Blood pressure is a cardinal vital sign that guides both acute and long-term clinical decision-making. Given its importance in directing care, it is essential to measure blood pressure accurately and consistently.
In general, 2 values are recorded during the measurement of blood pressure. The first, systolic pressure, represents the peak arterial pressure during systole. The second, diastolic pressure, represents the minimum arterial pressure during diastole. A third value, mean arterial pressure, can be calculated from the systolic and diastolic pressures.
The original method of determining blood pressure via the auscultation of Korotkoff sounds continues to be a mainstay in blood pressure measurement. This method utilizes a sphygmomanometer, a device comprised of an inflatable cuff that is connected to a pressure gauge (generally a column of mercury). To measure an individual’s blood pressure, the deflated cuff is placed around the arm and inflated sufficiently to occlude arterial flow. At this point, the pressure of the cuff exceeds the systolic pressure, and auscultation over the brachial artery reveals no sound due to complete obstruction of flow. The cuff is then gradually deflated while continuing to auscultate over the brachial artery. When the pressure in the cuff falls to the level of the systolic pressure, pulsatile blood flow begins to re-establish. The resulting turbulence produces characteristic tapping sounds known as Korotkoff sounds. As the cuff continues to deflate to the level of the diastolic pressure, pulsatile blood flow occurs smoothly, and Korotkoff sounds disappear. Thus, the systolic pressure is indicated by the origination of Korotkoff sounds, and the diastolic pressure is indicated by their disappearance.
Of note, medical professionals should be aware of the auscultatory gap, a phenomenon which can result in the premature recording of the diastolic pressure. In some patients, particularly those with wide pulse pressure, Korotkoff sounds can temporarily fade but then reappear as the cuff continues to deflate. Medical professionals should thus continue to auscultate over the brachial artery even when Korotkoff sounds disappear to eliminate the possibility of an auscultatory gap. Only the final disappearance of Korotkoff sounds should be used for recording of the diastolic pressure.
Automated devices can also be used to measure blood pressure. Rather than auscultate for Korotkoff sounds, these devices measure oscillations in blood flow as the cuff is deflated.
Device-specific algorithms are then used to calculate blood pressure indirectly. An advantage of automated measurements is that they require little user-knowledge and are thus suitable for use by laypersons in non-medical environments.
The most accurate method of obtaining blood pressure measurements is with the use of an invasive probe that is inserted directly into the lumen of an artery. An advantage of invasive monitoring is the ability to display blood pressure variations with each heartbeat. However, given the invasive nature of this method and the associated risks, its use is limited to critical care or operative settings. As such, the remainder of this review will focus on the previously mentioned methods of blood pressure measurement.
Blood pressure is remarkably labile. The most insignificant activities can result in substantial changes in blood pressure readings. As such, regardless of whether a manual or automated method is being used to measure blood pressure, medical professionals should always prepare the patient and environment properly before cuff inflation.
First, the patient should be questioned regarding recent caffeine consumption, exercise, or smoking. If any of these activities have occurred within the last 30 minutes, blood pressure measurement should be postponed until this period has passed. Next, the patient should be encouraged to empty his/her bladder. Upon return, the patient should be seated in a quiet room on a chair with back support. Both feet should be flat on the floor with the legs uncrossed, and this seated position should be maintained for at least 5 minutes.
At this time, a properly-sized cuff should be placed directly over the patient’s arm, no clothing should be underneath the cuff, and sleeves should not be rolled above the cuff. Once the cuff is in position, the patient’s arm should be supported so that the middle of the cuff is at the level of the right atrium. Initiation of blood pressure measurement can now occur. The patient should not speak or be spoken to while measurements are being taken.
There is significant variation among automated devices, and users should refer to instruction guides on how to initiate cuff inflation and blood pressure measurement. If a manual measurement is being performed, the bell or diaphragm of a stethoscope should be placed over the medial antecubital fossa over the approximate location of the brachial artery. The blood pressure cuff should be inflated 30 mm Hg beyond the point at which the radial pulse is no longer palpable. Deflation of the cuff should occur in a slow and controlled manner – a rate of 2 to 3 mm Hg per second is recommended. As mentioned previously, the appearance of Korotkoff sounds signifies the systolic pressure, while the cessation of these sounds indicates the diastolic pressure.
The most common source of error in blood pressure measurement is a failure to adhere to proper technique. Multiple studies have attempted to quantify the impact of common mistakes. Smoking within 30 minutes of measurement can raise the systolic blood pressure up to 20 mm Hg, while a distended bladder can raise both systolic and diastolic measurements by 10 to 15 mm Hg. Sitting in a chair lacking back support can raise systolic blood pressure up to 10 mm Hg, and a similar increase is observed when both legs are crossed. Cuff placement over clothing can affect measurements by an astonishing 50 mm Hg. Talking/listening during measurements can increase both systolic and diastolic measurements by 10 mm Hg. The use of an improperly sized cuff can affect blood pressure in either direction; a larger cuff results in falsely low measurements, while a smaller cuff results in falsely elevated measurements. Similarly, incorrect positioning of the arm also results in a bidirectional error; placing the arm below the level of the right atrium results in higher values, whereas placing the arm above the level of the right atrium generates lower values. The vast ranges of these errors highlight the importance of adhering to appropriate technique when measuring blood pressure.
In addition to the modifiable source of error above, there are also unavoidable sources of error. Multiple studies have found differences between measurements obtained in clinical settings and those taken in ambulatory settings. Hypertension can be inaccurately diagnosed in patients prone to “white-coat hypertension,” a phenomenon in which the apprehension of being in the presence of a medical professional results in a transient rise in blood pressure. These patients are normotensive in ordinary settings and are hypertensive only in clinical settings. Similarly, hypertension can be inadvertently overlooked in patients who appear to be normotensive in the clinic but are hypertensive throughout other points of the day. This “masked hypertension” may be due to lifestyle changes made before medical appointments.
When clinically feasible, 2 or more measurements taken at different visits should be used to make a diagnosis of hypertension. The threshold at which high blood pressure is diagnosed is constantly evolving in response to new evidence. Previous guidelines had defined hypertension beginning at a blood pressure of 140/90 mm Hg, but the most recent updates have lowered this level based on the benefits of early intervention. Current thresholds are listed in the table below:
Undiagnosed hypertension places patients at increased risk of developing coronary artery disease, stroke, and end-stage renal disease, among other complications. In the United States, hypertension is the second most preventable cause of death; worldwide, it is the leading cause of death and disability-adjusted life years. Thus, timely and accurate diagnosis of hypertension is essential to initiate treatment that prevents and/or reduces the occurrence of these complications.
Hypotension is less common than hypertension and is generally due to an identifiable cause, such as dehydration, illness, or medication side effect. While there is no formal threshold for the diagnosis of hypotension, a systolic pressure less than 90 mm Hg or a diastolic pressure less than 60 mm Hg is generally used by medical professionals. More commonly, the diagnosis is guided by patient symptoms, including light-headedness, dizziness, blurred vision, nausea, and weakness.
Some patients are normotensive at rest but experience symptoms of hypotension when standing. Such orthostatic hypotension is defined by a decrease in systolic blood pressure of 20 mm Hg or decrease in diastolic blood pressure of 10 mm Hg after three minutes of standing from a sitting or supine position.