Inflatable cuff devices are used in a variety of applications to measure such parameters as volume, change in volume, pressure, response to pressure, occlusion, and response to occlusion. See, for example, U.S. Pat. Nos. 4,747,415 and 5,692,520 to Lavoisier. These measurements are used widely in, but not limited to, the medical community to assay physiological parameters. Examples of such physiological parameters are blood pressure, cardiac cycle (plythesmography), blood flow, and pulse rate, to name a few. In each case, a bladder-based cuff device is placed around or in contact with the object to be measured (usually a body appendage such as an arm or a fingertip) and a gas, generally air, is pumped into the bladder, causing the bladder to inflate and thus reduce the inner diameter of the cuff. It should be noted that media other than air can be used; however, alternative media such as liquids have the disadvantage of being essentially non-compressible. The reduction of the inner diameter of the cuff results in an excess of bladder material being compressed into the area where the cuff engages the object to be measured. This excess of material creates folds, bends, wrinkles, and voids at the cuff/object interface. Depending upon the nature of the measurement, these irregularities generally result in a source of measurement error.
Also, there is a need for a device that accurately measures both the volume and the change in volume of a body appendage as blood or other fluids (such as water) pulse in and out or as fluid accumulates to cause swelling. For a device in which measurements of additional physiological parameters are made using optical emitters and detectors, the device preferably also holds the emitters and detectors against the body appendage in a predictable manner, simultaneous with measuring the volume or the volume changes in the body appendage. In holding the components against the body appendage, the bladder must not allow any air gaps or incorrect alignment of the emitters or detectors. The bladder must also be able to conform to the many different profiles of body appendages, while still allowing for accurate volume and optical measurements.
The prior art includes many devices in which a bladder inflates against the finger. Examples of such prior art are disclosed in U.S. Pat. Nos. 4,202,347 and 4,331,155 to Sacks, U.S. Pat. No. 5,025,793 to Richley et al., and U.S. Pat. No. 5,218,966 to Yamasawa. In each case, as the bladder inflates against the finger, gaps are caused by the folding of the bladder material. In addition, the bladders do not conform in a predictable way to each of the different shapes of fingers. These variances cause any measurements made by the bladders to be unpredictable and prone to inaccuracies. Through experimentation, we have concluded that it is not possible to build a bladder that simply inflates from a static, un-inflated position to a variety of body appendage shapes and sizes without having gaps, folds, bends, or wrinkles. Thus, the prior art devices inherently introduce errors into the measurements.
It is to the solution of these and other problems that the present invention is directed.