THEORY OF THE OSCILLOMETRIC MAXIMUM AND THE SYSTOLIC AND DIASTOLIC DETECTION RATIOS

It is proposed that the maximum in cuff pressure oscillations during o scillometry is due to the buckling of the brachial artery under a cuff . This theory is investigated by means of a mathematical model of osci llometry that includes the mechanics of the occlusive arm cuff, the ar terial pressure pulse waveform, and the mechanics of the brachial arte ry. A numerical solution is provided for the oscillations in cuff pres sure for one cycle of cuff inflation and deflation. The buckling press ure is determined from actual arterial data and the von Mises buckling criteria. The buckling of an artery under a cuff occurs near -2 to 0 mm Hg transmural pressure. This effect corresponds with a maximum arte rial compliance and maximum cuff pressure oscillations when cuff press ure is nearly equal to mean arterial pressure (MAP), in support of the suggested theory. The model was also found to demonstrate the basic c haracteristics of experimental oscillometry, such as an increasing and decreasing amplitude in oscillations as cuff pressure decreases, the oscillations that occur when cuff pressure is above systolic pressure, maximum oscillation amplitudes in the range of 1 to 4 mm Hg, and an o scillatory maximum at cuff pressure equal to MAP. These findings suppo rt the case that the model is representative of oscillometry. Finally, the model predicted values for the systolic and diastolic detection r atios of 0.593 and 0.717, respectively, similar to those found empiric ally. These ratios alter with blood pressure, but the tightness of the cuff wrap did not change their value.