2 Blood Pressure Assessment

What blood pressure represents

Blood pressure (BP) is the force blood exerts against arterial walls. It is recorded in millimeters of mercury as systolic pressure over diastolic pressure.

• Systolic blood pressure (SBP) is the peak arterial pressure during ventricular contraction.

• Diastolic blood pressure (DBP) is the lowest arterial pressure between contractions.

• Pulse pressure (PP) is the difference between systolic and diastolic pressure: $PP = SBP - DBP$. It is influenced by stroke volume and arterial stiffness.

• Mean arterial pressure (MAP) is the approximate average arterial pressure over one cardiac cycle. At ordinary heart rates, a useful estimate is:

For a pressure of $120/80\ \text{mm Hg}$, estimated MAP is about $93\ \text{mm Hg}$. This estimate is less accurate at unusually high heart rates or when the arterial waveform is abnormal. MAP is a useful indicator of perfusion pressure, but it does not by itself prove that organs are adequately perfused.

How blood pressure is determined

A simplified relationship connects mean arterial pressure with cardiac output and systemic vascular resistance:

Cardiac output is the volume of blood pumped by a ventricle per minute. It depends on heart rate and stroke volume:

More precisely, the pressure gradient driving systemic flow also reflects venous pressure. The simplified relationship is useful for understanding how changes in flow and resistance affect arterial pressure.

Stroke volume is affected by preload, contractility, and afterload. Preload describes ventricular filling and myocardial stretch at the end of diastole; venous return and circulating blood volume influence it. Low volume can reduce preload and stroke volume, while excessive volume may cause congestion.

Afterload is the resistance or pressure a ventricle must overcome to eject blood. Increased arterial resistance or pressure can raise afterload and, in some circumstances, reduce stroke volume. Systemic vascular resistance is strongly influenced by small-artery and arteriole diameter: vasoconstriction generally increases resistance, while vasodilation decreases it.

These factors interact. Vasodilation can result in low blood pressure despite normal or increased cardiac output. With blood loss, a person may initially maintain blood pressure through compensatory vasoconstriction and a faster heart rate.

Cuff measurement and technique

A manual measurement uses an inflatable cuff and stethoscope. As cuff pressure is released, the first Korotkoff sound indicates systolic pressure; in adults, disappearance of the sounds indicates diastolic pressure. Validated automated upper-arm devices use an oscillometric method to estimate blood pressure from cuff-pressure oscillations.

Use a properly maintained, validated device and a cuff that fits the measured arm. A cuff that is too small can produce a falsely high reading; one that is too large can produce a falsely low reading.

Reliable seated measurements

1. Have the person rest quietly for at least $5$ minutes. They should avoid talking or moving during the reading.

2. Seat the person with their back supported, feet flat on the floor, and legs uncrossed.

3. Place the correctly sized cuff on a bare upper arm, and support the arm at heart level.

4. Take two readings at least $1$ minute apart. Record the readings and which arm was used.

5. For ongoing home monitoring, use a validated upper-arm monitor and follow a consistent schedule. Repeated readings are more informative than one isolated value.

Recent activity, caffeine, nicotine, a full bladder, unsupported posture, talking, and an incorrectly sized cuff can affect results. If a reading is unexpected, check the technique and repeat it when clinically appropriate rather than relying on a single number.

Other monitoring methods

Orthostatic measurements involve measuring blood pressure and pulse after the person rests supine and again after standing, according to the applicable clinical protocol. Record the person's position, timing, symptoms, and readings. Dizziness or faintness on standing may indicate impaired compensation or reduced circulating volume.

Ambulatory BP monitoring uses an automated cuff to take repeated measurements during usual daily activities and sleep. It can help characterize blood pressure outside the clinic.

An arterial catheter provides continuous, beat-to-beat pressure measurement and may permit arterial blood sampling. Its fluid-filled system's transducer must be correctly leveled and zeroed, and the waveform checked. If the transducer is too high, the displayed pressure may be falsely low; if it is too low, the displayed pressure may be falsely high.

Other hemodynamic measurements may help assess circulation in critically ill patients. Central venous pressure and pulmonary artery occlusion pressure are pressure measurements used in context to estimate aspects of right- and left-sided filling. They are not direct measurements of blood volume or a stand-alone measure of fluid responsiveness. Interpret them alongside the clinical picture and other findings.

Interpreting adult readings

For adults, the 2025 AHA/ACC guideline categorizes office blood pressure using the average of at least two careful readings obtained on at least two occasions. When systolic and diastolic readings fall into different categories, use the higher category.

• Normal: systolic pressure is below $120\ \text{mm Hg}$ and diastolic pressure is below $80\ \text{mm Hg}$.

• Elevated: systolic pressure is $120\text{–}129\ \text{mm Hg}$ and diastolic pressure is below $80\ \text{mm Hg}$.

• Hypertension, stage 1: systolic pressure is $130\text{–}139\ \text{mm Hg}$ or diastolic pressure is $80\text{–}89\ \text{mm Hg}$.

• Hypertension, stage 2: systolic pressure is at least $140\ \text{mm Hg}$ or diastolic pressure is at least $90\ \text{mm Hg}$.

These adult categories do not replace age- and condition-specific assessment. A hypertension diagnosis should not be based on one reading alone. For example, a reading of $134/86\ \text{mm Hg}$ falls in stage 1 because either its systolic or diastolic value meets that category; diagnosis still requires appropriately repeated readings and clinical assessment.

Very high readings

For an adult with a reading above $180\ \text{mm Hg}$ systolic and/or $120\ \text{mm Hg}$ diastolic, repeat the measurement after at least $1$ minute. If it remains this high, contact a health professional promptly. If the reading is accompanied by chest pain, shortness of breath, weakness or numbness, vision change, difficulty speaking, or other concerning acute symptoms, call emergency services.

Recognizing deterioration

Assess the person, not just the monitor. A falling blood pressure or MAP, a new marked rise in heart rate, altered mental status, fainting, cool or mottled skin, weak pulses, reduced urine output, or increasing breathing difficulty can signal worsening perfusion or another acute problem.

A normal blood pressure does not rule out early deterioration. Compensatory responses can temporarily preserve pressure, and trends may reveal change before a single reading does. Recheck an unexpected measurement, assess symptoms and other vital signs, compare with the person's baseline, and promptly escalate concerning findings according to local protocols.

For example, a person with suspected blood loss may have near-normal blood pressure while becoming increasingly confused, tachycardic, and cool to the touch. Do not assume the person is stable on the basis of the blood pressure; reassess promptly and escalate care.

Worked examples and core relationships

A pressure of $126/78\ \text{mm Hg}$ gives an estimated MAP of about $94\ \text{mm Hg}$:

With a heart rate of $80$ beats per minute and stroke volume of $65\ \text{mL/beat}$, cardiac output is $5{,}200\ \text{mL/min}$, or $5.2\ \text{L/min}$:

A cuff that is too small may read falsely high. A transducer below the level of the right atrium may display a falsely high pressure; re-level it and confirm the system according to protocol. A transducer positioned too high may display a falsely low pressure.

Blood pressure reflects the interaction of cardiac output and vascular resistance. Cardiac output depends on heart rate and stroke volume, while preload, contractility, and afterload affect stroke volume. Accurate interpretation depends on proper measurement technique, repeated readings and trends, and consideration of symptoms, other vital signs, and perfusion findings.