renal physiology

MINIMALLY INVASIVE SURGERY OF THE KIDNEY A PROBLEM-ORIENTED APPROACH 0094-0143/00 $15.00 + .OO

RENAL PHYSIOLOGY Laparoscopic Considerations

Matthew D. Dunn, MD, and Elspeth M. McDougall, MD, FRCSC

The addition of laparoscopy to the urolo- gist’s surgical armamentarium has introduced the physiologic changes associated with these prolonged surgical procedures. An understanding of the effects of the pneumoperitoneum is especially important as the complex- ity of laparoscopic urologic procedures continues to increase. Laparoscopy has been used not only for diagnostic purposes but for therapeutic management (e.g., nephrectomy) and reconstruction (e.g., pyeloplasty) of urologic disease processes. As a minimally invasive method of surgical management, this form of treatment has been applied to the higher-risk, more debilitated patient who may be considered too sick for conventional open surgery. With the evolution of laparoscopy has come an extensive research experi- ence, from animal and clinical studies, evalu- ating the various systemic physiologic effects of insufflating gas into the body and elevating intra-abdominal or retroperitoneal pressures. In otherwise healthy patients, these changes may not be clinically significant. The elderly or debilitated patient, however, may have a diminished threshold for the effects of these physiologic changes. A clear understanding of the various physiologic effects of laparoscopic surgery allows the surgical team to min- imize the risks and to maximize the potential

advantages of this surgical approach in all patients.

INSUFFLANTS

Access to the abdominal cavity or retroperitoneal space may be performed by using a Veress needle or by using the Hasson cannula technique under direct visualization or after balloon dilation of the retroperitoneal space. Subsequently, insufflation of a gas is performed to develop and maintain a working space in the respective cavity. Gas insufflation is performed at pressures higher than those normally found in the intraperitoneal and retroperitoneal spaces, and these pressures are transmitted to all the associated and sur- rounding organs. The diaphragm also becomes displaced superiorly by the increased intra-abdominal or retroperitoneal pressure, exerting increased pressures on the intratho- racic organs.

Various gases have been used for devel- oping the pneumoperitoneum and the pneumoretroperitoneum: CO, argon, helium, and nitrous oxide. Carbon dioxide remains the most commonly used insufflating gas for laparoscopy,22, 34 is widely available in most operating rooms, is inexpensive, and is usu-

From the Department of Urology, University of Southern California School of Medicine, Los Angeles, California (MDD); and the Department of Urologic Surgery, Vanderbilt University Medical Center, Nashville, Tennessee (EMM)

UROLOGIC CLINICS OF NORTH AMERICA

VOLUME 27 - NUMBER 4 - NOVEMBER 2000 609

610 DUNN & McDOUGALL

ally well tolerated in most healthy patients. Because of its high diffusion capacity, it is readily absorbed and easily expired through the lungs.

Helium and argon are inert gases and have been used in place of CO, for patients with respiratory compromise and ventilatory prob- lems.'? Although they provide excellent expo- sure during penumoperitoneum and pneumoretroperitoneum and are noncombustible, they are not as readily diffusable by the body as CO, which may result in a prolonged du- ration of subcutaneous emphysema. An argon gas embolism has been reported with the use of the argon beam coagulator for he- mostasis during a laparoscopic cholecystectomy, raising the concern for that same potential during a pneumoperitoneumZ0; however, in this case, high-pressure jets of argon gas were applied to the open vascular bed of the liver where the gallbladder had been excised. Gas emboli can occur whenever a large amount of gas is administered directly into the vascular system.20

Among the other insufflants, nitrous oxide, if present in high concentrations, is well documented to support combu~tion.~ With the use of electrocautery and laser during laparoscopy, this gas becomes an unacceptable choice as an insufflant. Even when used during anesthetic induction, nitrous oxide diffuses from the bowel into the peritoneal cavity.28 Not surprisingly, such disasters as colonic explosion during laparoscopic colon surgery have been reported.' Nitrous oxide should be avoided routinely even as an anesthetic agent during any laparoscopic procedure.22

PNEUMOPERITONEUM

Two nonphysiologic situations occur during laparoscopy: insufflation of the abdominal cavity with gas, usually CO,; and the resultant elevation of the intra-abdominal pressure. Studies comparing the effects of insufflants on hemodynamic and respiratory function indicate that the type of gas used does not affect cardiac output significantly. Nevertheless, CO, has been demonstrated to have a negative effect on respiratory function, whereas argon and helium do not seem to have these limitations. Carbon dioxide gas also increases the central venous pressure and mean arterial pressure. Likewise, argon gas can increase the cardiac afterload markedly by elevating arterial pressures and systemic vascular resistance, even more so than CO,

gas. In contrast, these changes are only moderate with helium gas. Helium seems to be the best alternative to CO, as an insufflant because of its limited effect on hemodynamic function and essentially no effect on the respiratory function.lX Wolf et a1 reported that con- version to helium insufflation could reverse a significant respiratory acidosis caused by a CO, pneumoperitoneum successfully in a patient with severe chronic obstructive pulmonary disease during a laparoscopic radical ne- phrec t~my.~~

Aside from the choice of gas used for insufflation, the increased intra-abdominal pressure itself can lead to unfavorable hemodynamic changes, which are essentially pressure-dependent. The cardiovascular, pulmonary, and renal effects of elevated intra- abdominal pressure have been studied exten- sively in the laboratory and clinical setting.

Cardiopulmonary Effects

An elevated intra-abdominal pressure increases atrial filling pressures, increases systemic vascular resistance, decreases venous return, decreases cardiac output, and reduces stroke volume.6, 17, 30 Peak inspiratory pressures increase with the elevated abdominal pressure.30 These effects are pressure-dependent, usually not becoming evident until pressures reach 14 to 15 mm Hg.23,30 In animal studies comparing the effects of laparoscopic surgery on hemodynamic responses, the changes are minimal when intra-abdominal insufflation is performed in healthy well-hy- drated and hyperventilated animals.I5 For healthy patients in the clinical setting, these hemodynamic changes do not seem to have any significant adverse effect.'", 27 Clinically, the respiratory effect of the elevated intra- abdominal pressure is monitored by using pulse oximetry and capnography. The pulse oximeter saturation should be maintained at more than 93%, and the end-tidal CO, should be maintained between 35 and 45mm Hg, which usually ensures that the Paco, is less than 50 mm Hg. Increasing respiratory rate and tidal volume usually can control these levels satisfactorily. In the face of moderate pulmonary disease, despite an elevation of the arterial CO, pressure (Paco,) and respiratory acidosis, the impairment of pulmonary function usually causes no significant negative hemodynamic effects; however, in animal studies simulating sepsis, although laparos-

RENAL PHYSIOLOGY 611

copy can be performed, the hemodynamic compromise in the form of acidosis with cardiodepression is apparent. Therefore, the clinical application of laparoscopy in the patient with sepsis should be approached cau- tiously.I2

ALTERATIONS IN RENAL FUNCTION

A prolonged period of increased intra-abdominal pressure is associated with a decreased urine output, even to the point of anuria.22 This low urine output continues to be the most marked intraoperative renal effect observed from the pneumoperitoneum, and it is pressure-dependent. In a human study, the use of a lower intra-abdominal pressure (4mm Hg) with the aid of a retraction device for performing a laparoscopic cholecystectomy led to no significant change in urine output, effective renal plasma flow, or glomerular filtration rate (GFR), as opposed to the transient renal dysfunction that was noted with an intra-abdominal pressure of 12 mm Hg.26 Although this transient renal dysfunction has been well documented, various mechanisms have been described to ex- plain these changes. Proposed mechanisms include decreased cardiac output, renal vein compression, ureteral obstruction, renal parenchymal compression, and systemic hormonal effects.I4, l6

The choice of insufflant does not seem to have a direct effect on renal function.= In animal studies, comparing CO, to argon gas for pneumoperitoneum, no significant change in urine output was noted for intra-abdominal pressures less than 15 mm Hg. For intra- abdominal pressures greater than or equal to 15mm Hg, a similar impairment in urine output and GFR was seen for both types of gas.23

The body of evidence also eliminates a decrease in cardiac output as a direct causal factor responsible for the renal dysfunction.16 Although cardiac output has been reported to decrease to as much as 37% of normal at 40 mm Hg, normalizing cardiac output with plasma expanders failed to improve the diminished renal blood flow and GFR.14

Ureteral obstruction caused by extrinsic compression also does not seem to play a role in olig~ria.,~ Placement of ureteral stents during pneumoperitoneum does not improve urine output. Intraoperative urograms in animals with decreased urine output during

pneumoperitoneum also confirm the absence of ureteral obstruction.'6

To determine the cause of oliguria, Kirsch et all6 subjected rats to 5 mm Hg and 10 mm Hg of intra-abdominal pressure. Urine output did not diminish until the pressure reached 10 mm Hg. At that level of pneumoperitoneum, vena cava blood flow decreased 92%, and aortic blood flow decreased 46%, leading to the conclusion that the renal effect was caused by renal vascular insufficiency from central venous compression.'6 McDougall et al, using a porcine model, demonstrated a significant decrease in renal vein flow concomitant with a drop in urine output, but only at a pressure of 15mm Hg or more. Renal vein flow and creatinine clearance re- mained diminished even after 2 hours of desufflation, although no long-term effects were noted.23 Subsequent animal studies, using MR imaging to provide a noninvasive evaluation of renal vessel blood flow and parenchymal perfusion also demonstrated reduced cardiac output, reduced flow velocity in the renal vessels, decreased renal parenchymal perfusion, and a concomitant reduction in urine output. The changes seen in renal perfusion were similar in the cortex and medulla of the kidney, which confirmed that a shunting phenomenon was not occurring in the renal parenchyma during the time of the elevated pneumoperitoneum pressure.21

Perhaps the strongest evidence for the cause of the renal effects is that of direct renal parenchymal compression mimicking a Page kidney effect. Razvi et alZ9 placed a pressure cuff around canine kidneys, subjecting them to pressures of 15 mm Hg, which resulted in a decreased urine output of 63% in the treated kidney and decreased GFR and effective renal blood flow. As other investigators had observed, even 2 hours after desufflation, renal blood flow did not return to baseline levels. The control kidneys showed no significant changes in urine output or GFR. Aldosterone levels have been reported to be elevated during the oliguric period in the animal model, which correlates with the decreased urinary sodium and increased urinary potassium ob- ~ e r v e d . ~ This finding also supports the Page kidney effect as a significant component of the intraoperative oliguria observed during the pneumoperitoneum.

Although in healthy patients this acute renal dysfunction seems to resolve completely postoperatively after desufflation, there is concern that in patients with pre-existing re-


nal disease, these transient changes may be- come clinically significant. To address this, Cisek et al,5 in an animal model, performed renal reductive surgery to mimic chronic renal insufficiency. After being exposed to intra- abdominal pressures of 20 mm Hg for 6 hours, simulating a complex laparoscopic procedure, dramatic drops in urine output (SO%), GFR (63%), and renal blood flow (20%) were noted which did not return to baseline after 90 minutes of desufflation postproce- dure. An increase in the urinary P-D-acetylglucosaminidase (NAG) was observed, as was acute renal failure despite hydration and central venous pressure (CVP) monitoring. One week after the insufflation test, however, the GFR returned to the baseline, with the chronic renal failure level indicating that no long-term impact on the renal function from the acute insufflation was identified, even in the face of pre-existing renal ins~fficiency.~

These observed blood-flow changes have suggested the possibility of renal tubular damage secondary to ischemia as a cause of the oliguria associated with the pneumoperitoneum. P-D-acetylglucosaminidase is present in renal tubular cells and is released into the urine in response to tubular cell injury. Micali et a1,24 in a human study, measured preoperative and postoperative NAG levels in 31 patients undergoing laparoscopic surgery compared with 28 patients undergoing conventional open surgery. No differences in NAG were noted between the preoperative and the postoperative levels in either group or between the groups. They demonstrated that pneumoperitoneum was not associated with a change in the urinary concentration of NAG. Similarly, there was no correlation between urinary NAG levels and the total operative time. This observation suggests that significant ischemic renal injury is not associated with the laparoscopy-related oliguria. This biochemical finding has been confirmed histologically in animal models. In a rat study, after a 5-hour pneumoperitoneum at a pressure of 15 mm Hg, acutely and chronically, no significant histologic differences could be identified when compared with control rat kidneys.I9 McDougall et a123, in the porcine model, also confirmed a lack of histologic abnormality in kidneys rendered oliguric at pressures of 15 mm Hg.

Elevated intra-abdominal pressures also may act by stimulation of various systemic hormones that contribute to the previously mentioned hemodynamic and renal effects.

Independent of the type of gas used, exces- sive intra-abdominal pressures (2 20 mm Hg) are responsible for increasing serum catecholamine~. ,~ Endothelin, a potent vasoconstric- tor, also has been shown to increase in response to renal vein compression and during pneumoperitoneum in animal models. Com- pression of a unilateral renal vein leads to a decrease in the GFR and urine output in both kidneys and is associated with elevated renal vein endothelin concentrations, implicating it as a contributing factor to the oliguria observed during long laparoscopic cases.I3 In other animal studies, the administration of a vasopressin antagonist improved renal function when compared with control animals, suggesting that the endogenous release of ar- ginine vasopressin also contributes to the oliguria seen with increased intra-abdominal pre~sures.~

Methods to improve urine output during pneumoperitoneum, which include the use of ureteral stents, fluid hydration, and intrave- nous dopamine infusion, have been unsuc- cessful in changing urine output. Altering the temperature of insufflated CO, however, has been found to counteract the hemodynamic and renal effects of increased intra-abdominal pressure partially. A comparison of the use of warm (37°C) versus room-temperature CO, during prolonged (> 90 minutes) laparoscopic surgery demonstrated a higher core temperature, urine output, and cardiac index with warm insufflation, suggesting that local vasodilation in the compressed kidney may restore enough blood flow to maintain GFR and urine output. Despite the fact that long- term pathologic effects associated with the oliguria of pneumoperitoneum cannot be demonstrated, maintenance of urine output could prove beneficial to patients with bor- derline cardiac or renal function with respect to fluid management during these often prolonged cases.,

PNEUMORETROPERITONEUM

For the most part, gas insufflation into the retroperitoneal or extraperitoneal space has been associated with greater CO, absorption and hypercarbia as compared with intraperitoneal insufflation, especially in the presence of subcutaneous emphysema.”, 23, 33 The above observation, however, is not entirely uncon- troversial. Other animal and human studies

RENAL PHYSIOLOGY 613

have demonstrated the opposite. In a canine model, Wolf et a13' noted a higher increase in the Pacoz and greater drop in the serum pH in the intraperitoneal group compared with extraperitoneal insufflation. Conversely, the risk of thoracic dissection of gas was greater during extraperitoneal insufflation than during intraperitoneal insufflation. Wright et a1,34 in a human study, noted a more rapid increase of the Paco2 in the transperitoneal group than in the extraperitoneal group, although no significant difference in the overall magnitude of the rise was demonstrated. Re- gardless of the method of laparoscopy, however, appropriate ventilatory management with hyperventilation usually avoids any adverse sequelae of hypercarbia.

The renal and hemodynamic effects of a pneumoretroperitoneum are similar to those seen with a pneumoperitoneum. A unilateral pneumoretroperitoneum leads to elevated systolic and diastolic aortic pressures, although the effect is less than that seen with a pneum~peritoneum.~ In contrast, at a pressure of 15 mm Hg, the decrease in renal vein flow is similar to that seen with a pneumoper- i t ~ n e u m . ~ ~ An elevated retroperitoneal pressure also leads to oliguria, and when it is maintained for 2 hours, it leads to a gradual decrease in the contralateral kidney perfusion and a concomitant increase of the intra-abdominal pres~ure.~ This reduction in urine output is reversible after desufflation and leads to no pathologic renal abn~rmalit ies.~~

SUMMARY

Oliguria is a recognized component of the physiologic effect of increased intra-abdominal or retroperitoneal pressure. The cause is multifactorial, emanating from vascular and parenchymal compression, and is associated with systemic hormonal effects. Ureteral obstruction does not play a significant role. These changes are pressure-dependent and are usually not apparent until pressures reach 15 mm Hg or more. This effect is not associated with any histologic pathology or evidence of renal tubular damage. After the release of the pneumoperitoneum or pneumoretroperitoneum, the renal function and urine output return to normal with no long-term sequelae, even in patients with pre-existing renal disease. The entire operative team must understand the physiologic effects of CO, insufflation, which allows appropriate intraop-

erative monitoring and management and minimizes intraoperative and postoperative complications.

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renal physiology

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