What causes hypotension during dialysis in a patient with complex medical history and comorbidities?

Causes of Blood Pressure Drop During Dialysis

The primary cause of hypotension during dialysis is excessive ultrafiltration that removes fluid faster than the vascular compartment can refill from tissue spaces, compounded by impaired cardiovascular compensatory mechanisms. 1

Primary Mechanism: Volume Removal Exceeding Vascular Refill

• Rapid ultrafiltration depletes intravascular volume faster than plasma can refill from the interstitial space, creating a mismatch between fluid removal and vascular refilling capacity 1, 2
• Ultrafiltration rates exceeding 6 mL/h/kg are strongly associated with hypotension and increased mortality risk 3
• This occurs most commonly when interdialytic weight gain is excessive, forcing aggressive fluid removal during limited treatment time 3

Impaired Cardiovascular Compensation

Autonomic Dysfunction

• Diabetic patients and those with uremic neuropathy cannot mount adequate vasoconstriction in response to volume depletion 4, 1
• Paradoxical sympathetic withdrawal occurs in some patients, causing inappropriate vasodilation and increased venous pooling when vasoconstriction is needed 2
• The Bezold-Jarisch reflex may trigger sudden hypotension in patients with specific cardiac pathology 2

Defective Vascular Responsiveness

• Dialysis patients have impaired reactivity of both resistance vessels (arterioles) and capacitance vessels (veins) during hemodialysis 4
• This vascular dysfunction prevents normal compensatory vasoconstriction that should maintain blood pressure during volume removal 1

Cardiac Factors

• Left ventricular hypertrophy with diastolic dysfunction limits the heart's ability to maintain cardiac output when preload drops 4, 1
• Systolic dysfunction and congestive heart failure further compromise cardiovascular compensation 4
• Patients with diastolic dysfunction are particularly sensitive to reduced cardiac filling from hypovolemia 2

Dialysis Prescription Factors

Temperature-Related

• Standard dialysate temperature (37-38°C) increases core body temperature, promoting peripheral vasodilation that worsens hypotension 4
• Heat transfer from the dialysate circuit impairs vascular reactivity and reduces compensatory vasoconstriction 4

Dialysate Composition

• Acetate-containing dialysate (if used) causes inappropriate decreases in vascular resistance 3
• This contributes to hypotension through direct vasodilatory effects 3

Medication-Related Causes

Dialyzable Antihypertensives

• ACE inhibitors like enalapril and ramipril are removed during dialysis, but their removal paradoxically can worsen hypotension by eliminating blood pressure support 4
• Beta-blockers (atenolol, acebutolol, nadolol) are dialyzed out, removing their cardiovascular support 4

Excessive Antihypertensive Burden

• Four or more concurrent antihypertensive medications prevent compensatory vasoconstriction during ultrafiltration 3
• Carvedilol specifically blunts compensatory tachycardia and cardiac output increases needed during volume removal 3
• Nitrates taken before dialysis exacerbate hypotension risk 4

High-Risk Patient Populations

Specific patient groups are particularly vulnerable to intradialytic hypotension:

• Diabetic patients with autonomic neuropathy 4, 1
• Elderly patients ≥65 years with reduced cardiovascular reserve 4, 1
• Female patients (mechanism unclear but consistently observed) 4, 1
• Patients with predialysis systolic BP ≤100 mmHg 4, 1
• Those with severe anemia reducing oxygen-carrying capacity 4, 1
• Patients with poor nutritional status and hypoalbuminemia 4, 1
• Those requiring high ultrafiltration volumes (>3% body weight) 1

Target Weight Issues

• Setting the target weight too low causes hypotension as the patient is pushed below their true dry weight 4, 3
• Residual urine output may lead to underestimation of true dry weight, making the prescribed target inappropriately low 3
• The narrow therapeutic window between volume overload and depletion makes target weight determination challenging 4

Common Pitfalls to Recognize

• Routinely administering saline for every hypotensive episode perpetuates volume overload and creates a vicious cycle 3
• Attempting to achieve aggressive blood pressure targets may paradoxically increase hypotension frequency 1
• Continuing twice-weekly dialysis in patients with large interdialytic weight gains forces dangerously high ultrafiltration rates 3
• Food intake immediately before or during dialysis decreases peripheral vascular resistance and precipitates hypotension 1