Recognition and Management of Intraoperative Oliguria
Recognition and Management of Intraoperative Oliguria
Intraoperative oliguria is common in healthy surgical patients; it is caused by inadequate renal perfusion. Marked reduction or cessation of urine output may occur abruptly or developed gradually over several hours and can be related to several pathophysiologic processes. Accurate assessment and diagnosis of oliguria with early treatment may prevent deterioration in postoperative renal function. Hemodynamic changes (blood pressure, pulse rate and volume) and urine output are the two parameters generally monitored to assess renal function, but neither is ideal because they are indirect measures. Anesthetic agents affect renal functions directly and indirectly to cause oliguria. Inhalation agents decrease renal blood flow, glomerular filtration rate, and renal tubular function. When combined with surgical stress, they may stimulate release of antidiuretic hormone (ADH) and activate the renin angiotensin system. Anesthesia and mechanical ventilation alter renal function through their effects on the cardiovascular system, if the patient's cardiac output is reduced sufficiently to cause decreased renal blood flow.
Oliguria is usually defined as urine output less than 20 ml/hr in a normal adult. Oliguria can be diagnosed intraoperatively only via use of an indwelling urinary catheter. Lengthy surgical procedures, procedures that require deliberate hypotension and if large volume loss is anticipated are all indications for placement of a bladder catheter. When oliguria occurs, an effective approach to diagnosis is to consider the causes of pre-renal, renal, and post-renal oliguria.
When oliguria occurs, the first step in management is to determine if renal blood flow and glomerular filtration rate are decreased. Renal blood flow may be decreased by inhalation anesthetics, low cardiac output, hypoxia, hypercapnia, and positive pressure ventilation. Perhaps the most common cause of renal hypoperfusion is intravascular volume depletion related to intraoperative blood loss and/or dehydration. Rapid infusion of balanced salt solution is a useful diagnostic and therapeutic step during initial evaluation of oliguria. Additional fluids should be administered if an appropriate response to the initial bolus occurs and anesthetic- induced changes in renal blood flow can be reversed by discontinuing the agent. If fluid replacement is not successful in reversing oliguria, Use of an inotropic agent, such as dopamine, can increase cardiac output and there by improve renal blood flow. Use of diuretics in patients with oliguria secondary to hypovolemia is contraindicated, because further reduction in intravascular volume that results may aggravate changes in the kidney.
Oliguria related to renal causes may occur after admisntration of nephrotoxic durgs, including aminoglycosides and radio contrast dyes., In addition, oliguria may follow release of free hemoglobin during a hemolytic transfusion reactin.
Post-renal oliguria is usually caused by urinary tract obstruction or extravasation of urine following ureter damage. Urine flow can be blocked by obstruction of the catheter with mucus, blood, or bladder tissue or after inadvertent ureteral ligation by the surgeon. Also, a patient is steep head- down positions may not show adequate drainage of urine from the bladder. Each of these possible sites of obstruction must be evaluated when attempting to determine the cause of oliguria.