The History of Blood Pressure Monitoring

Invasive Catheterisation Method

In 1733, Reverend Stephen Hales inserted a long glass tube upright into an incision in a horse's artery. The pumping action of the heart generated a pressure force, causing the blood level to rise in the tube. These early surgical procedures were dangerous for patients, due to the risk of infection and excessive blood loss. Even today, invasive catheterisation procedures are seldom used solely for blood pressure measurement. Instead, non-invasive (i.e. non-surgical) methods sacrifice a degree of accuracy for patient safety, comfort, and convenience.

The Auscultatory Method

In 1905, Korotkoff described the auscultatory sounds which became the foundation for the auscultatory technique. This is the most common method of blood pressure measurement today.

An air-filled cuff is wrapped around the patient's upper arm. The cuff is inflated to occlude the brachial artery. As the cuff is allowed to deflate, a stethoscope is placed over the patient's brachial artery (distal to the cuff). The clinician uses the stethoscope to listen for the Korotkoff sounds as the cuff deflates. The beginning of Phase I is systolic pressure. There is some debate as to whether Phase IV, Phase V, or a combination of the two best represents diastolic pressure. This situation is complicated by the fact that some patients may not have audible Phase IV sounds, and in others Phase V may be difficult to determine.

The auscultatory technique is based on the ability of the human ear to detect and distinguish sounds. This is a great advantage since it allows the clinician to determine the quality of each measurement. However, inherent in this is the possibility for measurement error due to differences in hearing acuity from clinician to clinician. Unqualified or inexperienced personnel may be more susceptible to outside noise, other interference, or inconsistent assessment of Korotkoff sounds. In an attempt to increase reproducibility, some automated devices have replaced the human ear with a microphone.

The Automated Auscultatory Method

These devices apply sound-based algorithms to estimate SBP and DBP. By using a microphone, these devices lack validation ability. In addition to noise-artifact sensitivity, these sound-dependent algorithms may not adequately compensate for patient conditions such as hypotension (i.e. low blood pressure), where the Korotkoff sounds may be muted. To make automated measurement more reliable, oscillometric devices were created.

The Oscillometric Method

The term "oscillometric" refers to any measurement of the oscillations caused by the arterial pressure pulse. These oscillations are the direct results of the coupling of the occlusive cuff to the artery. This method allowed blood pressure measurement of critical care and intensive care (ICU) patients with muted Korotkoff sounds. These devices do not use microphones. Therefore, cuff placement and external noise are not significant problems. These devices are sensitive to patient movement and do not allow measurement validation. Following is the arterial waveform display by using the oscillometric method:

Unlike auscultatory techniques, which measure systolic and diastolic but estimate mean arterial pressure, oscillometric devices measure the mean but estimate systolic and diastolic. An air-filled cuff is wrapped around the patient's upper arm. The cuff is inflated to occlude the brachial artery. As the cuff is allowed to deflate, pressure data is recorded by the device. Over time, the pressure data looks like a waveform (see below). The point of maximum amplitude is considered mean arterial pressure. Systolic and diastolic are estimated from mean arterial pressure (MAP). Therefore, an erroneous determination of MAP may produce inaccurate values for systolic and diastolic.

Pulse Dynamics, the patented technology by Pulse Metric, is a variant of the oscillometric method. It combines the reliability of oscillometrics with the validation capability of the manual auscultatory method.

Alternative Methods

The majority of monitors on the market today are either oscillometric or auscultatory in nature. However, there are other types of devices. Some monitors employ both auscultatory and oscillometric methods, using one method as a primary measurement and the other for verification, to minimise the inherent disadvantages of each.

The infrasound technique attempts to improve on the auscultatory method by detecting low frequency Korotkoff vibrations below about 50 Hz, including sub-audible vibrations.

Ultrasound techniques are also occasionally used to determine blood pressure, usually in combination with other methods. Values recorded by ultrasound can be very operator dependent.

Another method, called impedance plethysmography, also measures the volumetric change associated with arterial distension. Volumetric changes cause changes in the electrical conductivity (impedance) of the measurement site. When graphed with time, an impedance waveform is produced, similar to the pressure-generated oscillometric waveform. Blood pressure is estimated in a manner similar to the oscillometric technique.

Arterial tonometry uses a very different approach. The artery is flattened by applying pressure non-invasively to squeeze the artery against bone. The applied pressures required to maintain the flattened shape are recorded. This is accomplished by using an array of sensors, each of which measures pressure. The result of this method is a waveform similar to catheter measurements, and an algorithm must be used to calculate pressures from that waveform.

Tonometry has several limitations. First, it is a measure of the peripheral circulation, which has different pressures from more centrally-located sites (such as the brachial artery). Second, tonometry has a high sensitivity to sensor position and angle . Therefore, inter-operator reproducibility may be low. Lastly, tonometry requires calibration via an initial blood pressure measurement obtained by an independent technique.

Ambulatory Blood Pressure Monitoring

Ambulatory blood pressure monitors measure patient blood pressure over a predetermined length of time (typically 24 hours) outside the clinic as the patients follow their normal daily routine.

Briefly described, the procedure is as follows: the patient receives instruction on the correct use of the monitor, and wears the device. Then, the patient leaves the clinic and follows a normal daily routine. Periodically, the monitor takes a measurement and stores the results. When the monitoring period is over, the patient returns to the clinic. The clinician downloads the data from the monitor to a computer for analysis. The clinician normally has between 70 and 100 blood pressure measurements available for analysis.

The purpose of ambulatory blood pressure monitoring (ABPM) is to obtain a profile of the patient's blood pressure under conditions that are more representative of the patient's lifestyle than those inherent in a clinical environment. It has been well documented that blood pressures measured in a clinic are not always representative of everyday pressures. This led to the identification of white coat hypertension and the circadian rhythm of blood pressure.

White coat hypertension is generally defined as "a persistently elevated clinic blood pressure and a normal pressure at other times." Although the general description of white coat hypertension is agreed upon, the exact definition varies. Between 20% to 40% of patients with mild to moderate hypertension in a clinical setting may actually be white coat hypertensive. Two salient issues that arise from white coat hypertension are: the effects of anti-hypertensive drugs on normotensive individuals, and the cost of administering those drugs. In order to avoid administering unnecessary therapy to white coat hypertensives, these individuals must be identified. ABPM is the only way to accomplish this.

ABPM has also led directly to the discovery of the circadian rhythm of blood pressure: a decrease in blood pressure levels from periods of wakefulness to period of sleep (for convenience, daytime and night-time will be used for wakefulness and sleep, respectively). Most people exhibit this circadian rhythm, which consists of a blood pressure decrease of approximately 15-25% during the night-time, with increases to daytime levels again in the morning. Clinical studies classify people as either "dippers" or "non-dippers", depending on whether their blood pressure exhibits the circadian rhythm or remains close to its daytime level during night-time hours. Since hypertensive "dippers" may be normotensive during night-time hours, ABPM may prove helpful in optimising treatment programs for hypertension.

Clinical research in the field of ABPM has led to the application of additional analysis techniques that may allow the clinician to obtain a clearer assessment of a patient's hypertensive condition.

ABPM may be cost effective by reducing the number of patients who are mislabelled as hypertensive and subsequently undergo hypertension management therapy. One twenty-four hour ABPM session using Pulse Dynamic technology may provide reliable information regarding blood pressure, arterial compliance, left ventricular contractility, and dipper/non-dipper classification. This decreases the need for exhaustive testing and allows quicker, easier diagnosis and treatment program development.