This chapter will cover clinical considerations concerning the renal system as they pertain to the practice of anesthesia.
CLINICAL ANESTHESIA CONSIDERATIONS
The function of the renal system, as it relates to the practice of anesthesia in the perioperative period, involves systemic removal and excretion of anesthetic medications, regulating hemoglobin levels, as well as maintaining acid-base and fluid balance.1–3 Diseases of the kidneys are common in surgical patients and are associated with increased risk of poor postoperative outcomes. Several risk factors that predict postoperative kidney dysfunction are discussed in the following sections.4,5
Renal blood flow (RBF) is approximately 20% of the total cardiac output. Two-thirds of RBF is taken up by the renal cortex.6 At arterial blood pressures 80 to 180 mm Hg, both glomerular filtration rate (GFR) and RBF remain constant through autoregulation. Autoregulation is achieved through arteriolar tone changes of the afferent vessels to the kidney, increasing or decreasing blood flow resistance. During episodes of moderate hypertension, the glomerular capillaries are protected by autoregulation and during modest hypotension, the renal tubular function and GFR are also protected. Once mean arterial pressure (MAP) is beyond the range of autoregulation, the blood flow to the kidney becomes pressure dependent. In conditions such as chronic hypertension, autoregulation may be reset. In diabetic kidney disease, autoregulation may be dysfunctional altogether.
The sympathetic nervous system and renin-angiotensin-aldosterone system (RAAS) also play a role in controlling RBF. The sympathetic nervous system can cause vasoconstriction or vasodilation which alters RBF even within the range of autoregulation. Low blood flow to the kidneys can release renin and activate the RAAS pathway, discussed later in this chapter.
Glomerular Filtration Rate
The ability of the glomerular membrane to filter the blood entering the glomerulus is measured via GFR and is reflective of glomerular function.6,7 GFR is dependent on glomerular filtration pressure (GFP) and normal GFR is 125 ml/min. GFP is dependent on the pressure in the renal artery, as well as glomerular oncotic pressure and arteriolar tone of the afferent and efferent vessels. Approximately 90% of the filtered fluid is returned to the systemic circulation through renal tubular reabsorption into peritubular capillaries.
The glomerular capillary hydrostatic pressure forces electrolytes and water into the Bowman’s space through the glomerular capillaries. Meanwhile, plasma oncotic pressure varies depending on the location. At the afferent arteriole, the plasma oncotic pressure is about 25 mm Hg, while at the efferent arteries the plasma oncotic pressure is about 35 mm Hg. The average rate at which the glomerular capillaries can filter plasma is 125 ml/min. However, GFR decreases with low MAP or low RBF. GFR can also be decreased through the afferent arteriolar constriction and increased by afferent arteriolar dilation. Mild constriction of the efferent vessels can increase both GFP and GFR.