Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-06-15
2004-04-13
Pelham, Joseph (Department: 3742)
Surgery
Diagnostic testing
Cardiovascular
C600S495000, C600S496000
Reexamination Certificate
active
06719703
ABSTRACT:
TECHNICAL FIELD
The present invention relates to non-invasive methods for determining the blood pressure of a subject. More particularly, the invention relates to an improved method and apparatus for making oscillometric measurements of systolic blood pressure.
BACKGROUND
Physicians and others monitor various physiological parameters in their patients and in other subjects. Such monitoring is an important tool in evaluating patients' health. The monitoring of cardiovascular function is particularly valuable and is performed on a very widespread basis. Accurate measurement of blood pressure (“BP”) and other physiological parameters allows for more precise diagnosis of medical problems. For example, accurate measurement of BP is important in the correct diagnosis of hypertension.
There are various ways to measure BP. For example, BP may be measured directly in the aorta or in other arterial blood vessels. This may be done, for example, by inserting into an arterial blood vessel a probe, such as a needle or catheter which bears, or is attached to, a pressure transducer. The transducer measures the actual pressure of the blood within the blood vessel. Although it is ideal to have directly-measured BP values for diagnostic purposes, procedures for directly measuring BP are invasive and are normally restricted to critical care environments such as operating rooms.
A variety of indirect or non-invasive techniques for measuring BP have been developed and include tonometric, auscultatory, and oscillometric methods. The tonometric method typically involves a transducer which includes an array of pressure sensitive elements positioned over a superficial artery. “Hold down” forces are applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure measured by the pressure sensitive elements is dependent upon the hold down pressure used to press the transducer against the skin of the patient.
Tonometric systems measure a reference pressure directly from a superficial artery such as the radial artery at the wrist and correlate this reference pressure with the arterial pressure. However, because the ratio of pressure outside the artery to the pressure inside the artery, known as “gain”, must be known and constant, tonometric systems are not reliably accurate. Furthermore, if a patient moves, recalibration of the tonometric system is required because the system may experience a change in gain. Because the accuracy of tonometric systems depends upon the accurate positioning of a transducer over the underlying artery, placement of the transducer is critical. Furthermore, achieving proper placement of the transducer in tonometric systems is time-consuming and prone to error.
The auscultatory method involves inflating a cuff placed around a limb of the patient. Following inflation of the cuff, the cuff is permitted to deflate. Systolic blood pressure (“SBP”) is taken to be the cuff pressure at which Korotkoff sounds begin to occur as the cuff is deflated. Diastolic blood pressure (“DBP”) is taken to be the cuff pressure at which the Korotkoff sounds become muffled or disappear. The auscultatory method requires a judgment to be made as to when the Korotkoff sounds start and when they stop. This judgment is made when the Korotkoff sounds are at their very lowest. As a result, the auscultatory method is subject to inaccuracies due to low signal-to-noise ratio. Another recognized major disadvantage of the auscultatory method is that its accuracy degrades severely with hypotension and obesity. It is also unreliable in infants and children.
The oscillometric method also involves the inflation of a cuff placed around a limb of the patient. In this method, the cuff is deflated slowly and the pressure within the cuff is continuously monitored. The oscillometric method measures and records the amplitude of pressure oscillations in the cuff caused by blood pressure pulses in underlying arteries. As the cuff is deflated, the pressure within the cuff exhibits a certain pressure versus time waveform (FIG.
1
A). The waveform can be separated into two components, a decaying component (the applied cuff pressure, C—
FIG. 1C
) and an oscillating component (the pressure pulse amplitudes, A—FIG.
1
B).
The oscillating component may be represented by a curve known by those in the art as the “oscillometric envelope” as shown in dotted line in FIG.
1
B. The oscillometric envelope starts at a low value when the cuff is inflated to a level beyond the patient's SBP and then increases to a peak value (A
max
) as the cuff pressure is reduced. Once the envelope has reached A
max
, the envelope then decays as the cuff pressure continues to decrease. At A
max
the mean pressure in the cuff is roughly equal to the patient's mean arterial blood pressure (“MAP”).
SBP, MAP and DBP values can be determined from the data obtained by monitoring the pressure within the cuff while the cuff is slowly deflated. Again, the mean arterial blood pressure value, MAP, can be estimated as the applied cuff pressure at the point in time when the peak, A
max
, of the oscillometric envelope occurs (FIG.
1
C). SBP may be determined as the cuff pressure corresponding to the amplitude on the systolic side (before peak amplitude A
max
) of the oscillometric envelope which is equal to a certain percentage of the peak amplitude A
max
. This percentage is known by those skilled in the art as the systolic Parameter Identification Point (“PIP”), and is generally considered to be about 55%. Similarly, DBP may be determined as the cuff pressure corresponding to the amplitude on the diastolic side (after peak amplitude A
max
) of the oscillometric envelope which is equal to a certain percentage of the peak amplitude A
max
. This percentage is known as the diastolic PIP, which is generally considered to be close to 72%.
So, the oscillometric method uses fixed PIP's to calculate SBP and DBP values from A
max
. Automated BP monitors using the oscillometric technique use these fixed PIP's in their algorithms to calculate these BP figures. It has been known for some time, however, that the oscillometric method has the disadvantage, using these fixed PIP's, of inaccuracy under the most important of circumstances, i.e., when measuring blood pressure of hypertensive patients. Specifically, using fixed PIP's, and especially a fixed systolic PIP, tends to cause most BP measuring devices to underestimate BP at higher pressures.
Baker et al. address the problem of using fixed PIP ratios in U.S. Pat. No. 5,339,819, “Method for Determining Blood Pressure Utilizing a Neural Network”, and also in WO 92/03966, “Method and apparatus for determining blood pressure”. Their solution to the problem is to train a neural network to recognize or map the relationship between sets of oscillometric envelope input data and the desired directly-measured blood pressure. The neural network is trained to analyse many data points on a single oscillometric envelope and thus has the advantage of not being entirely dependent upon a small number of parameters such as MAP and PIPs. However, neural networks have a disadvantage of being complex to design and train, and also costly to implement. A further disadvantage is that neural network performance is generally limited by the amount of training and the type of training data; neural networks may not perform well with new input data which they have not seen before.
Accordingly, an improved, more accurate method of blood pressure measurement which has the advantages of the oscillometric technique, but which does not underestimate higher pressures, is desirable.
SUMMARY OF INVENTION
The present invention provides an improved method and apparatus for measuring blood pressure in a subject, and in particular, systolic blood pressure.
A method according to a basic embodiment of the invention comprises the steps of: obtaining an estimate of mean arterial pressure (“MAP”) by measuring the cuff pressure at the peak of the oscillometric envelope using an oscillometri
Chen Yunquan
Strange Kevin Daryl
Zorn Anton Bogdan
Oyen Wiggs Green & Mutala
Pelham Joseph
VSM Medtech Ltd.
LandOfFree
Method and apparatus for measuring blood pressure by the... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for measuring blood pressure by the..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for measuring blood pressure by the... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3229102