Direct blood pressure monitoring.
Blood pressure (Measurement)
Blood pressure (Methods)
|Publication:||Name: Anaesthesia and Intensive Care Publisher: Australian Society of Anaesthetists Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2009 Australian Society of Anaesthetists ISSN: 0310-057X|
|Issue:||Date: July, 2009 Source Volume: 37 Source Issue: 4|
|Product:||Product Code: 3841281 Blood Pressure Monitors; 3841287 Blood Pressure Transducers NAICS Code: 334510 Electromedical and Electrotherapeutic Apparatus Manufacturing SIC Code: 3841 Surgical and medical instruments|
In 1876, Herbert Tomlinson strung 14 ft lengths of piano wire across a room and secured them with large hooks to blocks of wood (1). He connected the wires with a small piece of copper wire at one end and some wires in a Wheatsone-bridge arrangement at the other. Using a system of weights he stretched the wires and measured the resistance to the passage of an electric current. He reached several conclusions, the important one being that the increase in the resistance of the wire was proportional to the stretching force. Tomlinson had created the first strain gauge, albeit on a rather impractical scale. Given that electricity was still in its infancy, it is not surprising that it was many years before the strain gauge found commercial application.
Edward Simmons of Caltech invented the bonded strain gauge in 1936 (2). He presented it as part of his experimental work but it gained little attention. Arthur Ruge from Massachusetts Institute of Technology independently developed a bonded strain gauge and made it a commercial reality, establishing a business and taking the first order for 50,000 units in 1941. He approached the Massachusetts Institute of Technology Patent Committee and received the following response "... the Committee does not feel that the commercial use is likely to be of major importance" (3). Ultimately, it was Simmons who was awarded the patent in 1942.
Louis Statham, a physicist at the Curtiss-Wright Airplane Company, subsequently developed an unbonded strain gauge and pioneered a transducer manufacturing company using this technology. He designed a strain gauge for Edward Lambert and Earl Wood, physiologists at the Mayo Clinic. They were the first to adapt this technology to allow direct continuous blood pressure measurement. They were studying the effects of G forces on pilots in an experimental human centrifuge--work that required robust and accurate equipment. They recorded the signal using a Heiland galvanometer which was in common use by geophysicists and required no amplification; subsequently transistors were developed and a wide range of recorders and amplifiers became available to make this technology more accessible for other applications (4).
The transfer of this technology to medicine was originally made by cardiologists who adapted the techniques to physiological studies of the heart. Nobel Prize winning investigators such as Andre Cournand (5) and Werner Forssmann (6) developed cardiac catheterisation and used transducers to accurately measure pressures throughout the circulatory system. Early arterial puncture needles were called Cournand needles and based on those developed for use by cardiologists. Anaesthetists were quick to see the application to monitoring, but the technology was not immediately accessible and affordable. "Continuous and reliable blood pressure monitors are seldom available and although the technique of arterial puncture is relatively simple, the associated pressure transducers and amplifiers are costly." (7)
A plethora of devices were suggested for direct measurement of blood pressure without the use of expensive electronics. All essentially involved inserting a membrane at some point between the arterial cannula and the measuring device. The measuring device could be as simple as a column of fluid filled with dye and attached to a graduated scale or an anaeroid manometer attached to a syringe barrel with a rubber stopper. These systems were limited and often allowed only an approximation of mean arterial blood pressure, but they appear to have been widely used in the early years of arterial pressure monitoring (7-9).
Cannulation was not simple either; there were some specially designed cannulas but a variety of needles and techniques were used: "Arterial cannulation may be made with an 18-or 20-gauge Cournand needle or with Teflon catheters inserted after cutdown" (9). Even in the cardiac theatres, direct arterial cannulation was not common and anaesthetists usually monitored the blood pressure by attaching transducers to the surgeon's femoral catheters (R. Rippert, personal communication).
By the 1980s, direct blood pressure monitoring had a welldefined place in anaesthesia, paralleling the growth of the technology in intensive care units. Multimodal monitors were present in general theatres, catheters for direct arterial puncture were available and increasingly complex patients and operations demanded more intensive and invasive monitoring. Discussion in the literature shifted to analysis of monitoring systems and debates about the accuracy of equipment.
A catheter-transducer system has several important parameters (10). Firstly elasticity, which is dependent on the flexibility of the diaphragm, the presence of air bubbles and the compliance of the tubing or other elastic elements in the system. Secondly mass, principally the fluid mass in the catheter system and thirdly, friction due to the fluid in the system moving with the pulsations. The developments of the last 30 years have focussed on minimising the inherent inaccuracies in transducer systems due to these three parameters. The early dome transducers were separate components which had to be carefully primed with fluid to eliminate bubbles before connecting them to the tubing. Modern transducers are small, disposable, easier to prime and often integrated into the required tubing.
Today, with increasingly complex procedures, invasive pressure monitoring is common practice. A 1993 analysis of blood pressure monitoring in 2000 incident reports concluded "Invasive arterial monitoring is more likely to be an early detector of blood pressure change than non-invasive monitoring, and it is recommended that anaesthetists maintain a low threshold for deciding to use an arterial line in high risk cases" (11).
(1) Tomlinson H. On the increase in resistance to the passage of an electric current produced in wires by stretching. Proc R Soc Lond 1876; 25:451-453.
(2) Walter PL. The History of the Accelerometer: 1920s-1996--Prologue and Epilogue 2006. Sound and Vibration 2007.
(3) Water PL. Metal Strain Gages Remain Key Despite Initial Rejection. Sound and Vibration 2009; p. 4
(4) Comroe J. Retrospectroscope. Hydrogen, balloons and pressures. American Review of Respiratory Disease 1976; 113:73-76.
(5) Cournand AF. Nobel Lecture 1956. Control of the Pulmonary Circulation in Man with Some Remarks on Methodology. Nobel Lectures, Physiology or Medicine 1942-1962. Elsevier Publishing Company, Amsterdam 1964.
(6) Forssmann W. Nobel Lecture 1956. The Role of Heart Catheterization and Angiocardiography in the Development of Modern Medicine. Nobel Lecture, Physiology or Medicine 1942-1962. Elsevier Publishing Company, Amsterdam 1964.
(7) Blackburn JP. A disposable monitor for arterial blood pressure. Anaesthesia 1966; 21:109-110.
(8) Fink BR. Pneumatic monitor for arterial blood pressure. Anesthesiology 1963; 24:872-876.
(9) Cooperman LH, Mann P. A simple method for direct arterial pressure measurement. Anesthesiology 1957; 17:93-94.
(10) Gardner RM. Direct blood pressure measurement--dynamic response requirements. Anesthesiology 1981; 54:227-236.
(11) Cockings JGL, Webb RK, Klepper ID, Currie M, Morgan C. Blood pressure monitoring: applications and limitations: an analysis of 2000 incident reports: the Australian incident monitoring study. Anaesth Intensive Care 1993; 21:565-569.
C. BALL, R. N. WESTHORPE Geoffrey Kaye Museum of Anaesthetic History
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