An analysis of the method of measuring arterial blood pressure

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An analysis of the method of measuring arterial blood pressure

Description July 2, Pressure w. Tolles, Oyster Bay, N. The invention enables blood pressure to be measured continuously, and the apparatus is capable of automatic operation. In many fields of medicine there is a need for apparatus capable of continuously measuring arterial blood pressure in a simple and convenient manner.

The need is particularly keen in hospital operating rooms and the like, and in many experimental fields of medicine, for example in space medicine. In the latter field particularly, there is great need for continuous blood pressure measuring apparatus which will allow moderate activity by the subject and will not produce any general disco-Infort.

In conventional sphygmomanometry, the subject must remain relatively quiet for many seconds and withstand periodic circulatory occlusion.

Furthermore, the measurement gives only two pressure values i. Automatic methods of measuring blood pressure are known, based on the sphygmomanometer and digital plethysmograph principles. In addition to the limited information given, these methods are unsatisfactory for monitoring active subjects and the periodic occlusion of the blood circulation causes discomfort.

The weight and bulk of the required auxiliary air supply, and equip ment to calibrate for environmental pressure, are also disadvantages. Continuous measurement of blood pressure is possible by arterial cannulation, but serious drawbacks are present. In addition to the trauma of initial arterial puncture, movement of an active subject may cause secondary traumata and there is danger that the cannula may be dislodged from the artery.

Accordingly, it is an object of the present invention to provide a method and apparatus for continuously measuring arterial blood pressure without interfering with the blood circulation or unduly impairing the physical activity and general comfort of the subject.

It is a further object of this invention to provide apparatus which continuously and automatically records arterial blood pressure without the need for constant supervision by an attending physician.

An analysis of the method of measuring arterial blood pressure

The invention is based on the fact that the velocity of propagation of pressure waves in the blood stream varies with changes in arterial blood pressure.

Although the relationship is not linear, it is a single-valued function and hence changes in velocity of propagation can be used to indicate changes in blood pressure. As is well known, the beating of the heart causes heart pulse pressure waves to travel down the arteries.

Since the arterial walls are elastic, expansion and contraction of the walls take place as the pressure changes during a heart beat cycle. This results in transverse waves which propagate down the arterial blood stream with a velocity of propagation which varies with the blood pres- sure and the elastic condition of the arterial walls.

Commonly, velocities of propagation of tens of feet per second are encountered in humans, as contrasted with velocities of propagation of longitudinal compression waves of thousands of feet per second common in liquids. Inasmuch as the elastic condition of the arterial walls ice of an individual may be expected not.

Direct measurement of the time of travel of heart pulse waves between two spaced points along an artery has been proposed, but is subject to considerable inaccuracies. The heart pulse wave is quite complex, and it is diflicult to select precisely the same point on the Wave as it passes two spaced points so as to get an accurate measurement.

Also, the outputs of transducers used to pick up the heart pulse wave may be expected to vary in amplitude due to changes in the intimacy of contact with the artery over which they are placed, slight displacements thereof, etc. This further impairs accuracy of measurement. Proposals have also been made to apply single impact pulses, or a train of impact pulses, to the artery and measure the time of travel thereof between spaced points along the artery.

However, considerable distortion of the pulses occurs as they pass along the artery, so that measuring diificulties arise similar to those present in measuring the heart pulse waves.

Also, changes in the intimacy of contact and location of both the transmitting and receiving transducers cause amplitude variations, thereby impairing measurement accuracy.

In accordance with the present invention, alternating, substantially symmetrical continuous-wave pressure variations are impressed on the arterial blood stream at a frequency substantially higher than the frequency of hear-t contraction, and variations in the time of travel or phase of the continuous-wave pressure varia ions between points spaced along the arterial blood stream are measured.

Due to the changing propagation characteristics of the artery as a function of the blood pressure therein, the continuous-wave pressure variations become phase-modulated or frequency-modulated, since frequency is the time derivative of phase as a function of the blood pressure.

In effect, the applied continuous-wave pressure variation is a carrier which is phase-modulated by the blood pressure variations.

Accordingly a signal picked up by a suitable transducer mounted over the artery may then be amplitude-limited so as to eliminate errors due to varying amplitude, While at the same time preserving the necessary information as to blood pressure variations.

The phase-modulation may then be detected to obtain a signal varying with the blood pressure over a heart beat cycle.

Advantageously, changes in phase of the applied pressure wave between two spaced points along the arterial blood stream are measured. With a fixed-frequency applied pressure variation, changes in phase between two fixed points are inversely proportional to changes in velocity of propagation, and hence give an indication of changes in blood pressure.The standard location for blood pressure measurement is the brachial artery.

Monitors that measure pressure at the wrist and fingers have become popular, but it is important to realize that systolic and diastolic pressures vary substantially in different parts of the arterial tree with systolic.

Physical principles of intra-arterial blood pressure measurement, 08/06/ Page 5 of 8 Most commercially available systems have a natural frequency of around Hz but this is reduced by the addition of three-way taps, bubbles, clots and additional lengths of tubing.

May 17,  · Conclusion When blood pressure is measured in the first or second trimester of pregnancy, the mean arterial pressure is a better predictor for pre-eclampsia than systolic blood pressure, diastolic blood pressure, or an increase of blood pressure.

AcqKnowledge extracts various cycle-by-cycle measures from arterial blood pressure (ABP) and ECG signals. Use an NIBPD Noninvasive Blood Pressure system, or other, continuous blood pressure amplifier to record blood pressure, or use a catheter to record direct arterial blood pressure.

History of Arterial Pressure Waveform Analysis. accurate measurement of cardiac output in Unfortunately, measuring the cardiac output proved to be not as easy as measuring arterial blood pressure.

An analysis of the method of measuring arterial blood pressure

The advantage of this consecutive change analysis method is that it can be used to quantify the concordance (another way of looking at. Unfortunately, measuring the cardiac output proved to be not as easy as measuring arterial blood pressure.

The Fick method proved to be cumbersome and not very practical for most clinical situations.

Pressure Waveform Analysis | LiDCO – Hemodynamic Monitoring for the entire patient pathway