Chapter 35
Anesthetic Considerations for Renal Disease
Ins and outs of water and salts
Jane Quandt
College of Veterinary Medicine, University of Georgia, USA
- Q. How is blood flow to the kidneys regulated?
- A. Renal blood flow (RBF) is regulated by extrinsic nervous and hormonal control and by intrinsic autoregulation. Renal vasculature is highly innervated by the sympathetic nervous system that constricts blood flow in certain areas of the kidney under various physiologic conditions, including stress. Intrinsic autoregulation of RBF occurs when the mean blood pressure (MAP) is between 80 and 180 mmHg; in this pressure range the kidney can control blood flow by altering resistance in the glomerular afferent arterioles. At mean arterial blood pressures below 80 and above 180, autoregulation of RBF begins to fail. The kidneys receive 20–25% of the cardiac output. The kidneys have a high oxygen consumption, which makes them susceptible to ischemic insult if oxygen delivery is inadequate, either because of patient hypoxia or because of inadequate cardiac output and renal perfusion. This means that during anesthesia the respiratory and circulatory systems must be supported in order to adequately deliver oxygen to the kidneys [1].
- Q. What is the recommended pre-operative preparation and blood work for the renal patient?
- A. A complete physical exam should be performed in all patients undergoing anesthesia. In the renal patient a serum chemistry analysis, urinalysis, and assessment of urine output is essential. A blood urea nitrogen (BUN), creatinine, and creatinine clearance are used to assess the glomerular filtration rate (GFR), although these may be rather insensitive tests to subclinical reductions in GFR. A reagent test strip (e.g., Azostix) reading is a very poor substitute for a measured BUN. Renal tubular function can be evaluated via urine specific gravity and urine osmolarity. Electrolytes (most importantly, potassium) need to be evaluated and acid-base determined, ideally with an arterial blood gas analysis. A patient with suspected chronic renal disease needs evaluation of the PVC and TP. The kidney produces erythropoietin and chronic renal disease can lead to decreased production and subsequent anemia [1,2]. TP can be low 2o to proteinuria, which can occur with chronic glomerulo-nephropathies.
- Q. Why is azotemia harmful if my patient is going under anesthesia?
- A. Azotemia causes CNS depression, which will be additive with the CNS depressant effects of sedative and anesthetic drugs. Other reasons to correct azotemia include improvement of acid-base status; patients with azotemia are commonly acidotic which will increase the fraction of unbound drugs in the plasma. This would require the use of lower doses of highly protein bound injectable agents to avoid a relative drug overdose. Hyperkalemia is often seen with acidosis and azotemia. Hyperkalemia should be corrected prior to anesthesia due to its deleterious effect on cardiac rhythm. An effort should be made to reduce any level of azotemia prior to anesthesia. Pre-renal azotemia 2o to dehydration should be corrected and peri-operative hypovolemia needs to be avoided [2,3].
- Q. Are pre-anesthetic fluids recommended in patients with renal disease?
- A. Cardiovascular depression under anesthesia can lead to impaired renal function. To help minimize this effect IV crystalloid fluids can be given prior to anesthesia, during anesthesia, and into recovery. Pre-operative IV fluids will help to put the animal is a mild state of diuresis with the goal of having the urine output between 0.5 and 2.0 ml/kg/h. Ideally, IV balanced isotonic crystalloids should be administered for a 12–24 h period prior to anesthesia, at rates of ∼ 2 ml/kg/h. The rate should be tailored to the individual patient and fluid deficits should be corrected in addition to the maintenance rate of ∼ 2 ml/kg/h. The patient should be closely monitored for volume overload [1]. Patients with oliguric or anuric renal failure will not tolerate even these rates of fluid administration and are at a high risk for volume overload.
Anemia causes decreased oxygen delivery. A chronic renal failure patient with a PCV < 20% should prompt consideration for a pre-operative blood transfusion. Because most of these patients suffer from chronic anemia, they are somewhat more tolerant of a low PCV because of chronic shifts in Hb affinity for oxygen; specifically an increased Hb affinity for oxygen (left-shifted Bohr curve). Regardless, patients with a PCV < 15% will most certainly benefit from a pre-operative blood transfusion to increase their oxygen carrying capacity.
Renal failure patients may also suffer from hypo-proteinemia, which can lead to more free anesthetic drug available and to a lower colloid oncotic pressure (COP), which puts the patient at higher risk for anesthetic induced hypotension and relative hypovolemia, as fluids do not remain in the intravascular space when COP is low. These patients may require additional fluid therapy with a colloid. When giving a colloid concurrently with a crystalloid the amount of crystalloid is usually decreased by 40–50% to avoid volume overload [1,3]. Commonly available colloids (e.g., hetastarch) can be given at rates no higher than 50 ml/kg/day (see Chapter 9 for more details).
Stay updated, free articles. Join our Telegram channel
- Q. Why is azotemia harmful if my patient is going under anesthesia?
Full access? Get Clinical Tree
