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THE DESIGN OF MONITORING DEVICES FOR USE WITH A PUMP-OXYGENATOR
THE DESIGN OF M O N I T O R I N G DEVICES FOR USE W I T H A PUMP-OXYGENATOR Vernon
Montgomery,
M.D.,
Ph.D.,*
Bruce C. Paton, M.R.C.P.,
F.R.C.S.,
Jack Lucero, and Henry Swan, M.D., D.Sc, Denver, Colo.
T
HAS been much discussion, since the use of extracorporeal circulation became widespread, about the principles of perfusion that will give optimal results. In this hospital the problem has been approached, not on the basis of high or low flows, but by attempting to maintain the blood gas relationships within normal limits. It is felt that if sufficient oxygenated blood is provided by a pump-oxygenator to maintain a reasonable perfusion pressure, and if the quantity and rate of delivery of oxygenated blood results in a normal arterial-venous oxygen difference, then adequate perfusion has been attained. The actual amount of blood perfused in cubic centimeters per kilogram per minute varies from patient to patient, but as judged by the oxygen consumption of the patient and the adequacy with which this consumption is met, the perfusion is adequate. In order that perfusions may be conducted according to these principles, monitoring devices have been introduced into the extracorporeal circuit. It is necessary to monitor simultaneously and continuously arterial and venous oxygen tensions and arterial pH in the machine. In addition, in the patient, mean arterial and venous pressures, electroencephelogram, temperature, and electrocardiogram are also measured. The arterial oxygen tension and the pump speed determine the disc speed required. At a given pump speed, the greater the disc speed the greater the degree of oxygénation obtained. The pump speed is determined by the perfusion pressure, the venous oxygen tension, and the electroencephalogram. A small arterial-venous oxygen difference is taken to mean overperfusion and too large an arterial-venous oxygen difference indicates underperfusion. Control of carbon dioxide tension in arterial blood as estimated by p H is acomplished by altering the per cent carbon dioxide in the gas " r e s p i r e d " in the oxygenator. Two gas mixtures of oxygen and carbon dioxide containing 2 per cent and 4 per cent, respectively, are used. A total flow of about HERE
From the Halsted Laboratory of Experimental Surgery and the Department of Surgery, University of Colorado School of Medicine, Denver, Colo. Aided in part by a grant from U.S. Public Health Service and by a research contract (DA-49-007-MD-572) with the Surgeon General, United States Army. Received for publication April 10, 1959. •This work was done during the tenure of an Advanced Research Fellowship of the American Heart Association.
225
226
MONTGOMERY, ΡΛΤΟΝ, LUCERO, SWAN
J. Thoracic and Oirdiovas. Surg.
12 L. per minute is satisfactory and, by varying the proportions of each mixture, the percentage of carbon dioxide can be altered and, thus, the pH maintained within normal limits. The pH is measured by a Beckman Zeromatic meter which reads continuously and has a self-correcting device actuated every second. It is accurate to within plus or minus 0.05 pH and is thoroughly reliable and simple to operate. At the start of each perfusion, a sample of pump blood is taken off and its pH read on a Beckman Model G meter and the calibration of the Zeromatic meter is thereby checked. Connecting Plug
Solder Joint Adjustable Electrode Assembly
Silver W i r e Teflon Seal PlatinumWire Lock Nut
Electrode Chamber Mercury Filling Plug Sodium Chloride Solution Membrane
Collar
S c a l e d Cm.
=__
Pig. 1.—Membrane-covered
Beaded Platinum Wire Teflon Membrane polarogTaph.
The need for continuous monitoring of p 0 2 necessitated the design of special electrodes (Pig. 1). The electrodes used are modifications of the Clark 1 polarograph. The electrode unit is in two parts; an inner core containing the electrodes and an outer chamber enclosing the electrode assembly. The platinum electrode lies in the center of the core and is divided at its middle. The separated ends dip into a sealed pool of mercury and in this way electrical contact is maintained while the differences between the coefficients of expansion of Teflon and platinum are eliminated. The silver
Vol. 39, No. 2 February, 1960
MONITORING DEVICES FOR PUMP-OXYGENATOR
227
electrode lies along the outside of the core and its tip encircles the Teflon rod. A Teflon membrane, 0.002 inch thick and 0.5 inch in diameter, is held in place over the end of the electrode chamber by a removable collar. The use of Teflon rather than other plastics permits the electrode to be autoclaved. This is done with the electrode disassembled. To reassemble the unit, the membrane is first secured in position and the electrode chamber filled with normal saline solution. During filling, a small screw plug is removed from the side of the chamber through which air bubbles may escape while the electrode assembly is being screwed into position. When the tip of the platinum electrode is sufficiently advanced beyond the end of the chamber to make the membrane tense and slightly tented, the lock nut on the assembly is tightened. No further change can then occur in the position of the electrode tip. Blood Inlet
Tubing Connector Nut
Blood Inlet
Pig. 2.—Multi-electrode adaptor.
The polarographs are connected with circuit control units so calibrated as to register oxygen tensions directly. The circuit used in this unit is similar to that described by Severinghaus and Bradley, 2 but has been modified to deliver a potential difference of 0.7 volt across the electrodes. Under both experimental and clinical conditions, these polarographs have been found to react almost instantly to changes in oxygen tension, but are not so sensitive as to be difficult to control. They are not affected by changes in temperature or pressure in the blood, but slight changes occur in the readings with variations in blood flow rates. In order to introduce these electrodes into the circuit, special adaptors have been designed to house them (Pig. 2). At the venous end, the adaptor consists of a simple tube with a side hole into which the polarograph is screwed. This adaptor also has another attachment to which a short-circuiting tube from the arterial end of the circuit for use during the period of equilibration before perfusion may be attached. At the arterial end the adaptor is more complicated and is located between the outflow tube from the oxygenating
228
MONTGOMERY,
PATON,
LUCERO,
SWAN
J. Thoracic and Cardiovas. Surg.
cylinder and the arterial pump. It consists of a Y junction rejoining the blood flows from the two oxygenating cylinders and in the sides of which are screw plugs containing a polarograph, thermistor, and two p H meter electrodes. The dimensions of the adaptor are given in Table I. TABLE I j
Length External diameter Internal diameter
VENOUS
ADAPTOR
5 !%e in. 1 % 6 in. % in.
j
ARTERIAL ADAPTOR
3 % in. 1 in. % in.
SUMMARY
The principles of perfusion used at the University of Colorado and the consequent need for continuous monitoring devices have been described. A plea is made for attempting to achieve "physiologic" perfusion. Details are given of an autoclavable polarograph and multi-electrode adaptor for the continuous monitoring of extracorporeal perfusion. REFERENCES
1. Clark, L. E., Jr., Wolf, C. R., Granger, D., and Taylor, Z.: Continuous Recording of Blood Oxygen Tension by Polarography, J . Appl. Physiol. 6: 189, 1953. 2. Bradley, A. F., and Severinghaus, J . W. : Personal communication.
Montgomery,
M.D.,
Ph.D.,*
Bruce C. Paton, M.R.C.P.,
F.R.C.S.,
Jack Lucero, and Henry Swan, M.D., D.Sc, Denver, Colo.
T
HAS been much discussion, since the use of extracorporeal circulation became widespread, about the principles of perfusion that will give optimal results. In this hospital the problem has been approached, not on the basis of high or low flows, but by attempting to maintain the blood gas relationships within normal limits. It is felt that if sufficient oxygenated blood is provided by a pump-oxygenator to maintain a reasonable perfusion pressure, and if the quantity and rate of delivery of oxygenated blood results in a normal arterial-venous oxygen difference, then adequate perfusion has been attained. The actual amount of blood perfused in cubic centimeters per kilogram per minute varies from patient to patient, but as judged by the oxygen consumption of the patient and the adequacy with which this consumption is met, the perfusion is adequate. In order that perfusions may be conducted according to these principles, monitoring devices have been introduced into the extracorporeal circuit. It is necessary to monitor simultaneously and continuously arterial and venous oxygen tensions and arterial pH in the machine. In addition, in the patient, mean arterial and venous pressures, electroencephelogram, temperature, and electrocardiogram are also measured. The arterial oxygen tension and the pump speed determine the disc speed required. At a given pump speed, the greater the disc speed the greater the degree of oxygénation obtained. The pump speed is determined by the perfusion pressure, the venous oxygen tension, and the electroencephalogram. A small arterial-venous oxygen difference is taken to mean overperfusion and too large an arterial-venous oxygen difference indicates underperfusion. Control of carbon dioxide tension in arterial blood as estimated by p H is acomplished by altering the per cent carbon dioxide in the gas " r e s p i r e d " in the oxygenator. Two gas mixtures of oxygen and carbon dioxide containing 2 per cent and 4 per cent, respectively, are used. A total flow of about HERE
From the Halsted Laboratory of Experimental Surgery and the Department of Surgery, University of Colorado School of Medicine, Denver, Colo. Aided in part by a grant from U.S. Public Health Service and by a research contract (DA-49-007-MD-572) with the Surgeon General, United States Army. Received for publication April 10, 1959. •This work was done during the tenure of an Advanced Research Fellowship of the American Heart Association.
225
226
MONTGOMERY, ΡΛΤΟΝ, LUCERO, SWAN
J. Thoracic and Oirdiovas. Surg.
12 L. per minute is satisfactory and, by varying the proportions of each mixture, the percentage of carbon dioxide can be altered and, thus, the pH maintained within normal limits. The pH is measured by a Beckman Zeromatic meter which reads continuously and has a self-correcting device actuated every second. It is accurate to within plus or minus 0.05 pH and is thoroughly reliable and simple to operate. At the start of each perfusion, a sample of pump blood is taken off and its pH read on a Beckman Model G meter and the calibration of the Zeromatic meter is thereby checked. Connecting Plug
Solder Joint Adjustable Electrode Assembly
Silver W i r e Teflon Seal PlatinumWire Lock Nut
Electrode Chamber Mercury Filling Plug Sodium Chloride Solution Membrane
Collar
S c a l e d Cm.
=__
Pig. 1.—Membrane-covered
Beaded Platinum Wire Teflon Membrane polarogTaph.
The need for continuous monitoring of p 0 2 necessitated the design of special electrodes (Pig. 1). The electrodes used are modifications of the Clark 1 polarograph. The electrode unit is in two parts; an inner core containing the electrodes and an outer chamber enclosing the electrode assembly. The platinum electrode lies in the center of the core and is divided at its middle. The separated ends dip into a sealed pool of mercury and in this way electrical contact is maintained while the differences between the coefficients of expansion of Teflon and platinum are eliminated. The silver
Vol. 39, No. 2 February, 1960
MONITORING DEVICES FOR PUMP-OXYGENATOR
227
electrode lies along the outside of the core and its tip encircles the Teflon rod. A Teflon membrane, 0.002 inch thick and 0.5 inch in diameter, is held in place over the end of the electrode chamber by a removable collar. The use of Teflon rather than other plastics permits the electrode to be autoclaved. This is done with the electrode disassembled. To reassemble the unit, the membrane is first secured in position and the electrode chamber filled with normal saline solution. During filling, a small screw plug is removed from the side of the chamber through which air bubbles may escape while the electrode assembly is being screwed into position. When the tip of the platinum electrode is sufficiently advanced beyond the end of the chamber to make the membrane tense and slightly tented, the lock nut on the assembly is tightened. No further change can then occur in the position of the electrode tip. Blood Inlet
Tubing Connector Nut
Blood Inlet
Pig. 2.—Multi-electrode adaptor.
The polarographs are connected with circuit control units so calibrated as to register oxygen tensions directly. The circuit used in this unit is similar to that described by Severinghaus and Bradley, 2 but has been modified to deliver a potential difference of 0.7 volt across the electrodes. Under both experimental and clinical conditions, these polarographs have been found to react almost instantly to changes in oxygen tension, but are not so sensitive as to be difficult to control. They are not affected by changes in temperature or pressure in the blood, but slight changes occur in the readings with variations in blood flow rates. In order to introduce these electrodes into the circuit, special adaptors have been designed to house them (Pig. 2). At the venous end, the adaptor consists of a simple tube with a side hole into which the polarograph is screwed. This adaptor also has another attachment to which a short-circuiting tube from the arterial end of the circuit for use during the period of equilibration before perfusion may be attached. At the arterial end the adaptor is more complicated and is located between the outflow tube from the oxygenating
228
MONTGOMERY,
PATON,
LUCERO,
SWAN
J. Thoracic and Cardiovas. Surg.
cylinder and the arterial pump. It consists of a Y junction rejoining the blood flows from the two oxygenating cylinders and in the sides of which are screw plugs containing a polarograph, thermistor, and two p H meter electrodes. The dimensions of the adaptor are given in Table I. TABLE I j
Length External diameter Internal diameter
VENOUS
ADAPTOR
5 !%e in. 1 % 6 in. % in.
j
ARTERIAL ADAPTOR
3 % in. 1 in. % in.
SUMMARY
The principles of perfusion used at the University of Colorado and the consequent need for continuous monitoring devices have been described. A plea is made for attempting to achieve "physiologic" perfusion. Details are given of an autoclavable polarograph and multi-electrode adaptor for the continuous monitoring of extracorporeal perfusion. REFERENCES
1. Clark, L. E., Jr., Wolf, C. R., Granger, D., and Taylor, Z.: Continuous Recording of Blood Oxygen Tension by Polarography, J . Appl. Physiol. 6: 189, 1953. 2. Bradley, A. F., and Severinghaus, J . W. : Personal communication.