What causes hypotension in patients with end-stage renal disease (ESRD) during hemodialysis?

Causes of Low Arterial Pressure During Hemodialysis

Intradialytic hypotension results from two fundamental mechanisms: insufficient intravascular volume to support the ultrafiltration rate and inadequate cardiovascular compensatory responses to volume removal. 1

Primary Pathophysiologic Mechanisms

Volume-Related Causes

• Excessive ultrafiltration is the most common cause, occurring when fluid removal exceeds the rate of vascular refilling from the interstitial compartment. 2
• Ultrafiltration rates as low as 6 mL/h/kg can be associated with higher mortality risk, indicating that even modest rates can overwhelm compensatory mechanisms in vulnerable patients. 1
• Inadequate dry weight assessment leads to chronic hypovolemia, triggering compensatory mechanisms including increased thirst and paradoxically higher interdialytic weight gains. 1

Impaired Cardiovascular Compensation

• Autonomic dysfunction is particularly prevalent in diabetic patients and those with long-standing ESRD, resulting in blunted sympathetic responses to volume depletion. 2, 1
• Impaired neurohormonal responses include blunted activation of the renin-angiotensin-aldosterone system and sympathetic nervous system during intravascular volume loss. 1
• Defective vascular reactivity affects both resistance and capacitance vessels during hemodialysis sessions, preventing adequate compensatory vasoconstriction. 2

Dialysis Treatment-Related Factors

Dialysate Composition and Temperature

• Acetate-containing dialysate contributes to hypotension by decreasing total vascular resistance, increasing venous pooling, and increasing myocardial oxygen consumption. 1
• Elevated dialysate temperature (37°C) promotes peripheral vasodilation and reduces sympathetic tone, whereas the optimal temperature of 34-35°C increases peripheral vasoconstriction through enhanced sympathetic activity. 1, 3
• Bicarbonate-buffered dialysate should replace acetate-based solutions to minimize hypotensive episodes. 2, 3

Rapid Solute and Thermal Shifts

• Increased core body temperature during standard dialysis is related to heat load from the extracorporeal system or secondary to volume removal, increasing the risk for hypotension. 2
• Urea and osmotic shifts can contribute to hemodynamic instability, with protracted hypotension exaggerating urea rebound and compromising dialysis adequacy. 2, 1

Patient-Specific Risk Factors

High-Risk Populations

• Diabetic patients with autonomic neuropathy show exaggerated drops in systolic and diastolic blood pressures compared to those without autonomic dysfunction. 2
• Elderly patients (≥65 years) have increased susceptibility to intradialytic hypotension. 2
• Patients with pre-dialysis systolic blood pressure <100 mmHg, including anephric patients and those on long-term dialysis. 2
• Patients with cardiovascular disease, including left ventricular hypertrophy with diastolic dysfunction, systolic dysfunction with heart failure, valvular disease, or pericardial disease. 2

Cardiac Dysfunction

• Left ventricular dysfunction (both systolic and diastolic) impairs the heart's ability to maintain cardiac output during volume removal. 2
• Endothelial dysfunction assessed by flow-mediated dilation is independently associated with intradialytic hypotension, with every 5% decrease in flow-mediated dilation associated with a 7.5 mmHg decrease in systolic blood pressure. 4

Medication-Induced Hypotension

• Antihypertensive medications taken before dialysis prevent compensatory vasoconstriction and cardiac responses, exacerbating hypotension. 1, 3
• Dialyzable antihypertensive agents including enalapril, ramipril, methyldopa, atenolol, acebutolol, nadolol, minoxidil, and nitroprusside are removed during hemodialysis, potentially causing paradoxical blood pressure changes. 2
• Nitrates used before dialysis sessions increase the risk of hypotension in patients with coronary artery disease. 2

Nutritional and Metabolic Factors

• Food intake immediately before or during hemodialysis causes splanchnic vasodilation and decreased peripheral vascular resistance, redirecting blood flow away from the peripheral circulation. 1
• Severe anemia reduces oxygen-carrying capacity and impairs cardiovascular compensation, though correction to hemoglobin ≥11 g/dL can reduce hypotension incidence. 1, 3
• Hypoalbuminemia and poor nutritional status are associated with increased risk of intradialytic hypotension. 2

Critical Clinical Pitfalls

• Intradialytic hypotension occurs in 25% to 50% of all hemodialysis treatments, making it the most common acute complication. 2, 5
• Recurrent hypotension creates a cascade of end-organ ischemia affecting the heart, brain, liver, gut, and kidneys, contributing to increased morbidity and mortality. 1
• Premature termination of dialysis due to hypotension results in inadequate dialysis dose (suboptimal Kt/V) and failure to meet ultrafiltration goals. 2
• Long-term consequences include volume overload from suboptimal ultrafiltration, left ventricular hypertrophy with associated morbidity and mortality, and interdialytic hypertension. 2