BRIAN C. MORTON,* MD, CM, FRCPjIC] Hemodynamic monitoring in the critically ill patient requires the use of sophisticated electronic devices. To use this equipment one should have a general understanding of the principles involved and the requirements of a reliable system. This communication serves to explain the requirements of the various components of a hemodynamic monitoring system and to demonstrate how they interact to produce accurate and safe electronic signals from mechanical wave forms obtained from the patient. La surveillance hemodynamique du patient gravement malade requiert l'emploi d'appareils electroniques perfectionnes. Pour utiliser cet equipement on devrait avoir une connaissance g6n6rale des principes impliqu6s et des exigences d'un systeme fiable. Cette communication est destinee a expliquer les exigences des diverses composantes d'un systeme de surveillance h6modynamlque et A d6montrer comment elles agissent l'un sur l'autre afin de produire des signaux pr6cis et sOrs A partir d'ondes m6caniques obtenues du patient.
One aspect of modern medicine for which many are not well prepared is encountering the panoply of intricate electronic equipment that surrounds an ill patient. However, doctors and nurses working in intensive care settings need to understand how this equipment functions (or malfunctions) since the patient's care and safety may be at *Assistant professor of medicine, University of Ottawa Reprint requests to: Dr. Brian C. Morton, University of Ottawa cardiac unit, Qttawa Civic Hospital, 1053 Caning Ave., Ottawa, Ont. K1Y 4E9
stake. To this end, this paper reviews the devices used in hemodynamic pressure monitoring. The goal of a pressure monitoring system is to sample the wave form inside a blood vessel or heart chamber and to display it externally, where it may be used to guide treatment. The simplest and earliest system was devised by Steven Hales, an English scientist and clergyman, who in 1733 first directly measured the arterial pressure of a horse. He inserted a brass rod inside the carotid artery of a mare and connected it through the flexible trachea of a goose to a glass
manometer rod that measured the height to which the blood rose. Though there has been considerable improvement in equipment design, the concept of today's system (Fig. 1) is not dissimilar. Instead of a brass rod a flexible, semirigid plastic catheter is now used; rather than a goose trachea the catheter is attached to more rigid manometric tubing. Reverend Hales' glass manometer would yield only a mean arterial pressure; today's systems require instantaneous responses to pressure changes throughout the cardiac cycle. A variety of methods of recording and displaying these phasic changes are available. For the most part, electromechanical transducers are used that convert the mechanical energy of a pressure wave into an electrical signal. This small signal is amplified and displayed as a series of waves on a cathode ray tube (by bombardment of the phosphor-coated tube with electrons) and also on a pressure meter in direct numeric terms. If only a mean pressure is required, as in conventional central venous pressure (CPV) monitoring, then a manometer tube is adequate.
- IV fluid
Pressure monitor
Connecting tube Three-way. stopcock transducer
2811 _____
mit Transducer cable between catheter, pressure transducer and amconnections system; pressure monitoring FIG. 1-Hemodynamic plifier shown. (From Schroeder and Daily' with permission.) CMA JOURNAL/OCTOBER 6, 1979/VOL. 121 879
Mean
Original
E 0 1st harmonic
.z.;..t
-.
.
2fldharrnon,c 3rd harmonic h harmonic
. 5th harmonic th
6 harmonic Mean E
0
Synthetic
to marked distortion of the output wave form. What is required is a frequency response high enough to give all the desired pressure components without distortion due to resonance. This critical degree of damping is illustrated in Fig. 3, where the percentage of amplitude response is plotted against the percentage of natural resonant frequency. The curve labelled 0.7 demonstrates a roughly linear amplification over a wide range. The other curves show an excess of amplitude response at high levels of natural resonant frequency giving rise to either excessive damping (curve labelled 1.0) or excessive oscillation (curve labelled 0.2). Occasionally, even in the intensive care unit, the tracing may be underdamped; a narrow bore tubing is then inserted between the catheter and the stopcock to reduce overshoot due to resonance. For details on how to gauge critical damping the reader can consult Mendel's text on cardiac catheterization.3 Overdamping, the more common problem, needs to be continually combated by ensuring water-tight connections in the lines, assiduous removal of air from the lines and transducers, and frequent rapid
flushing to prevent blood coagulation in the catheter. The more frequent the use of the monitoring line for sampling blood, the more difficulty that can be expected with overdamping because of presence of air or blood in the lines. Components of a pressure recording system
The components of a pressure recording system are depicted in Fig. 1. From the patient's blood vessel they are, in order, the intravascular catheter, the three-way stopcock, the connecting tubing, the pressure transducer and intraflow device, the transducer cable and the oscilloscope in the monitor. Knowing now what is expected from this overall system, we can examine the individual components in more detail. Catheters used for pressure measurement range from short, fairly rigid tubes in peripheral arteries to long, flexible tubes for flow-directed placement in the pulmonary circulation. The catheter's construction is a compromise between the flexibility needed to negotiate vessels safely and the rigidity required to transmit pressure waves faith-
fully. As mentioned before, the tendency to damping or limitation of the frequency response resides largely in the catheter and the manometric tubing that couples the catheter to the transducer. As damping varies directly with the square root of the catheter length and inversely with the cube of the catheter radius, the catheter radius is far more important. Therefore, larger-bore catheters produce less damping of transmitted pressure wave forms. Similar principles apply to the manometric tubing. Fig. 4 outlines the components of the strain-gauge pressure transducer. The strain-gauge transducer is not as formidable as it may first seem. The catheter is connected via a stopcock through a port on the fluid-filled dome. The other port on the dome is opened or closed to atmospheric pressure via a second stopcock. Pressure waves are transmitted through the fluid-filled catheter to the fluid-filled dome. The dome communicates directly with the diaphragm of the pressure transducer. The transmitted pressure waves move the diaphragm up and down. The diaphragm is connected to four wires arranged so that upward or downward movement of the diaphragm will stretch
300
CATHETER -TO IV 0I IA C
0
200
S
'DOME
-DIAPHRPOM
E0 0 6l 0'
0
c4,
100
U 4,
a.
0
-fliF'" WHEATSTONE SRSGE
two wires and compress the other two, altering the electrical resistance. The four wires are connected to form a Wheatstone bridge; when their relative resistance is changed a small current is induced to flow through one arm of the bridge or the other. The current generated is proportional to the extent to which the diaphragm is moved. In this way the mechanical energy of a pressure pulse is transformed into an electrical signal. Finally, the display portion of the system should be considered. Usually the display is both graphic (the wave form) and numeric. The former is mandatory both to ensure high fidelity of the tracing (that it is neither overdamped nor underdamped) and for inspection of the contour of the pressure wave form, which is often diagnostically useful. The phosphor trace of the original "bouncing-ball" oscilloscopic signal is relatively short-lived. The design of recent oscilloscopes permits comparison of one beat with the next by electronic renewal of the signals so that a sliding panel of wave forms is viewed. A modification of this is retention of the wave form on half of the screen while one is viewing current data on the other half. At the same time, some feature of the wave form (for example, systolic peak, diastolic trough or cyclic mean) is usually displayed on .a meter. In Fig. 5 are examples of the meter display and digital display preamplifiers that are available. The top panel shows a digital display of pressure; the selector switch to the left indicates that it is a mean pressure. The second and fourth panels show a meter display of systolic and diastolic pressures. The digital display of systolic and diastolic pressures is shown in the third panel. It is immaterial whether the meter or the digital display is chosen provided the pressure wave form (that is, the analogue signal) is displayed and the system can be calibrated.
display. The numbers displayed there, however, are no better than the care with which the information is collected. Before accepting these deceptively simple numbers at face value it is vital to ensure that the pressure tracing from which the numbers are generated is of high fidelity, the transducer is properly positioned and balanced and the system is properly calibrated. As important as the absolute numbers is their reproducibility over time. This can only be ensured when the
measurement technique is constant, thus permitting observation of pressure trends over time. Recent advances in transducers - the development of microtransducers and the introduction of intraflow devices that allow continuous flushing of the recording system - have simplified pressure monitoring. Before inserting the catheter one should appropriately prepare the transducer by filling the dome with fluid and taking care to evacuate bubbles, which may overdamp
lUAU
fiR
Met', 'flats
4RMAN
Prntt.on
O@AflOU RALAKE
.
*AIE
MEAN
0
CAL
SAL
-WI
I
M2101 (PDM) ThAU
MEIf
. . fl lEAN RAIANCI
ALARM
*
£ 'men."
0
*AeW
MEAN
(t
JAL
OutPut
M2102 (PAM) IDES
tRACt
MEAN
04*
Wi
SALANCR
I
SAL
_
CAL
*
0
-,
0
1
M2103 (0DM)
Preparation of a pressure measurement system In the hated and demanding
atmosphere that surrounds the critically ill patient, decision-making may sometimes be based on brief glances at a digital pressure
FIG. 5-Examples of digital and analogue pressure modules, with display of pressure data. The same analogue meter may have different ranges, depending on the sensitivity used.
882 CMA JOURNAL/OCTOBER 6, 1979/VOL. 121
the system. The slow continuous flushing provided by an intraflow device, which is attached beyond the transducer, allows microscopic amounts of fluid under high pressure to be forced through the line to ensure patency. When preparing a pressure recording system for use one must allow the pressure transducer to warm up for 15 or 20 minutes after filling the dome in order to reach a constant operating temperature. At this time the transducer and amplifier can be balanced and calibrated. All intravascular pressures are measured with reference to the atmospheric pressure at the middle of the right atrium, this being the arbitrary zero point. The transducer must now be positioned so that the port that is open to air will lie approximately at the zero level. All future measurements must be made with the transducer at the same level. When the transducer dome port is opened to air and the Wheatstone bridge is exactly balanced, the electrical and mechanical zeros will coincide. This is accomplished simply by pushing a button on the amplifier casing. To ensure linearity of future measurements a series of electrical signals designed to simulate certain mechanical pressures is introduced and the numeric response on the pressure display module is checked against this. In addition to this electrical calibra-
tion, independent mechanical cali- such as optical recorders or inkbration may be done with an jet writers with minimal inertia, are aneroid manometer. Since the sensi- used. If a great deal of biologic tivity of pressure transducers may information is to be accumulated, a vary, a different predetermined magnetic tape recorder and comcalibration factor is used for each. puter facilities may prove to be Fig. 6 illustrates the use of this desirable. system. When measurements are beThe same recording technique ing taken the catheter port of the can be used to display the curve transducer dome is opened and the generated when the cardiac output atmospheric port is closed (Fig. 6B). is determined by the indicator diluWhen the mechanical zero of the tion technique. With the use of ansystem is being rechecked the other form of preamplifier (a directcatheter port is closed and the at- current signal coupler), the temmospheric port is opened (Fig. 6A). perature change recognized by the Once the system is recalibrated the thermistor at the tip of a Swanstopcocks are returned to the posi- Ganz thermodilution cardiac output tion for pressure monitoring (Fig. catheter can be plotted as a function 6B). The two ports of the trans- of time and displayed as a temducer dome should not be closed perature-time curve. It is equally at the same time since only minimal important when using cardiac output expansion of the fluid in the dome computers to be able to see the due to mechanical or temperature curve generated by the temperature changes can damage the diaphragm. change. This verifies the digital disOnce the pressure measurement play of the cardiac output given system is set up, the digital data it by most computers by ensuring provides are usually recorded in the that the computer is integrating the patient's chart. Alternatively, one area under an appropriate curve. may wish to have a permanent Once this facility for measurerecord of the wave form. The sim- ment and recording of hemodyplest, most durable and least ex- namic data is ready, it is important pensive device is the hot-stylus or to appreciate that potential elecstrip-chart recorder. A galvanome- trical hazards exist and require certer drives the heated stylus across tain safety precautions. When more chemically treated, heat-sensitive than one electrical device is conpaper to make the recording. When nected the chances of a potentially particularly high frequencies or dangerous electrical current being simultaneous pressure channels are conducted through the patient's needed, more elaborate devices, heart increase. This is especially Catheter open to transducer
fluids Open to air
Closed to air
A
B For pressure measurement
On IV infusion FIG. 6-Arrangement of stopcocks on
atmospheric and
catheter
ports
of
pressure
transducer.
The
atmospheric
port is open to air when the system is recalibrated. (From Schroeder and Daily1 with permission.) CMA JOURNAL/OCTOBER 6, 1979/VOL. 121 883
true when these devices penetrate the electrical barrier provided by the skin and reside within the heart. Therefore, all electrical devices connected to a patient should be attached to a common ground with proper ground wires and threepoint plugs. Most newer devices isolate the patient input circuitry from the power supply, thus providing extra protection should the ground fail. Finally, careful inspection of electrical equipment and connections, and correction of deficiencies
are prudent measures to ensure patient safety. Though nurses and doctors cannot be expected to acquire or be burdened by the expertise of biomedical engineering, some understanding of the use of hemodynamic monitoring equipment may circumvent frustration and wasted time, and contribute to safe and improved management of the critically ill. A more detailed discussion of the issues raised may be found in the references.
I thank Mr. Brian Winchester for technical advice and Mrs. Lolita Sutarno and Mrs. Janis Lukey for secretarial help.
References 1. SCHROEDER JS, DAILY EK: Techniques
in Bedside Hemodynamic Monitoring, Mosby, St Louis, Mo, 1976 2. PIEMME TE: Pressure measurement: electrical pressure transducers. Prog Cardioi'asc Dis 5: 574, 1963 3. MENDEL D: A Practice of Cardiac Catheterization, 2nd ed, Blackwell Sci Publ, Oxford, 1974
Hemodynamic monitoring: catheter insertion techniques, complications and trouble-shooting RONALD S. BAIGRIE, MD;* CHRISTOPHER D. MORGAN, MDt
Hemodynamic monitoring is an important aspect of contemporary intensive care of the critically ill patient. The potential problems associated with invasive monitoring fall into two general categories: those related to technical pitfalls and those related to patient complications. An awareness of these problems combined with technical expertise and an understanding of cardiovascular physiology can minimize complications and make hemodynamic monitoring a safe and useful procedure. La surveillance hemodynamique est un aspect important des soins intensifs actuels des patients gravement malades. Les problemes potentiels associes aux techniques envahissantes de surveillance se classent en deux categories generales: ceux qui sont relies aux embflches techniques et ceux qui sont relies aux complications du patient. Une connaissance de ces problemes associee a une expertise technique et a une comprehension de Ia physiologie cardiovasculaire peut minimiser les complications et faire de Ia surveillance hemodynamique une procedure sOre et utile.
Invasive hemodynamic monitoring has become an important aspect of modern intensive care.1'2 The techniques involved are relatively safe and easy in experienced hands. However, there are potential problems, which can be classified as * Director, coronary care unit, Toronto General Hospital and assistant professor of medicine, University of Toronto tOntario Heart Foundation research fellow in cardiology, Toronto General Hospital Reprint requests to: Dr. Ronald S. Baigrie, Toronto General Hospital, 101 College St., CW 1-104, Toronto, Ont. M5G 1L7
technical sources of error and patient-related complications; it is to these two areas that this paper is addressed. Technical sources of error Equipment failure There is always the possibility of failure of the technical equipment required for hemodynamic monitoring. This unfortunate occurrence is not only a source of frustration for personnel but also a potential hazard to the patient. It is difficult to justify the insertion of indwelling catheters when the time, effort and expense are wasted because of ma-
chine breakdown. There are a number of ways to minimize equipment failure: (a) always check all electrical systems for malfunction immediately before inserting catheters; (b) have medical engineering personnel carry out frequent preventive maintenance servicing on the equipment; and (c) either keep back-up equipment on hand or ensure that the supplier can quickly service or replace malfunctioning parts. It is important to be sure that the commercial product that is initially purchased is reliable, well recommended and supported by a service contract and personnel who can react in a reasonable time. Newer monitoring systems have a modular design of amplifiers and preamplifiers that allows for quick replacement of nonfunctioning modules. Obviously monitoring equipment must be carefully treated and abusive handling avoided. This may be achieved by staff education programs and in-service courses given by the manufacturer's sales representatives at the time of installation and periodically thereafter. Much of the apparent machine malfunction that occurs is due to the attempts of untrained personnel to calibrate or ad-
CMA JOURNAL/OCTOBER 6, 1979/VOL. 121 885
One aspect of modern medicine for which many are not well prepared is encountering the panoply of intricate electronic equipment that surrounds an ill patient. However, doctors and nurses working in intensive care settings need to understand how this equipment functions (or malfunctions) since the patient's care and safety may be at *Assistant professor of medicine, University of Ottawa Reprint requests to: Dr. Brian C. Morton, University of Ottawa cardiac unit, Qttawa Civic Hospital, 1053 Caning Ave., Ottawa, Ont. K1Y 4E9
stake. To this end, this paper reviews the devices used in hemodynamic pressure monitoring. The goal of a pressure monitoring system is to sample the wave form inside a blood vessel or heart chamber and to display it externally, where it may be used to guide treatment. The simplest and earliest system was devised by Steven Hales, an English scientist and clergyman, who in 1733 first directly measured the arterial pressure of a horse. He inserted a brass rod inside the carotid artery of a mare and connected it through the flexible trachea of a goose to a glass
manometer rod that measured the height to which the blood rose. Though there has been considerable improvement in equipment design, the concept of today's system (Fig. 1) is not dissimilar. Instead of a brass rod a flexible, semirigid plastic catheter is now used; rather than a goose trachea the catheter is attached to more rigid manometric tubing. Reverend Hales' glass manometer would yield only a mean arterial pressure; today's systems require instantaneous responses to pressure changes throughout the cardiac cycle. A variety of methods of recording and displaying these phasic changes are available. For the most part, electromechanical transducers are used that convert the mechanical energy of a pressure wave into an electrical signal. This small signal is amplified and displayed as a series of waves on a cathode ray tube (by bombardment of the phosphor-coated tube with electrons) and also on a pressure meter in direct numeric terms. If only a mean pressure is required, as in conventional central venous pressure (CPV) monitoring, then a manometer tube is adequate.
- IV fluid
Pressure monitor
Connecting tube Three-way. stopcock transducer
2811 _____
mit Transducer cable between catheter, pressure transducer and amconnections system; pressure monitoring FIG. 1-Hemodynamic plifier shown. (From Schroeder and Daily' with permission.) CMA JOURNAL/OCTOBER 6, 1979/VOL. 121 879
Mean
Original
E 0 1st harmonic
.z.;..t
-.
.
2fldharrnon,c 3rd harmonic h harmonic
. 5th harmonic th
6 harmonic Mean E
0
Synthetic
to marked distortion of the output wave form. What is required is a frequency response high enough to give all the desired pressure components without distortion due to resonance. This critical degree of damping is illustrated in Fig. 3, where the percentage of amplitude response is plotted against the percentage of natural resonant frequency. The curve labelled 0.7 demonstrates a roughly linear amplification over a wide range. The other curves show an excess of amplitude response at high levels of natural resonant frequency giving rise to either excessive damping (curve labelled 1.0) or excessive oscillation (curve labelled 0.2). Occasionally, even in the intensive care unit, the tracing may be underdamped; a narrow bore tubing is then inserted between the catheter and the stopcock to reduce overshoot due to resonance. For details on how to gauge critical damping the reader can consult Mendel's text on cardiac catheterization.3 Overdamping, the more common problem, needs to be continually combated by ensuring water-tight connections in the lines, assiduous removal of air from the lines and transducers, and frequent rapid
flushing to prevent blood coagulation in the catheter. The more frequent the use of the monitoring line for sampling blood, the more difficulty that can be expected with overdamping because of presence of air or blood in the lines. Components of a pressure recording system
The components of a pressure recording system are depicted in Fig. 1. From the patient's blood vessel they are, in order, the intravascular catheter, the three-way stopcock, the connecting tubing, the pressure transducer and intraflow device, the transducer cable and the oscilloscope in the monitor. Knowing now what is expected from this overall system, we can examine the individual components in more detail. Catheters used for pressure measurement range from short, fairly rigid tubes in peripheral arteries to long, flexible tubes for flow-directed placement in the pulmonary circulation. The catheter's construction is a compromise between the flexibility needed to negotiate vessels safely and the rigidity required to transmit pressure waves faith-
fully. As mentioned before, the tendency to damping or limitation of the frequency response resides largely in the catheter and the manometric tubing that couples the catheter to the transducer. As damping varies directly with the square root of the catheter length and inversely with the cube of the catheter radius, the catheter radius is far more important. Therefore, larger-bore catheters produce less damping of transmitted pressure wave forms. Similar principles apply to the manometric tubing. Fig. 4 outlines the components of the strain-gauge pressure transducer. The strain-gauge transducer is not as formidable as it may first seem. The catheter is connected via a stopcock through a port on the fluid-filled dome. The other port on the dome is opened or closed to atmospheric pressure via a second stopcock. Pressure waves are transmitted through the fluid-filled catheter to the fluid-filled dome. The dome communicates directly with the diaphragm of the pressure transducer. The transmitted pressure waves move the diaphragm up and down. The diaphragm is connected to four wires arranged so that upward or downward movement of the diaphragm will stretch
300
CATHETER -TO IV 0I IA C
0
200
S
'DOME
-DIAPHRPOM
E0 0 6l 0'
0
c4,
100
U 4,
a.
0
-fliF'" WHEATSTONE SRSGE
two wires and compress the other two, altering the electrical resistance. The four wires are connected to form a Wheatstone bridge; when their relative resistance is changed a small current is induced to flow through one arm of the bridge or the other. The current generated is proportional to the extent to which the diaphragm is moved. In this way the mechanical energy of a pressure pulse is transformed into an electrical signal. Finally, the display portion of the system should be considered. Usually the display is both graphic (the wave form) and numeric. The former is mandatory both to ensure high fidelity of the tracing (that it is neither overdamped nor underdamped) and for inspection of the contour of the pressure wave form, which is often diagnostically useful. The phosphor trace of the original "bouncing-ball" oscilloscopic signal is relatively short-lived. The design of recent oscilloscopes permits comparison of one beat with the next by electronic renewal of the signals so that a sliding panel of wave forms is viewed. A modification of this is retention of the wave form on half of the screen while one is viewing current data on the other half. At the same time, some feature of the wave form (for example, systolic peak, diastolic trough or cyclic mean) is usually displayed on .a meter. In Fig. 5 are examples of the meter display and digital display preamplifiers that are available. The top panel shows a digital display of pressure; the selector switch to the left indicates that it is a mean pressure. The second and fourth panels show a meter display of systolic and diastolic pressures. The digital display of systolic and diastolic pressures is shown in the third panel. It is immaterial whether the meter or the digital display is chosen provided the pressure wave form (that is, the analogue signal) is displayed and the system can be calibrated.
display. The numbers displayed there, however, are no better than the care with which the information is collected. Before accepting these deceptively simple numbers at face value it is vital to ensure that the pressure tracing from which the numbers are generated is of high fidelity, the transducer is properly positioned and balanced and the system is properly calibrated. As important as the absolute numbers is their reproducibility over time. This can only be ensured when the
measurement technique is constant, thus permitting observation of pressure trends over time. Recent advances in transducers - the development of microtransducers and the introduction of intraflow devices that allow continuous flushing of the recording system - have simplified pressure monitoring. Before inserting the catheter one should appropriately prepare the transducer by filling the dome with fluid and taking care to evacuate bubbles, which may overdamp
lUAU
fiR
Met', 'flats
4RMAN
Prntt.on
O@AflOU RALAKE
.
*AIE
MEAN
0
CAL
SAL
-WI
I
M2101 (PDM) ThAU
MEIf
. . fl lEAN RAIANCI
ALARM
*
£ 'men."
0
*AeW
MEAN
(t
JAL
OutPut
M2102 (PAM) IDES
tRACt
MEAN
04*
Wi
SALANCR
I
SAL
_
CAL
*
0
-,
0
1
M2103 (0DM)
Preparation of a pressure measurement system In the hated and demanding
atmosphere that surrounds the critically ill patient, decision-making may sometimes be based on brief glances at a digital pressure
FIG. 5-Examples of digital and analogue pressure modules, with display of pressure data. The same analogue meter may have different ranges, depending on the sensitivity used.
882 CMA JOURNAL/OCTOBER 6, 1979/VOL. 121
the system. The slow continuous flushing provided by an intraflow device, which is attached beyond the transducer, allows microscopic amounts of fluid under high pressure to be forced through the line to ensure patency. When preparing a pressure recording system for use one must allow the pressure transducer to warm up for 15 or 20 minutes after filling the dome in order to reach a constant operating temperature. At this time the transducer and amplifier can be balanced and calibrated. All intravascular pressures are measured with reference to the atmospheric pressure at the middle of the right atrium, this being the arbitrary zero point. The transducer must now be positioned so that the port that is open to air will lie approximately at the zero level. All future measurements must be made with the transducer at the same level. When the transducer dome port is opened to air and the Wheatstone bridge is exactly balanced, the electrical and mechanical zeros will coincide. This is accomplished simply by pushing a button on the amplifier casing. To ensure linearity of future measurements a series of electrical signals designed to simulate certain mechanical pressures is introduced and the numeric response on the pressure display module is checked against this. In addition to this electrical calibra-
tion, independent mechanical cali- such as optical recorders or inkbration may be done with an jet writers with minimal inertia, are aneroid manometer. Since the sensi- used. If a great deal of biologic tivity of pressure transducers may information is to be accumulated, a vary, a different predetermined magnetic tape recorder and comcalibration factor is used for each. puter facilities may prove to be Fig. 6 illustrates the use of this desirable. system. When measurements are beThe same recording technique ing taken the catheter port of the can be used to display the curve transducer dome is opened and the generated when the cardiac output atmospheric port is closed (Fig. 6B). is determined by the indicator diluWhen the mechanical zero of the tion technique. With the use of ansystem is being rechecked the other form of preamplifier (a directcatheter port is closed and the at- current signal coupler), the temmospheric port is opened (Fig. 6A). perature change recognized by the Once the system is recalibrated the thermistor at the tip of a Swanstopcocks are returned to the posi- Ganz thermodilution cardiac output tion for pressure monitoring (Fig. catheter can be plotted as a function 6B). The two ports of the trans- of time and displayed as a temducer dome should not be closed perature-time curve. It is equally at the same time since only minimal important when using cardiac output expansion of the fluid in the dome computers to be able to see the due to mechanical or temperature curve generated by the temperature changes can damage the diaphragm. change. This verifies the digital disOnce the pressure measurement play of the cardiac output given system is set up, the digital data it by most computers by ensuring provides are usually recorded in the that the computer is integrating the patient's chart. Alternatively, one area under an appropriate curve. may wish to have a permanent Once this facility for measurerecord of the wave form. The sim- ment and recording of hemodyplest, most durable and least ex- namic data is ready, it is important pensive device is the hot-stylus or to appreciate that potential elecstrip-chart recorder. A galvanome- trical hazards exist and require certer drives the heated stylus across tain safety precautions. When more chemically treated, heat-sensitive than one electrical device is conpaper to make the recording. When nected the chances of a potentially particularly high frequencies or dangerous electrical current being simultaneous pressure channels are conducted through the patient's needed, more elaborate devices, heart increase. This is especially Catheter open to transducer
fluids Open to air
Closed to air
A
B For pressure measurement
On IV infusion FIG. 6-Arrangement of stopcocks on
atmospheric and
catheter
ports
of
pressure
transducer.
The
atmospheric
port is open to air when the system is recalibrated. (From Schroeder and Daily1 with permission.) CMA JOURNAL/OCTOBER 6, 1979/VOL. 121 883
true when these devices penetrate the electrical barrier provided by the skin and reside within the heart. Therefore, all electrical devices connected to a patient should be attached to a common ground with proper ground wires and threepoint plugs. Most newer devices isolate the patient input circuitry from the power supply, thus providing extra protection should the ground fail. Finally, careful inspection of electrical equipment and connections, and correction of deficiencies
are prudent measures to ensure patient safety. Though nurses and doctors cannot be expected to acquire or be burdened by the expertise of biomedical engineering, some understanding of the use of hemodynamic monitoring equipment may circumvent frustration and wasted time, and contribute to safe and improved management of the critically ill. A more detailed discussion of the issues raised may be found in the references.
I thank Mr. Brian Winchester for technical advice and Mrs. Lolita Sutarno and Mrs. Janis Lukey for secretarial help.
References 1. SCHROEDER JS, DAILY EK: Techniques
in Bedside Hemodynamic Monitoring, Mosby, St Louis, Mo, 1976 2. PIEMME TE: Pressure measurement: electrical pressure transducers. Prog Cardioi'asc Dis 5: 574, 1963 3. MENDEL D: A Practice of Cardiac Catheterization, 2nd ed, Blackwell Sci Publ, Oxford, 1974
Hemodynamic monitoring: catheter insertion techniques, complications and trouble-shooting RONALD S. BAIGRIE, MD;* CHRISTOPHER D. MORGAN, MDt
Hemodynamic monitoring is an important aspect of contemporary intensive care of the critically ill patient. The potential problems associated with invasive monitoring fall into two general categories: those related to technical pitfalls and those related to patient complications. An awareness of these problems combined with technical expertise and an understanding of cardiovascular physiology can minimize complications and make hemodynamic monitoring a safe and useful procedure. La surveillance hemodynamique est un aspect important des soins intensifs actuels des patients gravement malades. Les problemes potentiels associes aux techniques envahissantes de surveillance se classent en deux categories generales: ceux qui sont relies aux embflches techniques et ceux qui sont relies aux complications du patient. Une connaissance de ces problemes associee a une expertise technique et a une comprehension de Ia physiologie cardiovasculaire peut minimiser les complications et faire de Ia surveillance hemodynamique une procedure sOre et utile.
Invasive hemodynamic monitoring has become an important aspect of modern intensive care.1'2 The techniques involved are relatively safe and easy in experienced hands. However, there are potential problems, which can be classified as * Director, coronary care unit, Toronto General Hospital and assistant professor of medicine, University of Toronto tOntario Heart Foundation research fellow in cardiology, Toronto General Hospital Reprint requests to: Dr. Ronald S. Baigrie, Toronto General Hospital, 101 College St., CW 1-104, Toronto, Ont. M5G 1L7
technical sources of error and patient-related complications; it is to these two areas that this paper is addressed. Technical sources of error Equipment failure There is always the possibility of failure of the technical equipment required for hemodynamic monitoring. This unfortunate occurrence is not only a source of frustration for personnel but also a potential hazard to the patient. It is difficult to justify the insertion of indwelling catheters when the time, effort and expense are wasted because of ma-
chine breakdown. There are a number of ways to minimize equipment failure: (a) always check all electrical systems for malfunction immediately before inserting catheters; (b) have medical engineering personnel carry out frequent preventive maintenance servicing on the equipment; and (c) either keep back-up equipment on hand or ensure that the supplier can quickly service or replace malfunctioning parts. It is important to be sure that the commercial product that is initially purchased is reliable, well recommended and supported by a service contract and personnel who can react in a reasonable time. Newer monitoring systems have a modular design of amplifiers and preamplifiers that allows for quick replacement of nonfunctioning modules. Obviously monitoring equipment must be carefully treated and abusive handling avoided. This may be achieved by staff education programs and in-service courses given by the manufacturer's sales representatives at the time of installation and periodically thereafter. Much of the apparent machine malfunction that occurs is due to the attempts of untrained personnel to calibrate or ad-
CMA JOURNAL/OCTOBER 6, 1979/VOL. 121 885
