Causes of Low Arterial Pressure During Dialysis
Intradialytic hypotension results from excessive ultrafiltration rate overwhelming the patient's cardiovascular compensatory mechanisms, combined with impaired vascular reactivity and inadequate refilling of the intravascular compartment from interstitial fluid. 1
Primary Pathophysiologic Mechanisms
Excessive ultrafiltration is the most common cause, occurring when fluid removal exceeds the rate at which fluid can shift from the interstitial space back into the vascular compartment. 1 Ultrafiltration rates as low as 6 mL/h/kg are associated with higher mortality risk, indicating that even modest rates can overwhelm compensatory mechanisms in vulnerable patients. 1
Inadequate cardiovascular compensation occurs when the normal physiologic responses fail—these include insufficient increases in heart rate, myocardial contractility, peripheral vasoconstriction, and splanchnic blood flow redistribution. 1 Patients with autonomic dysfunction show exaggerated blood pressure drops compared to those with intact autonomic function. 2, 1
Dialysis Treatment-Related Factors
Dialysate composition significantly impacts blood pressure stability:
Acetate-containing dialysate decreases total vascular resistance, increases venous pooling, and increases myocardial oxygen consumption. 2, 1 Conversion to bicarbonate-containing dialysate minimizes hypotension and associated symptoms like headaches, nausea, and vomiting. 2
Elevated dialysate temperature (37°C) promotes peripheral vasodilation and reduces sympathetic tone. 2 Standard dialysis increases core body temperature, which further increases hypotension risk. 2 Reducing dialysate temperature to 34-35°C increases peripheral vasoconstriction and cardiac output through enhanced sympathetic activity, decreasing symptomatic hypotension from 44% to 34%. 2, 3
Dialysate sodium concentration affects vascular refilling—low sodium dialysate can worsen hypotension, while sodium profiling (starting at 148 mEq/L and gradually decreasing) helps maintain vascular stability. 2
Impaired vascular responsiveness occurs during hemodialysis sessions, with defective reactivity of both resistance and capacitance vessels. 2 The exact mechanism is not fully understood, but data from isolated ultrafiltration and hemodiafiltration show that vascular responses remain intact when these modalities are not associated with increased core body temperature. 2
Patient-Specific Risk Factors
High-risk patient subgroups include: 2, 4, 1
- Patients with pre-dialysis systolic blood pressure ≤100 mmHg (5-10% of dialysis patients), including anephric patients and those on long-term dialysis
- Diabetic patients with autonomic neuropathy
- Elderly patients (≥65 years)
- Patients with cardiovascular disease: left ventricular hypertrophy with diastolic dysfunction, systolic dysfunction with heart failure, valvular heart disease, or pericardial disease
- Patients with poor nutritional status and hypoalbuminemia
- Patients with severe anemia
- Patients requiring high-volume ultrafiltration (>3-4% body weight per session)
Medication-Induced Hypotension
Antihypertensive medications taken before dialysis prevent compensatory vasoconstriction and cardiac responses. 1, 5 The evidence on this is contradictory—some studies show strong correlation between antihypertensive use and hypotension, while others do not. 2 However, this should be evaluated individually for problematic patients. 2
Dialyzable antihypertensive agents are removed during hemodialysis, potentially causing paradoxical blood pressure changes. These include enalapril, ramipril, methyldopa, atenolol, acebutolol, nadolol, minoxidil, and nitroprusside. 1
Nitrates used before dialysis sessions substantially increase hypotension risk in patients with coronary artery disease. 2, 4, 1
Nutritional and Behavioral Factors
Food intake immediately before or during hemodialysis causes splanchnic vasodilation and decreased peripheral vascular resistance, redirecting blood flow away from the peripheral circulation. 2, 1
Severe anemia reduces oxygen-carrying capacity and impairs cardiovascular compensation, though correction to hemoglobin ≥11 g/dL can reduce hypotension incidence. 2, 1
Excessive interdialytic weight gain from inadequate fluid restriction forces higher ultrafiltration rates that exceed vascular refilling capacity. 2
Critical Clinical Implications
Intradialytic hypotension occurs in 25-50% of all hemodialysis treatments, making it the most common acute complication. 1 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 and failure to meet ultrafiltration goals, perpetuating volume overload and interdialytic hypertension. 1