Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-10-05
2003-10-14
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S500000, C600S480000, C600S503000
Reexamination Certificate
active
06632181
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for the non-invasive determination of blood pressure.
Direct measurement of blood pressure with a pressure measuring device such as a tonometer is difficult in a clinical setting. A problem with tonometer readings is that although the times of the systolic and diastolic pressures are correct, the pressure readings may have an incorrect scaling or have an offset in the recorded pressure. Tonometer measurements can depend on the position of the tonometer, artery and bone structure behind the artery.
Another prior art system used to determine arterial pressures is an automated oscillometric device called a “Dinamap” (device for indirect non-invasive mean arterial pressure). This device is described in a paper entitled “Arterial Pressure Monitoring: Automated Oscillometric Devices”; M. Ramsey III;
Journal of Clinical Monitoring
; Volume 7, No. 1; January 1991; pp. 56-67. This system uses a cuff to supply an external pressure to an artery. The cuff pressure is stepped in increments from a pressure believed to be above the systolic pressure to a pressure believed to be below the diastolic pressure. An arterial volumetric indication is monitored by the system. For example, a pressure transducer attached to the cuff will give some indication of the volume of the artery, since the pressure in the cuff will be greater when the artery volume is high. When the mean value of the arterial blood pressure is about the same as the external cuff pressure, the amplitude of the variations of the volumetric indication will be the greatest. In this way, an indication of the mean arterial pressure can be obtained. A disadvantage of this prior art system is the considerable time it takes to obtain the arterial pressure information. Many cardiac cycles are needed to obtain the data required to determine a blood pressure.
An alternative system is described in “Vibration Technique for Indirect Measurement of Diastolic Arterial Pressure in Human Fingers”; Shimazu, et al.;
Medical and Biological Engineering in computing
; March 1999; Volume 27; pp. 130-136. This paper describes a method for obtaining a diastolic pressure which is somewhat similar to the oscillometric technique used with the Dinamap. In the Shimazu, et al. system, a small oscillation is placed on the cuff pressure. A plethysmograph is used to get a volumetric indication of the volume of the artery. The output of the plethysmograph will show a high-frequency component imposed on a pulsatile component. The cuff pressure is ramped or stepped in a manner similar to the Dinamap system. In the cardiac cycle where the cuff pressure is roughly equal to the diastolic pressure, the amplitude of the high-frequency component of the volumetric indication will be greater in the diastolic period of that cycle than at the diastolic period of any other cycle. In this way, the diastolic pressure can be determined. Like the Dinamap system, the Shimazu, et al. system is relatively slow. Many cardiac cycles are required to determine a single blood pressure value.
Another prior art system is described in Penàz U.S. Pat. No. 4,869,261. Penàz describes a vascular unloading system. Vascular unloading systems attempt to cause the external applied pressure to be equal to the arterial blood pressure at all times. These systems use a plethysmograph and a feedback loop in order to adjust the external pressure so that it tracks the arterial pressure. A disadvantage of this system is that, when the external pressure tracks the arterial pressure, the mean applied pressure is relatively high. For this reason, the system described in Penàz may be uncomfortable or painful to use. Additionally, vascular unloading systems tend to produce a pressure signal that is off from the real arterial pressure by a DC offset.
The systems of Palti U.S. Pat. No. 4,660,544 and Sramek U.S. Pat. No. 4,343,314 use very fast ramped external pressures. A disadvantage of these systems is that the required very fast ramped pressures may be impractical to produce. In particular, it may be difficult to use a cuff to apply the external pressures because of the relatively long periods of time required to inflate or deflate a cuff. Additionally, the quick external pressure ramp could be uncomfortable.
Therefore, it is desired to have a method and apparatus for obtaining a blood pressure that can avoid long measurement times or high applied external pressures.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for quickly determining a blood pressure value at a specific time. An external pressure is supplied to the artery so that the artery experiences a range of transmural pressures. The external pressure is set so that a known event, or marker, will occur during a measurement period. The measurement period is a short period of time that typically can be a cardiac cycle or a few cardiac cycles. The value of the external pressure at the time of the event allows for the calculation of an arterial pressure associated with the time of the event or an earlier or later time in another cardiac cycle.
By allowing a range of transmural pressures within a cardiac cycle, the transmural pressure is not clamped at zero like in the apparatus of the Penàz patent. In one embodiment, the range in the transmural pressure is mainly due to arterial pressure variations.
Examples of events, or markers, that can be detected include a peak in the arterial compliance curve at the transmural pressure about equal to zero or an opening or closing of an artery. Other markers that can be used include a pressure dependant change in attenuation of a propagating pressure wave; frequency dependent effects (i.e. level of attenuation of a propagating wave versus frequency or a compliance versus frequency relationship); changes in a compliance versus attenuation relationship; a change in the viscoelastic properties of the artery or other portion of the body; or a change in the flow of blood through the artery.
Several of the preferred embodiments of the invention use, as a marker, the peak in the compliance of the artery. The peak is, by definition, the transmural pressure (or pressure across the artery wall) where the slope of the pressure-volume relationship of the artery is the steepest. The pressure-volume relationship relates the pressure across the artery wall to the volume in the lumen of the artery. The peak in the compliance indicates that, at that transmural pressure, a given change in transmural pressure will cause a larger change in the volume of the artery than at any other transmural pressure. This peak is thought to occur at a transmural pressure of zero because the artery wall is at the least amount of stress at this pressure.
It is possible that the peak in compliance does not occur at zero transmural pressure, but rather at some transmural pressure close to zero. These slight variations could be caused by a number of factors including: arterial size, presence of disease states, age, gender, etc. The system of the present invention can produce relatively accurate blood pressure values when it assumes the compliance peak occurs at the transmural pressure of zero. If significant variations in the transmural pressure of peak compliance are found to occur, this invention would still function adequately if an appropriate compensating factor were added to the measured pressure.
The time that an event (i.e. transmural pressure approximately zero) occurs is used to produce an indication of the arterial blood pressure at a specific time. For example, if the transmural pressure is equal to zero at time T
1
, the system knows that the arterial pressure is equal to the cuff pressure at time T
1
and/or at a different time with the same position in a subsequent or prior cardiac cycle as T
1
.
Within as little as single cardiac cycle, all of the data required to determine a blood pressure at a certain time can be obtained. This is much quicker than the many cycles required in the Dinamap and Shimazu systems.
The information of a b
Caro Richard G.
Flaherty Bryan Patrick
Sher Mark Henry
Masimo Corporation
Nasser Robert L.
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