DynaPulse monitors are classified as oscillometric devices because they measure the oscillations (due to the arterial pulse) that occur from the coupling of a cuff to an artery. However, while other oscillometric devices use empirically-derived algorithms, DynaPulse monitors utilize Pulse Dynamic technology to measure blood pressure and to obtain additional cardiovascular information.
An inherent element of Pulse Dynamic technology is the graphical display of the waveform. That waveform, displayed with each measurement, is a digitally accurate record of the measurement which plots arterial pressure oscillations against both cuff pressure and time. SBP, DBP, and MAP are then determined based on the physical principles of blood flow, not empirically-generated criteria.
In order to perform a blood pressure measurement, the cuff is first inflated to occlude the artery. Next, the cuff is allowed to deflate over a span of approximately thirty seconds. The physical events that correspond to particular sections of the waveform are outlined below, in chronological order:
Cuff pressure exceeds SBP. This is considered the "super-systolic" portion
of the waveform. During this period, the artery remains fully occluded. Pressure waves generated by cardiovascular activity create pulsatile arterial distention proximal to the occlusion. This distention is sensed by the transducer,producing the initial oscillations displayed on the waveform (see diagram above). Unlike later oscillations, the occlusion prevents blood flow from contributing to the measured pressures (i.e. the pressures are dominated by forces directly generated by the heart). Therefore, these oscillations represent aortic activity and may be a source of additional cardiovascular information.
When cuff pressure decreases to a point just below SBP, the pressure built up from previous heartbeats forces blood through the artery at a high velocity,creating a Bernoulli effect: a force acting inward on the arterial wall. This inward force creates a measurable shift in the pressure wave, that shift causes the transducer to generate a signal. The shift is a time-dependent signal which exhibits different time-dependent characteristics from the original, pulsatile signal. The time dependent signal may be detected by visually inspecting the waveform and observing the gradual pulse-to-pulse change in trough shape over time. SBP is determined by using a digital pattern-recognition algorithm to identify the time-dependent signal characteristic of SBP (marked by the first triangle icon on the waveform).
As the cuff deflates, an increasing portion of the cardiovascular cycle generates pressures that exceed cuff pressure, resulting in an increasing volume of blood flow through the artery. The increase in blood flow volume causes an increase in oscillometric amplitude and a progressive shape change in the time-dependent signal.
When cuff pressure reaches MAP, the forces produced by the Bernoulli effect balance the cuff pressure, and the corresponding time-dependent signal results in a symmetrically-shaped triangular trough (marked by the second triangle icon).
Cuff pressure continues to decrease, and is no longer sufficient to occlude the artery. The resultant alleviation of the driving pressure causes the magnitude of the arterial wall distention to decrease. This is reflected in the decreasing magnitude of the waveform oscillations.
When the time-dependent signal reaches a characteristic DBP point, the
pattern recognition algorithm identifies DBP (marked by the third triangle icon).
At pressures lower than diastolic, the cuff does not occlude the artery. Therefore, blood flow is no longer impeded. This final portion of the waveform is known as the sub-diastolic range of measurement. In this region, the forces on the arterial wall are dominated by hemodynamic factors. Therefore, any arterial wall distention can be used for calculations of arterial compliance (change in volume/change in pressure), or blood flow based on changes in hemodynamic parameters.
Since DynaPulse blood pressures are determined by a physical consequence of blood flow (i.e. the time-dependent signal), as opposed to the abstract criteria used in traditional oscillometric techniques, blood pressures measured using oscillometric Pulse Dynamics correlate very well to invasive, "gold standard" catheter measurements taken at the aorta. For further information on the correlation of the Pulse Dynamic technique to both catheter and auscultatory blood pressure please refer to DynaPulse Clinical Data. (link to research/validation)
Another advantage of the waveform is the high degree of confidence provided by its measurements. While most monitors provide final measurements, without any indication of measurement quality, DynaPulse monitors provide "full disclosure" by displaying a waveform for each measurement. Signal noise (artifact) due to patient movement, inadequate cuff placement, or other factors will produce waveform irregularities that may be visually detected. This allows the operator to quickly establish the quality of individual measurements for validation.
The occlusion applied by the cuff during the measurement process causes the brachial and associated central arteries (including the aorta) to completely fill with blood. In this state, the arteries transmit cardiovascular pressure waves and fluctuations from the heart to the brachial artery, where they result in an output signal from the transducer. This allows additional cardiovascular information to be extracted from the waveform. For example, ectopic arrhythmia and other cardiac rhythm abnormalities may be detected by visually examining the waveform for missing or distorted oscillations.
The above plot is a graphical display of arrhythmia from the Pulse Dynamic technology. With this information, the physician could refer to traditional diagnostic techniques to determine the actual nature of the arrhythmia.