Blood Pressure Measurement & Interpretation

A blood pressure reading of 142/88 in the right arm and 118/76 in the left arm — is this normal variation or a vascular emergency? The difference changes everything.

Blood pressure reflects cardiac output multiplied by systemic vascular resistance. Normal adult BP is less than 120/80 mmHg. Elevated is 120-129/<80. Stage 1 hypertension is 130-139 or 80-89. Stage 2 is ≥140 or ≥90. Hypotension becomes clinically significant below 90/60 or when symptomatic. Proper technique requires the patient to rest 5 minutes, feet flat on the floor, arm supported at heart level, back supported. The cuff bladder must cover 80% of the arm circumference; a too-small cuff falsely elevates readings and a too-large cuff falsely lowers them. The first Korotkoff sound marks systolic pressure, and the fifth (disappearance) marks diastolic. An auscultatory gap — a silent interval between systolic and diastolic sounds — can cause falsely low systolic readings if you don't palpate first to estimate systolic before auscultating. A difference greater than 10 mmHg between arms is abnormal and warrants repeat measurement; differences ≥20 mmHg are more strongly associated with coarctation of the aorta or subclavian stenosis and require further vascular evaluation. Use the arm with the higher reading for subsequent measurements. Orthostatic hypotension is confirmed by a systolic drop ≥20 mmHg or diastolic drop ≥10 mmHg within 3 minutes of standing, often with a compensatory heart rate increase ≥20 bpm.

Key Distinctions

Don't confuse a too-small cuff (falsely high reading) with a too-large cuff (falsely low reading) — the error direction matters. Students mix up the auscultatory gap with equipment malfunction; it's a physiologic phenomenon common in hypertensive and elderly patients. Orthostatic vitals require position changes from supine to sitting to standing with 1-3 minutes between each — not just one standing reading.

Clinical Pearl

Wrong cuff, wrong answer. Small cuff reads high, large cuff reads low — think of it as a tight cuff squeezing the reading upward.