Mechanics of the occlusive arm cuff and its application as a volume sensor

Although a common medical instrument, the mechanical function of an occlusive arm cuff has not been fully described in an engineering sense. The occlusive arm cuff is examined here using a mathematical mechanics model and experimental measurements. Cuff stretch was modeled by a nonlinear pressure-volume function. Air compression was represented by Boyle's law. An apparatus was developed to measure pressure due to the air volume pumped into the cuff for fixed arm volume. Data were obtained for two different cuff designs, and reveal a nonlinear cuff pressure-volume relationship that could be represented accurately by the mathematical model. Calibration constants are provided for the two types of occlusive cuff. Thus, the cuff pressure was found to consist of a balance between that produced by stretch of the elastic cuff bladder and that of the compression of the air contained within the bladder. The use of the gas law alone was found to be inadequate to represent the cuff mechanics. When applying the cuff to measure change in arm volume, such as during plethysmography or oscillometry, it cannot be assumed that the cuff sensitivity is constant. More precisely, it was found that the occlusive cuff is a transducer with a volume sensitivity that increases with cuff pressure and volume until it becomes nearly constant at high levels of cuff pressure (150 mmHg). A hypothetical case of a linear elastic artery with constant pulse pressure was used as input to the cuff model to illustrate the change in cuff pressure oscillations that occurs while cuff pressure is released.(ABSTRACT TRUNCATED AT 250 WORDS)