The Pulmonary Artery Catheter: Accuracy and Safety

James G Ramsay MD, FRCPC

Department of Anesthesiology
Emory University School of Medicine, Atlanta GA


Introduction

The &accuracy" of the pulmonary artery catheter (PAC) in this summary will be defined as the true representation of pressures and waveforms in the heart and pulmonary artery, and the correct interpretation of their meaning. Cardiac output is discussed in a separate summary. Accuracy of the PAC is a function of proper set-up (de-airing of all lumens; correct levelling, zeroing, and calibration procedures for the transducer), appropriate waveform identification, and physiologically correct waveform interpretation.


Set up

In setting up a PA catheter, the most likely source of introduced error is incorrect levelling of the transducer (the air-fluid interface) to the left atrium. Damping of the waveform due to the presence of air bubbles is visually detected; zeroing and calibration are usually accomplished by the push of a button, and may be verified by testing devices or a mercury manometer. The only method to detect incorrect transducer level is direct observation; this must be performed frequently if the transducer is fixed while the patient is moved. A more difficult-to-detect error is inadequate "dynamic response". This refers to the appropriate matching of damping to the natural frequency of the catheter transducer system (1). "Overdamping" or "underdamping" produce characteristic artifact on the pressure tracings and are more likely to affect systolic pressure than diastolic or mean pressures.


Waveform identification

While clinicians may be confident they can identify waveforms from the PAC, a recent survey suggested that many can not. Forty seven per cent of 496 physicians could not correctly identify the pulmonary artery occlusion pressure (PAOP) from a clear tracing (2). In order to correctly identify the PAOP, one must be able to determine the phase of respiration (all pressures should be read at end-expiration), and the components of the normal PAOP. This requires the ability to identify the "a" and "v" waves which occur in the left atrium. Either hard copy (i.e. printout of the waveform) or a monitor screen which can be "frozen" is required for adequate visualization. As the PAOP is an estimate of left ventricular end diastolic pressure (LVEDP), the peak "a" wave height is probably most reflective of this pressure.


Waveform interpretation

There can be little doubt that misinterpretation of pressures or waveforms from the PAC occurs commonly; this subject has been covered in extensive reviews (3,4). For example, while the PAOP waveform may be correctly identified, the pressure may not equivalent to LVEDP (table). Or, the presence of a large "v" wave on the PAOP tracing may be attributed to mitral regurgitation when in reality there is simply a noncompliant left atrium (5). Although clinicians use the PAOP to estimate preload, true ventricular preload is fiber stretch before shortening, or end diastolic volume. Acute reductions in ventricular compliance (e.g. myocardial ischemia) may cause acute increases in LVEDP (and PAOP), but the preload may actually decrease. In cardiac surgery patients LV pressures and volumes correlated poorly both before and after bypass (6).


Derived indices

Some clinicians believe that combinations of factors yield more information than the factors themselves. Examples include calculated vascular resistances, and "work" indices (e.g. LV stroke work index). The vascular resistances are computed by dividing the pressure decrease across a circuit (e.g. PA mean pressure minus PAOP) by the cardiac output. A major problem with this concept is that very small individuals (i.e. with lower cardiac output) will have very high resistances. A problem with the concept of pulmonary "resistance" calculated in this way is that the pulmonary circulation is "recruitable": vascular beds may be opened or closed when flow and/or pressure are changed.


Safety of the PA catheter

The above discussion should lead to the conclusion that with the very many potential areas for inaccuracy or misintepretation, a major concern in PA catheter safety should be mismanagement. While there are no scientific studies of this phenomenon, the study referred to above (2) showed that 55% of the 496 respondents scored less than 70% correct answers to questions regarding interpretation of PAC data, and 43% of the respondents scored less than 70% correct answers to questions about treatment based on PAC data. Some of the reports that have failed to detect a benefit from the use of the PAC may simply be documenting this phenomenon (7). Journals representing the Canadian and American Medical Associations, and the European Society of Intensive Care Medicine have recently published reviews recommending some combination of restricted availability, the development of practice guidelines, utilization management, as well as research directed at identifying patient groups likely to benefit from use of the PAC (8,9,10).


Complications

As an invasive procedure, PA catheter insertion has been associated with a host of mechanical complications. Any structure within needle range of the various approaches has been entered, catheters have perforated vessels and cardiac structures, and have knotted with other invasive lines or wires. Most cardiac anesthesiologists will have had the experience of a PAC being inadvertently sewn to the heart at the time of surgery, requiring a return to the operating room for removal (this may be avoided by verifying free movement of a PAC before chest closure). Thrombosis around even heparin bonded PAC's occurs commonly, and may progress to major vein thrombosis; infections may occur at the site of insertion, or may progress to sepsis or endocarditis. Recent studies have shed doubt on the widely practised "scheduled" replacement of PACs (e.g. every 3 days) as a means to reduce infection; it seems the main result of this practice is to increase the incidence of mechanical complications due to insertion (11,12). Pulmonary infarction may occur distal to a permanently wedged PAC, and massive hemoptysis as a result of PA rupture during balloon inflation may also occur. Fortunately most serious complications occur infrequently, but this fact is of little comfort when the patient is your own.

Table:  Situations where mean PAOP may misrepresent LVEDP

PAOP > LVEDP					PAOP < LVEDP

Positive Pressure Ventilation +/- PEEP		Noncompliant left ventricle
Increased intrathoracic pressure		Aortic regurgitation
PAC not in West zone III			Reduced area of pulmonary vessels (e.g. embolus)
Obstructive airways disease                   
Tachycardia
Increased pulmonary vascular resistance
Mitral valve disease
Pulmonary venous compression
						(from Tuman et al3)

References

  1. Gardner RM, Hollingsworth KW: Optimizing the electrocardiogram and pressure monitoring. Crit Care Med 1986; 14:651-658
  2. Iberti TJ, Fischer EP, Leibowitz AB et al: A multicenter study of physicians' knowledge of the pulmonary artery catheter. JAMA 1990;264:2928-2932
  3. Tuman KJ, Carroll GC, Ivankovitch AD: Pitfalls in interpretation of pulmonary artery catheter data. J Cardiothorac Anesth 1989;5:625-641
  4. Raper R, Sibbald WJ: Misled by the wedge? The Swan-Ganz catheter and left ventricular preload. Chest 1986;89;427-434
  5. Fuchs RM, Heuser RR, Yin CP, Brinker JA: Limitations of pulmonary wedge V waves in diagnosing mitral regurgitation. AM J Cardiol 1982:49:849-854
  6. Douglas PS, Edmunds LH, Sutton MS et al: Unreliability of hemodynamic indexes of left ventricular size during cardiac surgery. Ann Thorac Surg 1987; 44:31-34
  7. Gore JM, Goldberg RJ, Spodick DH et al: A community-wide assessment of the use of pulmonary artery catheters in patients with acute myocardial infarction. Chest 1987;92:721-727
  8. Technology Subcommittee of the Working Group on Critical Care, Ontario Ministry of Health: Hemodynamic monitoring: a technology assessment. Can Med Associ J 1991;145:114-121
  9. Naylor CD, Sibbald WJ, Sprung CL et al: Pulmonary artery catheterization: can there be an integrated strategy for guideline development and research promotion? JAMA 1993;269;2407-2411
  10. Bennett D, Boldt J, Brochard L et al: Expert Panel: The use of the pulmonary artery catheter. Intensive Care Medicine 1991;17:I-VIII
  11. Cobb DK, High KP, Sawyer RG et al: A controlled trial of scheduled replacements of central venous and pulmonary artery catheters. N Engl J Med 1992;327:1062-1068
  12. Eyer S, Brummitt C, Crossley K et al: Catheter-related sepsis: prospective, randomized study of three methods of long-term catheter maintenance. Crit Care Med 1990; 18:1073-1079

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