Background

The most popular method for blood pressure measurement today is the auscultatory method. An inflatable cuff is applied around the upper arm. The cuff is then inflated to occlude the brachial artery. The pressure in the cuff is then released gradually, while the clinician, using a stethoscope, listens at the brachial artery for characteristic Korotkoff sounds. These sounds are used to obtain systolic and diastolic blood pressure.

Although the auscultatory method is the most popular, direct (invasive) measurement is considered the gold standard for blood pressure accuracy. To obtain these measurements, a catheter must be inserted surgically into the patient's artery and guided to the measurement site. Due to patient risk and discomfort, non-invasive measurements are generally used.

Pulse Dynamics is a patented non-invasive measurement technology based on the oscillometric method. The oscillometric method, like the auscultatory method, requires an occlusive cuff applied around the upper arm. However, instead of using microphones (or stethoscopes) to listen for characteristic sounds, oscillometric devices use the pressure signal generated from the cuff to determine blood pressure. The pressure signal is caused by the interaction between the cuff and the blood flow through the brachial artery. This signal is analysed to measure mean arterial pressure. From this measurement, systolic and diastolic blood pressures are estimated.

Pulse Dynamics uses the same measurement technique, but analyses the pressure signal differently from other oscillometric devices. The pressure signal appears as a waveform when displayed visually. By using pattern recognition technology, Pulse Dynamics allows independent measurement of mean arterial, systolic, and diastolic pressures from the waveform. Additionally, the waveform is displayed for measurement validation and analysis. Pulse Dynamics is one of the few non-invasive techniques in the world to measure mean, systolic, and diastolic pressures without estimation. This technology has been patented by Pulse Metric and is used in all DynaPulse products.

The Pulse Dynamic waveform is directly affected by variations in blood flow and blood pressure. Therefore, the waveform reflects, to a certain extent, the state of the cardiovascular system. Recent studies have validated the use of Pulse Dynamics in the measurement of arterial compliance and left ventricular contractility. Traditionally, these measurements were obtained by surgical procedures, ultrasound, or other costly procedures. With Pulse Dynamics, the same measurement that provides blood pressure can also provide these parameters. For more information on Pulse Dynamics research, see Pulse Metric Clinical Research at this site.

Measurement Process

After the cuff is wrapped around the patient's upper arm and inflated to occlude the brachial artery, the cuff deflates while the DynaPulse records the pressure signal used to determine blood pressure. When displayed, the pressure signal appears as a waveform. The physical events that correspond to particular sections of the waveform are outlined below, in chronological order:

Section 1: Cuff Pressure Exceeds Systolic Blood Pressure
During this period, the artery remains fully occluded. Pressure waves generated by the heartbeat create the pulsatile pressure signal shown. The occlusion of the artery prevents blood flow from contributing to the measured pressures. Therefore, these initial oscillations are highly representative of aortic activity and may be a source of additional cardiovascular information.
Section 2: Cuff Pressure Between Systolic and Mean Pressures
When cuff pressure decreases to a point just below systolic pressure, the blood spurts through the artery at a high velocity. This creates a Bernoulli effect causing a phase shift in the pressure signal. Pattern recognition is used to determine systolic blood pressure. Systolic pressure is marked by the first triangle icon. As the cuff deflates, increasing portions of the cardiac cycle generate pressures that exceed cuff pressure. Therefore, an increasing volume of blood flows through the artery. The increased flow causes an increase in the amplitude and phase shift of the pressure signal.
Section 3: Cuff Pressure Between Mean and Diastolic Pressures
When cuff pressure reaches mean arterial pressure, the forces produced by blood flow balance the cuff pressure. Mean arterial pressure is the average pressure in the artery, and is marked by the second triangle icon. As cuff pressure continues to decrease, it is no longer sufficient to occlude the artery. The alleviation of cuff pressure causes the magnitude of the pressure signal to decrease. This is reflected in the decreasing magnitude of the waveform. The third triangle icon marks diastolic pressure.
Section 4: Cuff Pressure Less than Diastolic Pressure
At pressures below diastolic, the cuff does not occlude the artery. Therefore, blood flow is no longer impeded. At this point, forces on the arterial wall are dominated by haemodynamic factors. Therefore, this region is useful for calculations of arterial compliance or blood flow based on changes in haemodynamic factors.

Accuracy of Pulse Dynamics

In clinical trials, Pulse Dynamics correlated well with both auscultatory and invasive methods. Pulse Dynamics produces measurements that are usually higher than auscultatory measurements for the same patient. It is accepted that oscillometric techniques produce higher measurements than auscultatory techniques. When using a product such as the DynaPulse, differences of 5 -15 mmHg systolic and 5 - 10 mmHg diastolic between Pulse Dynamics and auscultatory measurements may be expected.
Pulse Dynamics compares very well with invasive measurements, which are considered the gold standard for accuracy. Clinical studies produced correlation coefficients of 0.94 for systolic, 0.95 for mean, and 0.91 for diastolic. The published studies are included in the Pulse Dynamics research guide.

Products Using Pulse Dynamics

DynaPulse products, manufactured by Pulse Metric, use Pulse Dynamics to measure blood pressure. In addition accuracy, ease of measurement, and reliability, these products incorporate a software package for waveform display and analysis. Movement artifact and certain arrhythmia are easily detectable by glancing at the measurement waveform. This allows clinicians to discard suspicious measurements, and gives them more insight into the cardiovascular condition of the patient.
The unique capabilities of Pulse Dynamics are particularly useful in ambulatory monitoring. For example, after a patient completes an ambulatory session, the doctor can quickly validate the measurement set to verify the accuracy of each measurement. Since one of the goals of ambulatory monitoring is to obtain a more accurate profile of the patient's blood pressure behaviour, the ability to validate each measurement is invaluable. The DP5000A, the DynaPulse ambulatory monitor, uses Pulse Dynamics for precisely this reason.