3 ± 5.317 ± 3.814 ± 1.620 ± 5.218 ± 4.8
PAOP (mm Hg)8 ± 2.411 ± 3.39.4 ± 2.28.3 ± 2.18 ± 2.910.2 ± 3.88 ± 3.113.4 ± 5.712.6 ± 4.5
Cl (L/min/m2)1.8 ± 0.11.8 ± 0.11.8 ± 0.31.7 ± 0.41.8 ± 0.41.8 ± 0.31.7 ± 0.11.7 ± 0.51.9 ± 0.4
SVI (mL/beat/kg)0.53 ± 0.050.54 ± 0.040.54 ± 0.10.5 ± 0.130.54 ± 0.10.55 ± 0.110.51 ± 0.040.5 ± 0.160.57 ± 0.14
Do2 (mL/min/m2)262 ± 36250 ± 25248 ± 55248 ± 74254 ± 63272 ± 73246 ± 25257 ± 67321 ± 102b
o2 (mL/min)16.6 ± 4.814.3 ± 4.416.1 ± 7.016.9 ± 5.115.0 ± 10.514.6 ± 5.815.2 ± 2.210.2 ± 515.8 ± 4.2
O2 extraction ratio0.2 ± 0.050.18 ± 0.060.21 ± 0.110.22 ± 0.090.18 ± 0.140.16 ± 0.090.19 ± 0.040.14 ± 0.070.16 ± 0.08
SVRI (dynes•sec/cm5•m2)2,919 ± 4993,348 ± 6632,989 ± 4683,005 ± 6263,721 ± 6413,224 ± 6392,873 ± 6504,697 ± 1,733a,b3,702 ± 843
Pulmonary vascular resistance index (dynes•sec/cm5•m2)283 ± 50263 ± 40278 ± 57444 ± 365472 ± 474286 ± 104288 ± 86309 ± 105249 ± 122

RR = Respiratory rate. Spo2 = Oxygen saturation as measured with pulse oximetry.

Within a row, value differs significantly (P < 0.05) from baseline value for given IAP.

Within a row, value differs significantly (P < 0.05) from value for 4 mm Hg at same measurement point.

Within a row, value differs significantly (P < 0.05) from value for 8 mm Hg IAP at same measurement point.

More significant changes occurred at an IAP of 15 mm Hg, although no differences from baseline or values measured at lower IAPs were evident in HR or respiratory rate. Blood gas analysis revealed that cats were more acidotic at an IAP of 15 mm Hg than at baseline as well as at the lower IAPs of 4 and 8 mm Hg. Values of Paco2 at 15 mm Hg were also greater at the 30-minute measurement point than at baseline or the lower IAPs; however, Pao2 at 15 mm Hg did not differ from other IAPs at any point. The Cao2 was greater after 30 minutes at 15 mm Hg than at baseline and at the same time point at 4 mm Hg. At 15 mm Hg, MAP was greater than at baseline at 2 and 30 minutes and greater than at 4 mm Hg for the same time points. Cardiac index, SVI, PAP, PAOP, o2, and oxygen extraction ratio did not differ among IAPs at any measurement point. The DO2 was significantly greater after 30 minutes at 15 mm Hg than at the same time point at 4 mm Hg. At 15 mm Hg, SVRI was significantly greater at 2 minutes than at baseline or the same time point at 4 mm Hg.

Comparison of insufflator and manometry IAP readings—Insufflator IAP readings were not significantly different from manometry-measured IAP readings at 4 mm Hg (Table 2). However, at insufflator IAP readings of 8 and 15 mm Hg, the manometry-measured IAPs were significantly (P < 0.05) higher than those measured with the insufflator.

Table 2—

Mean ± SD measured IAPs and abdominal cavity dimensions of the cats in Table 1.

IAP via insufflator (mm Hg)IAP via manometry (mm Hg)Abdominal width (cm)Abdominal height (cm)Abdominal circumference (cm)
0NA17.5 ± 1.58.3 ± 0.543.4 ± 2.3
44.2 ± 0.817.7 ± 0.911.5 ± 0.9a47.8 ± 3.4a
88.6 ± 0.6b17.4 ± 0.512.9 ± 0.5a,c50.1 ± 2.1a,c
1516.5 ± 0.7b17.6 ± 0.613.8 ± 0.45a,c51.2 ± 2.6a,c,d

NA = Not available.

Within a column, value differs significantly (P < 0.05) from baseline value (IAP insufflator pressure, 0 mm Hg).

Value for IAP measured by manometry differs significantly (P < 0.05) from insufflator IAP value.

Within a column, value differs significantly (P < 0.05) from value at an IAP insufflator pressure of 4 mm Hg.

Within a column, value differs significantly (P < 0.05) from value at an IAP insufflator pressure of 8 mm Hg.

Working space evaluation—Measurements of the cats’ abdominal walls revealed that the circumference changed significantly as the IAP increased, but that this increase was principally caused by an increase in abdominal height and not width. Abdominal width was not significantly different from baseline at any induced IAP. Abdominal height and circumference at an IAP of 4 mm Hg were significantly (P < 0.01) higher than at baseline. At 8 mm Hg, height and circumference were significantly greater than at 4 mm Hg and at baseline. At 15 mm Hg, circumference was significantly greater than at all lower pressures, but there was no significant difference in abdominal height between 8 and 15 mm Hg.

Subjective assessment of still images and video segments obtained during the experiment revealed progressive increases in working space as IAP increased, with the most noteworthy differences evident between an IAP of 4 and 8 mm Hg and subjectively smaller, although noticeable, differences between 8 and 15 mm Hg (Figure 1). At an IAP of 4 mm Hg, a considerable amount of working space was present that would, in the authors’ opinion, allow many basic minimally invasive procedures to be performed with adequate laparoscopic visualization.

Figure 1—
Figure 1—

Cranial abdominal laparascopic images of each of 2 healthy cats after induction of pneumoperitoneum at IAPs of 4 mm Hg (A and D), 8 mm Hg (B and E), and 15 mm Hg (C and F).

Citation: American Journal of Veterinary Research 74, 10; 10.2460/ajvr.74.10.1340

Discussion

The present study was designed to evaluate for the first time the effect of intentionally induced pneumoperitoneum on cardiorespiratory variables in healthy cats. Intra-abdominal pressures of ≤ 15 mm Hg were assessed, which our clinical experience suggests would provide sufficient working space within the peritoneal cavity to perform all currently practiced feline laparoscopic procedures. Studies1,2 have shown that considerable cardiorespiratory changes occur in dogs at IAPs as high as 40 mm Hg. At those pressures, profound decreases in cardiac output have been documented, caused principally by the compressive effect of pneumoperitoneum on the vena cava, which compromises venous return and therefore cardiac preload.3 Because IAPs of this magnitude are not clinically necessary, 15 mm Hg was chosen for the study cats as the highest IAP for evaluation, and consequently, milder changes in cardiorespiratory variables were observed.

At IAPs of 4 and 8 mm Hg, few cardiorespiratory changes were evident, and no adverse effects on the cats were noticed. The findings agreed largely with results from a study3 in which dogs were evaluated at similar IAPs. In that study, cardiac output, MAP, systemic vascular resistance, and vena caval and splanchnic blood flow were all unchanged at 8 mm Hg and even 12 mm Hg, compared with at baseline (no pneumoperitoneum). At 15 mm Hg IAP, the study cats had some significant cardiorespiratory changes, although the impact on hemodynamic variables was less than has been reported for dogs.3,4 Cardiac index remained unchanged throughout our experiment because of an almost unchanged HR and SVI. In canine studies3,4 in which an IAP of 15 mm Hg was evaluated, a significant decrease in cardiac output was identified, presumably mediated by an increase in peripheral resistance and impairment of venous return. Another study5 in dogs found that cardiac output was preserved by an increase in HR and maintenance of stroke volume. An initial transient increase in cardiac output has also been identified in dogs, possibly attributable to the increase in pressure propelling blood from the splanchnic bed and spleen back to the heart.2 In the study cats, SVRI was inconsistently increased, and it is possible that the significant increase from baseline observed in SVRI 2 minutes after initiation of pneumoperitoneum was caused by an extremely high reading in 1 cat (7,904 dyne•s/cm5•m2). Measurement of peripheral resistance in dogs at IAPs at or close to 15 mm Hg has yielded conflicting results. In 1 study,3 a progressive increase was observed in SVRI over 3 hours, whereas no change in SVRI was evident over a similar period in another study.5 However, an increase in SVRI would partially explain the increase in MAP in the study cats that began to increase at 8 mm Hg and was significantly increased at both time points at 15 mm Hg. Others have found minimal to no significant changes in MAP at comparable pressures.3–5

Blood gas analysis in the present study revealed several alterations in cats at an IAP of 15 mm Hg. The Paco2 was significantly increased relative to baseline 30 minutes after pneumoperitoneum induction. Hypercapnia develops readily in patients undergoing CO2-induced pneumoperitoneum both as a result of rapid CO2 absorption across the peritoneal membrane and the compromising effect of diaphragmatic compression on tidal volume. The Paco2 likely increased as a result of these mechanisms. Arterial oxygen concentration was stable at all IAPs throughout the study, although Cao2 and Do2 were both high at an IAP of 15 mm Hg. Oxygen delivery is a product of cardiac output and Cao2 and may have increased as a combination of the nonsignificant increase in CI that was identified after 30 minutes at an IAP of 15 mm Hg IAP or by increases in arterial hemoglobin content that were noted at 15 mm Hg, compared with at 4 and 8 mm Hg.

One limitation of the present study is that pneumoperitoneum was sustained at each IAP for only a 30-minute period. Most basic laparoscopic procedures can be performed within this period, and we would expect adequate hemodynamic and respiratory equilibration to have occurred within 30 minutes after gas insufflation. However, some studies3,5 involving much longer periods of sustained pneumoperitoneum showed that certain detrimental physiologic changes may only occur after a more prolonged period. Although values of most measured and calculated hemodynamic variables changed within 15 minutes after initiation of pneumoperitoneum in a study5 involving dogs, certain blood gas variables appeared to be detrimentally affected after a longer period than in our study cats; those dogs became acidotic and developed significant decreases in Pao2 only after 60 minutes of insufflation at 15 mm Hg. Minimal changes developed in the cats of the present study when evaluated at IAPs of 4 and 8 mm Hg for a 30-minute period; however, greater changes might occur during procedures requiring longer periods of pneumoperitoneum at these pressures. An additional limitation is the use of healthy young cats. Physiologic responses to pneumoperitoneum may be different in older or diseased cats, which is the population in which laparoscopic surgery may be indicated.

Creation of a working space within the abdominal cavity is a fundamental requirement for performing laparoscopic surgery. It can be achieved by insufflation with various gases such as CO2 and helium or by use of lift or gasless laparoscopy.16,17 Use of different gases for induction of pneumoperitoneum has been investigated extensively; CO2, however, has remained the insufflation gas of choice in most centers because it is generally inert, noncombustive, and inexpensive.16 Lift laparoscopy, which has been described in small animals, involves the physical elevation of the abdominal wall by an abdominal wall lift device.16,17 The procedure is generally associated with less cardiorespiratory depression than pneumoperitoneum induction18,19 but may not create as large a working space.

We found that at an IAP of 4 mm Hg, a considerable amount of working space was created in the abdomen of the healthy study cats. As IAP increased to 8 mm Hg, the enlargement within the abdomen continued, with dimensions of abdominal height and circumference being significantly greater than at an IAP of 4 mm Hg. Interestingly, abdominal width did not change from baseline, indicating that pneumoperitoneum increases abdominal circumference through increasing height, without a concomitant widening of the abdominal cavity even at an IAP of 15 mm Hg. At 15 mm Hg, height did not increase significantly, compared with at 8 mm Hg, and despite abdominal circumference being significantly greater at 15 mm Hg than at 8 mm Hg, mean values only increased from 50.1 ± 2.1 cm to 51.2 ± 2.6 cm, which is a difference likely to have limited clinical importance. These results suggested that little advantage in working space is gained by increasing the degree of induced pneumoperitoneum beyond an IAP of 8 mm Hg. Direct visual inspection of the abdominal cavity through the laparoscope to assess working space yields only subjective data, and therefore, limited conclusions can be drawn from findings obtained in that manner. The clinical relevance of the working space provided and the procedures that could reasonably be performed with any given amount of working space will depend on instrumentation and surgeon experience as well as the nature and location of the organ or lesion requiring surgery. Subjective laparoscopic inspection showed a difference between the volume of working space at 4 and 8 mm Hg, whereas a less discernible difference was seen between 8 and 15 mm Hg. In the authors’ opinion, simpler laparoscopic procedures could be performed at an IAP of 4 mm Hg, whereas more complex procedures perhaps requiring greater visualization might be easier to perform at an IAP of 8 mm Hg. In this regard, we agree with the observations of Van Nimwegen8 that laparoscopic ovariectomy could readily be performed at an IAP of 4 mm Hg. The evidence that induction of an IAP of 8 mm Hg caused little to no changes in values of cardiopulmonary or metabolic variables over a 30-minute period and the finding that working space was visibly similar at 8 and 15 mm Hg argue against the use of IAPs > 8 mm Hg in cats for laparoscopic procedures. However, additional research in older or diseased cats is needed to support this supposition.

ABBREVIATIONS

BE

Base excess

Cao2

Arterial oxygen content

Cl

Cardiac index

Do2

Oxygen delivery rate

HR

Heart rate

IAP

Intra-abdominal pressure

MAP

Mean arterial blood pressure

Paco2

Partial pressure of arterial carbon dioxide

Pao2

Partial pressure of arterial oxygen

PAOP

Pulmonary arterial occlusion pressure

PAP

Pulmonary arterial pressure

SVI

Stroke volume index

SVRI

Systemic vascular resistance index

o2

Oxygen consumption rate

a.

Atropine sulphate, West-ward Corp, Eatonton, NJ.

b.

Buprenorphine hydrochloride, Reckitt Benckiser Pharmaceuticals Inc, Richmond, Va.

c.

Propofol, Abbott Animal Health, Chicago, Ill.

d.

Piramal Healthcare Ltd, Andrah Pradesh, India.

e.

Arrow AK-04650-E, Teleflex Medical, Durham, NC.

f.

Arrow CP-07511-P, Teleflex Mecical, Durham, NC.

g.

Arrow AI-07044, Teleflex Medical, Durham, NC.

h.

Ternamian Endotip cannula, Karl Storz Veterinary Endoscopy, Goleta, Calif.

i.

Endoflator, Karl Storz Veterinary Endoscopy, Goleta, Calif.

j.

Hopkins II, Karl Storz Veterinary Endoscopy, Goleta, Calif.

k.

Research Randomizer, Social Psychology Network, Middletown, Conn. Available at: www.randomizer.org. Accessed Nov 14, 2011.

l.

Datex-Ohmeda S/5 compact anesthesia monitor, GE Healthcare Technologies, Madison, Wis.

m.

COM-1, American Edwards Laboratories, Irvine, Calif.

n.

ABL 800, Radiometer, Copenhagen, Denmark.

o.

Myotape, Accufitness Inc, Greenwood Village, Colo.

p.

Intercooled Stata, version 12.1, Stata Corp, College Station, Tex.

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