Phoenix Ambulatory Blood Pressure Monitor
Geddes Inventory of Blood Pressure Measurement Methods
The following is a summary of the techniques documented by Leslie Alexander Geddes in 1970, in his authoritative history of blood pressure techniques up to that date.
Geddes, L.A. The Direct and Indirect Measurement of Blood Pressure. Chicago: Year Book Medical Publishers, 1970.
Geddes's work forms the baseline of the methods considered by the Phoenix Project.
Direct measurement of blood pressure requires a sensor that is in direct contact with the blood. Most of the techniques documented by Geddes employ some sort of flexible membrane or diaphragm, and then detect the deflection of the membrane as blood pressure changed.
Measurements are of arterial pressure with cannula needle in an artery (usually radial, femoral, dorsalis pedis or brachial)
To be clinically useful, the manometer should be placeable at the tip of a catheter or in a cannula needle. This makes size a serious constraint; not all the methods listed here can fit that criteria.
Direct measurement is usually taken at the radial (wrist), femoral (inner thigh), dorsalis pedis (lower leg) or brachial (upper arm) artery.
photoelectric detection of the deflection of an elastic diaphragm
One configuration used two fiber optic bundles running the length of a catheter and a diaphragm at the catheter's tip. One fiber optic bundle transmitted light to the diaphragm and the other bundle carried the reflected light back to the photodetector. The light source and photodetector were at the opposite end of the catheter from the tip.
detects the deflection of an elastic diaphragm by measuring capacitance changes between the diaphragm and a fixed plate next to the diaphragm
The transducer is composed of a flexible plane, which is the diaphragm, and a fixed plane. The two planes would be made of conductive metal, such as nickel or silver.
A strain gauge is a wire the electrical resistance of which changes as it is stretched. In a manometer, strain gauges would be attached to a diaphragm.
A miniature four-winding differential transformer mounted inside the tip of catheter. Two windings constituted the primary coil to which a electrical oscillator was connected. The other two windings constituted the secondary coil to which the measuring apparatus was connected. An armature core ran through the coils and was attached to a rubber membrane at the tip of the catheter. Deflection of the member, and thus of the armature, caused a voltage to display.
A small triode vacuum tube, in which the anode is connected to a small protruding pin, which in turn was coupled to an elastic metal diaphragm. The pin deflects with the diaphragm. creating a voltage change between a cathode fixed within the tub and the pin-connected anode.
Geddes mentions two indirect methods of calibrating pulse detector placed over superficial arteries:
To date, the only methods of indirectly determining the pressure within an artery with reasonable accuracy employ pressurized occluding devices and therefore permit sampling, not continuous monitoring of systolic and diastolic blood pressure. [p. 128]
All the indirect methods documented by Geddes usually involve an occluding cuff. Such a cuff can appear as an arm-band; as an annular cuff on a finger; or as a headband, when applied to the a temporal artery.
With all of the occlusive methods, one of the most important factors that influences the accuracy attainable is the length of the arterial segment which is compressed. This, in turn, is determined by the width of the occluding cuff. Thus, there is an important relationship between the width of the occluding cuff and the size of the member to which it is applied. [p. 101]
The relationship between the size of the cuff and the member to which it is applied is especially important when the indirect method is applied to children and infants. [p. 103]
Uses the hands to examine a patient's radial (wrist), femoral (thigh), or carotid (neck) pulse.
Put three fingers upon the radial pulse. The finger nearest the hand prevents the recurrent pulse through the palmar arch, the middle finger feels the pulse and the finger nearest the heart compresses the artery until the middle finger can no longer feel the pulse. The tension is estimated by the greater or less amount of pressure required to extinguish the pulse. [p. 70]
The different sites cannot be used interchangeably. The minimum systolic pressure at which a pulse appears varies between the radial, femoral, or carotid arteries.
Palpation can also be combined with an artery-occluding cuff that substitutes for the finger nearest the heart. Furthermore, connecting the cuff to a manometer (making it a sphygmomanometer) enables measurement of systolic pressure when the pulse appears.
By placing the sphygmomanometer on the brachial (upper arm) artery, some practitioners can also perceive a transition identifying diastolic pressure by palpating the brachial artery distally from the cuff.
The method first constricted blood flow to the finger to the point of it becoming blanched. Using an occluding cuff attached to a manometer and placed proximally from the fingertip, the occluding pressure was then slowly lowered and the blanched finger was observed until color started to return. At this point, the occluding pressure was just below systolic pressure, and the pressure was recorded as systolic.
Oscillometric methods of blood pressure measure usually employ an occluding cuff placed over a blood vessel, with something like a manometer for reading the pressure within the cuff. Intra-arterial pulsation is transmitted to the occluding cuff, causing oscillations in the sphygmomanometer cuff pressure. Starting with the cuff inflated to the point of occluding any pulse and then gradually reducing the counterpressure, systolic pressure is signaled when the pulses (oscillations) appear and diastolic pressure is signaled when the oscillations reach their maximum amplitude.
At least that was the theory in the early twentieth century. Comparisons of measurements from the oscillometric, auscultatory, and palpatory methods indicate the following:[p. 93]
Despite the fact that the oscillatory criteria have not been validated satisfactorily (at least as of 1970) the method has been used to determine the pressure in arteries other than the brachial, such as in the temporal and opthalmic arteries. A photoelectric sensor has been used in conjunction with the oscillatory method to determine the pressure in the auricular artery, which is in the pinna -- the ear outside the head. The photoelectric devices works because the pulsatile arterial blood flow varies the optical density of the light path.
The auscultatory method (from the Latin word for "listening") uses a stethoscope and a sphygmomanometer. This comprises an inflatable cuff placed around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The mercury manometer, considered the gold standard, measures the height of a column of mercury, giving an absolute result without need for calibration and, consequently, not subject to the errors and drift of calibration which affect other methods. The use of mercury manometers is often required in clinical trials and for the clinical measurement of hypertension in high-risk patients, such as pregnant women.
A cuff of appropriate size is fitted smoothly and snugly, then inflated manually by repeatedly squeezing a rubber bulb until the artery is completely occluded. Listening with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood just starts to flow in the artery, the turbulent flow creates a "whooshing" or pounding (first Korotkoff sound). The pressure at which this sound is first heard is the systolic blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound), at the diastolic arterial pressure.
The auscultatory method is the predominant method of clinical measurement. [Wikipedia, "Blood Pressure"]
The auscultatory method could be applied at any of multiple sites.
This method uses ultrasound to measure vessel-wall movement during deflation of a pneumatic cuff. Two small flat ultrasound (8 MHz) transmitting and receiving crystals are mounted to a piece of Velcro cloth, placed on the brachial artery, and held in place with a pneumatic cuff. During cuff deflation, when cuff pressure is just below systolic pressure, the vessel opens and then closes when the arterial pulse falls below cuff pressure. Two closely spaced wall moved signals can be obtained, one for the opening, the other for the closing of the vessel. As pressure is further reduced, the time between the opening and closing signals increases. As cuff pressure approaches the diastolic pressure, the closing and the next opening signal merge and then disappear because the vessel is open throughout the pulse.
Cuff pressure is measured. The Doppler signals indicate the attainment of first systolic and the diastolic pressure.
Body movements affect measurement accuracy by changing the distance between the ultrasound transducers and the underlying vessel.
This page is maintained by Christopher J. Adams.
It was last updated 30 December 2013.Copyright (c) 2013 Christopher J. Adams
This work is licensed under a Creative Commons Attribution 3.0 Unported License
