Clinical Skills · General Survey, Anthropometric Measurement, and Vital Signs
Blood Pressure
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In 30 seconds
Blood pressure (BP) is the force that circulating blood exerts against the walls of the arteries. It is reported as two numbers written as a fraction: the systolic pressure (peak pressure during the heart's contraction) over the diastolic pressure (lower pressure while the heart relaxes and refills). A reading is expressed in millimeters of mercury (mmHg), for example 120/80 mmHg, and documented as "120 over 80."
Blood pressure is not fixed — it fluctuates with activity, position, emotion, and time of day, so a single reading is a snapshot, not a verdict. The pattern over time matters: a baseline taken under consistent conditions, compared with later readings taken the same way, gives the care team information about perfusion, fluid status, medication effects, and risk.
Why this matters
Blood pressure reflects how hard the heart is working and how well blood reaches the organs. Sustained elevations are a major risk factor for heart disease, stroke, and kidney damage; low blood pressure can signal inadequate perfusion. It is also one of the easiest vital signs to measure inaccurately — cuff size, arm position, preparation, and technique can each shift the numbers meaningfully. Understanding what creates blood pressure and what distorts its measurement helps the nurse protect the person from both missed problems and unnecessary alarm.
The college version
Core Concepts
What the two numbers mean
systolic pressure The peak arterial pressure during heart contraction. Full entry → is the force during ventricular contraction — the peak the arteries experience with each heartbeat. diastolic pressure The lower arterial pressure while the heart relaxes. Full entry → is the force during ventricular relaxation; the difference between them is the pulse pressure (e.g., 120 − 80 = 40 mmHg). A useful derived value is the mean arterial pressure (MAP) Estimated average pressure perfusing the organs, ≈ (SBP + 2×DBP) ÷ 3. Full entry →, the average pressure driving blood into the organs, commonly estimated as (systolic + 2 × diastolic) ÷ 3 — about 93 mmHg for 120/80. MAP matters most for organ perfusion.
What creates blood pressure
Blood pressure is produced by two main forces: cardiac output Volume of blood pumped by the heart per minute (heart rate × stroke volume). Full entry → (blood pumped per minute = heart rate × stroke volume) and systemic vascular resistance The resistance blood meets in the arteries. Full entry → (resistance in the arteries, influenced by vessel diameter and tone). Blood volume and vessel elasticity also matter — stiffer arteries produce higher systolic pressures. Anything that increases cardiac output, narrows vessels, or expands volume tends to raise blood pressure; the opposite lowers it. That is why pain, anxiety, and cold can raise a reading, and blood loss or vasodilation can lower it.
Korotkoff sounds and the auscultatory method
The classic manual technique uses a sphygmomanometer The cuff-and-gauge device used to measure blood pressure. Full entry → (cuff + gauge) and a stethoscope over the brachial artery at the antecubital area. The cuff is inflated above the point where blood flow stops, then deflated slowly while the nurse listens for Korotkoff sounds Sounds heard over the artery while the cuff deflates. Full entry →. The first clear tapping sound marks systolic pressure; the sounds then muffle and disappear — disappearance marks diastolic pressure. A rare but important trap is the auscultatory gap A silent interval between systolic and diastolic sounds. Full entry →: a silent interval between systolic and diastolic sounds that can cause a falsely low systolic reading if the cuff was not inflated high enough.
Cuff size, positioning, and preparation
Measurement errors usually come from the technique, not the machine. The cuff bladder should fit the arm: a commonly taught sizing principle is that bladder width should be about 40% of arm circumference and long enough to encircle most of the arm. A cuff that is too small tends to overestimate pressure; one that is too large tends to underestimate it. Commonly taught preparation for a resting measurement: no caffeine, smoking, or vigorous exercise for about 30 minutes beforehand; sitting quietly, back supported, feet flat, legs uncrossed; arm supported at heart level on the bare arm; no talking; slow deflation (about 2–3 mmHg per second). Follow your facility's procedure for exact steps.
Sources of variation and the bigger picture
Blood pressure varies with age, position, pain, anxiety, exertion, medications, and time of day. Some people show white coat hypertension Elevated readings in clinical settings that aren't seen at home. Full entry → (elevated readings in clinical settings that are not present at home) or the reverse, masked hypertension (normal readings in clinic, elevated at home) — another reason trends matter more than single readings. Orthostatic (postural) hypotension is a drop in pressure on moving from lying to sitting or standing, often with dizziness; it is assessed by comparing readings in different positions. Automated devices are convenient but can disagree with manual readings; use them per policy and confirm manually when a reading seems inconsistent with the person's condition. Abnormal readings are rechecked before reporting, and thresholds for action follow current guidelines and institutional policy.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Systolic pressure | Diastolic pressure | Systolic is during contraction (top number); diastolic is during relaxation (bottom number). |
| Too-small cuff effect | Too-large cuff effect | Too small overestimates; too large underestimates — remember the direction for exam questions. |
| First Korotkoff sound | Muffling/disappearance of sounds | First sound = systolic; disappearance = diastolic. |
| A single high reading | Hypertension | One reading is a snapshot; diagnosis and thresholds follow guidelines, trends, and provider judgment. |
| Blood pressure | Heart rate | BP is force on vessel walls; heart rate is beats per minute — related, but distinct findings. |
| Korotkoff silence (auscultatory gap) | True diastolic | The gap can hide the true systolic; always inflate well above expected pressure. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Blood pressure is how hard the blood pushes against the walls of the tubes it travels through — like water pressing against a garden hose. The first number is the push when the heart squeezes, and the second number is the push when the heart rests. A nurse checks it with a squeeze cuff to make sure the push is strong enough to reach everywhere it needs to go, but not so strong that it strains the pipes.
Worked example
Mr. Okafor, a 45-year-old person, is seen for a routine check. The regular adult cuff is snug on his upper arm, suggesting a larger cuff would fit better. Using the snug cuff, the first reading is 148/92 mmHg. The nurse notes the cuff-size concern, lets him rest quietly, rechecks with a properly sized cuff on the bare arm at heart level, and obtains 132/84 mmHg.
The nurse documents both readings with cuff size and arm used, and reports per institutional policy so the care team can interpret the trend — including that the first reading used an undersized cuff, which may have overestimated the pressure. (Educational illustration: no treatment starts from a single reading; thresholds and next steps follow current guidelines and provider direction.) The key lesson: technique and cuff fit can change the numbers by more than enough to matter.
Key takeaways
- BP = cardiac output × systemic vascular resistance (with blood volume and vessel elasticity also involved).
- Systolic = peak pressure during contraction; diastolic = pressure during relaxation; report as a fraction, e.g., 120/80 mmHg.
- MAP ≈ (systolic + 2 × diastolic) ÷ 3 — the average pressure driving organ perfusion.
- Cuff size matters: a too-small cuff overestimates; a too-large cuff underestimates.
- Standard prep (commonly taught): rest quietly, no caffeine/smoking ~30 min before, arm supported at heart level, no talking during measurement.
- Systolic is identified by the first Korotkoff sound; diastolic by the disappearance of the sounds.
- Watch for the auscultatory gap — inflate the cuff well above the expected systolic pressure.
- One reading is a snapshot: interpret trends, account for white coat and masked hypertension, and follow institutional policy for abnormal readings and reporting.
- Orthostatic hypotension is assessed by comparing lying, sitting, and standing readings.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Define systolic and diastolic pressure and identify which number is heard first during deflation.
Show answer
Systolic is the peak pressure during heart contraction; diastolic is the pressure during relaxation. The first Korotkoff sound marks systolic; disappearance marks diastolic.
Why can a cuff that is too small produce a falsely high reading?
Show answer
A narrow cuff compresses the arm tissue and artery unevenly, so the gauge reads a higher pressure than the true arterial pressure — the classic "too small overestimates" error.
How is MAP estimated, and why is it clinically useful?
Show answer
MAP ≈ (systolic + 2 × diastolic) ÷ 3; it estimates the average pressure driving blood to the organs, so it is watched closely in acute care.
List three elements of standard patient preparation before a resting blood pressure measurement.
Show answer
Any three commonly taught elements, e.g.: rest quietly for several minutes; no caffeine, smoking, or vigorous exercise ~30 minutes prior; arm supported at heart level; back supported, feet flat, legs uncrossed; no talking during measurement.
Ms. Lee's blood pressure is 118/74 lying, and 104/68 standing, and she feels dizzy on standing. What is this pattern called, and how was it detected?
Show answer
Orthostatic (postural) hypotension — detected by comparing readings taken lying, sitting, and standing, along with the symptom of dizziness.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- systolic pressure
- The peak arterial pressure during heart contraction.
- diastolic pressure
- The lower arterial pressure while the heart relaxes.
- cardiac output
- Volume of blood pumped by the heart per minute (heart rate × stroke volume).
- systemic vascular resistance
- The resistance blood meets in the arteries.
- mean arterial pressure (MAP)
- Estimated average pressure perfusing the organs, ≈ (SBP + 2×DBP) ÷ 3.
- sphygmomanometer
- The cuff-and-gauge device used to measure blood pressure.
- Korotkoff sounds
- Sounds heard over the artery while the cuff deflates.
- auscultatory gap
- A silent interval between systolic and diastolic sounds.
- orthostatic hypotension
- A drop in pressure (often with dizziness) on position change.
- white coat hypertension
- Elevated readings in clinical settings that aren't seen at home.
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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