Swan-Ganz catheterization: Interpretation of tracings


Use of the flow-directed pulmonary artery (or Swan-Ganz) catheter provides the clinician with the ability to measure pressures and sample blood from the right atrium, right ventricle, and pulmonary artery [1,2]. Left atrial pressure can also be assessed indirectly from the pulmonary artery wedge (also called pulmonary artery occlusion or pulmonary capillary wedge) tracing. Careful observation of pressure waveforms permits evaluation of mechanical events within the atria and ventricles and offers important diagnostic information in a wide variety of cardiopulmonary disease states. Information derived from the use of this catheter can provide important insights into the assessment of:

This card will review the interpretation of Swan-Ganz catheter pressure tracings in normals and in a variety of pathophysiologic states. The technique of catheter insertion and indications for catheter use are discussed separately.).

ZEROING AND REFERENCING — The catheter must be appropriately zeroed and referenced for accurate diagnostic information to be obtained. Although zeroing and referencing are done in one step, they represent two separate processes:

It is important to note that this "phlebostatic level" changes with differences in position of the patient

Placement of the catheter — The pulmonary artery catheter is positioned using pressure waveform or fluoroscopic guidance.). The latter is often required in patients with marked right atrial or ventricular dilatation or severe tricuspid regurgitation.

Once in position, the catheter allows simultaneous recording of pressure waveforms from the right atrium and pulmonary artery or pulmonary artery wedge pressure. Pulmonary artery wedge pressure measurements are made by inflating the balloon at the distal tip of the catheter with approximately 1.0 to 1.5 mL of air, thereby occluding a segment of the pulmonary artery. This waveform reflects the transmitted pressure of the left atrium. It does not reflect true left ventricular preload (ie, left ventricular end-diastolic volume), nor does it reflect capillary hydrostatic pressures [4] or transmural pressures (see below)

CALCULATION OF CARDIAC OUTPUT — In addition to providing pressure measurements, the pulmonary artery catheter facilitates measurement of cardiac output via the indicator thermodilution method or the assumed Fick method Indicator dilution method — The indicator dilution principle predicts that, when an indicator substance is added to a stream of flowing blood, the flow rate will be inversely proportional to the mean concentration of the indicator at a downstream site. In the case of thermodilution, the indicator used is approximately 5 mL of either dextrose or saline that is cooler than blood.

The indicator is injected as a bolus through the proximal port of the pulmonary artery catheter and mixes with blood in the right ventricle. This mixing lowers the temperature of flowing blood, which is carried to the distal thermistor port. The thermistor records the temperature change and can electronically display a temperature-time curve. The area under this curve is inversely proportional to the flow rate in the pulmonary artery. This flow rate should be equal to cardiac output in the absence of an intracardiac shunt.

The indicator dilution method has been well validated when compared with calculation of cardiac output using the Fick method. There are, however, several important sources of error:

Newer catheters — Newer catheter designs incorporate continuous oximetric monitoring of pulmonary artery oxygen saturation (by fiberoptic reflectance spectrophotometry), thereby enabling continuous cardiac output estimation. These catheters have been shown to have reasonable correlation with measured mixed venous oxygen saturation by co-oximetry [10]. Continuous thermodilution catheters are also available and correlate reasonably well with bolus thermodilution methods [11].

In addition to calculating cardiac output, the Swan-Ganz catheter facilitates estimation of systemic vascular resistance and pulmonary vascular resistance PRESSURE WAVEFORMS — Waveforms can be monitored from several locations.

Right atrial waveform ミ In the presence of a competent tricuspid valve, the right atrial (RA) pressure waveform reflects both venous return to the right atrium (during ventricular systole) and right ventricular end-diastolic pressure. There are normally three positive components and two negative deflections in the RA waveform (The a wave reflects contraction in atrial systole, while the x descent reflects the fall in right atrial pressure following this event.

Electrocardiographic (ECG) correlation is required for correct identification of these events. In general, due to the length of the tubing in system, there is an 80 to 100 ms delay in the timing of mechanical events from their appearance on the ECG. Normal right atrial pressures vary from 0 to 7 mmHg.

Elevations in RA pressure are seen in a number of conditions. These include:

Tricuspid regurgitation produces elevated RA pressures as well as prominent, tall v waves as blood is regurgitated into the RA during ventricular systole (show figure 4).

A number of cardiac rhythm disturbances can also produce characteristic abnormalities in the RA waveform:

Finally, elevation and equalization of RA pressure with both right ventricular end-diastolic pressure and pulmonary artery wedge pressure is seen in cardiac tamponade, constrictive pericardial disease, and restrictive cardiomyopathies.).

Right ventricular waveform — Two pressures are typically measured in the right ventricle (RV): the peak right ventricular systolic pressure; and the right ventricular end-diastolic pressure immediately after the a wave Ventricular diastole is made up of an early rapid filling phase (during which approximately 60 percent of filling occurs), a slow phase (during which another 25 percent of filling occurs), and an atrial systolic phase (which produces the a wave in the RV tracing).

If measurement of RV end-diastolic pressure is required, as in the diagnosis of cardiac tamponade, restriction, or constriction, recordings are made from the distal tip of the catheter during initial catheter insertion. Routine serial monitoring of RV pressure is usually not clinically necessary.

Normal right ventricular systolic pressure varies from 15 to 25 mmHg, and right ventricular end-diastolic pressure varies from 3 to 12 mmHg. An increase in RV systolic pressure is seen in disorders in which there is pulmonary hypertension or pulmonic stenosis; in the latter condition, there is also a systolic pressure gradient from the right ventricle to the pulmonary artery. Acute pulmonary embolism can also produce elevations in the RV systolic pressure, although RV systolic pressures rarely exceed 40 to 50 mmHg in this setting.

An increase in RV end-diastolic pressure is seen in many forms of cardiomyopathy, as well as in right ventricular ischemia, infarction, and cardiac constriction or tamponade.

Pulmonary artery waveform — The main components of the pulmonary artery (PA) tracing are the systolic and diastolic pressures and the dicrotic notch, which represents closure of the pulmonic valve. The PA tracing is similar in appearance to the systemic arterial pressure tracing, except that the pulmonary arterial pressures are normally much lower. Normal pulmonary artery systolic pressures vary from 15 to 25 mmHg, while pulmonary artery diastolic pressures vary from 8 to 15 mmHg.

Elevations in PA pressures are seen with volume overload or with a variety of conditions in which pulmonary vascular resistance is elevated. These include:

As noted above, right ventricular and pulmonary artery systolic pressures rarely exceed 40 to 50 mmHg with acute pulmonary embolism. Fluctuation in the PA pressures may be seen in irregular heart rhythms such as atrial fibrillation, owing to different diastolic filling periods.

Pulmonary artery wedge pressure waveform — The pulmonary artery wedge pressure tracing is obtained by inflating a balloon at the distal tip of the catheter to obstruct forward blood flow through that particular branch of the pulmonary artery. This creates a static column of blood between the catheter tip and the left atrium, allowing the pressure at both ends of the column to equilibrate. The pressure at the distal end of the catheter is then equal to that of the left atrium, and is termed the pulmonary artery (or pulmonary capillary) wedge pressure (PAWP) [1].

The PAWP is considered to reflect the left ventricular end-diastolic pressure when there is no obstruction to flow between the left atrium and left ventricle. However, there are significant limitations to this assumption:

The PAWP tracing is similar in general configuration to that seen in the right atrium. The a wave reflects contraction in atrial systole, while the x descent reflects the fall in left atrial pressure that follows.

The electrical and mechanical correlation between ECG and PAWP tracings is similar to that seen in the right atrium, but the electromechanical delay is longer, due to the time necessary for left atrial mechanical events to be transmitted through the pulmonary vasculature to the distal tip of the catheter.

Elevations in the a wave of the PAWP tracing can be seen with increased resistance to left ventricular filling of any cause. These include:

Elevations in the v wave of the PAWP tracing represent either mitral regurgitation (show figure 10) or an acute volume load to the left atrium (due, for example, to a ventricular septal defect complicating myocardial infarction). Severe mitral regurgitation is often associated with large v waves in the PAWP tracing, but they are neither sensitive nor specific for this condition [12].

LUNG ZONES — The accuracy of the PAWP is dependent upon a continuous fluid column between the left atrium and the distal catheter tip. If the pressure in the surrounding alveoli exceeds capillary pressures and compresses the capillaries, the pressure at the catheter tip will reflect alveolar pressure and not left atrial pressure. This concept has been used to divide the lungs into three physiologic zones of blood flow, which are based upon the relationship between alveolar pressure, mean pulmonary artery pressure, and the pulmonary capillary pressure The PAWP is an accurate estimate of left atrial pressure only when the pulmonary capillary pressure exceeds the mean alveolar pressure (zone 3). This zone is located in the most dependent portion of the lung, where vascular pressures are the highest (due to gravity). Thus, the tip of the catheter should ideally be positioned below the level of the left atrium, which only occurs in approximately 60 percent of catheter insertions. Indicators of non-zone 3 catheter site placement include abnormal position on lateral chest x-rays, marked respiratory variation in the PAWP tracing, and increases in PAWP of more than 50 percent of the amount of PEEP applied (see below).

RESPIRATORY EFFECTS — During normal spontaneous ventilation, alveolar pressure (relative to atmospheric pressure) decreases during inspiration and increases during expiration. These changes are reversed with positive-pressure ventilation: alveolar pressure increases during inspiration and decreases during expiration. The changes in pleural pressure are transmitted to the cardiac structures and are reflected by changes in pulmonary artery and PAWP measurements during inspiration and expiration.

At end-expiration, pleural and intrathoracic pressures are equal to atmospheric pressures, regardless of the mode of ventilation. Thus, the true transmural pressure and therefore the PAWP should be measured at this point.

Most intensive care units use electronic pressure monitors that are designed to measure pressure in time intervals of four seconds and to display three different pressures: systolic (peak); diastolic (trough); and electronic mean pressure. The wedge pressure can be followed serially by selecting the systolic pressure for those breathing spontaneously, and by selecting the diastolic pressure for those on positive pressure ventilation. Use of these settings avoids false depression or elevation of intravascular pressure measurements due to superimposed fluctuations in pressure during respiration. Alternatively, newer monitors allow manual selection of the pulmonary artery wedge pressure via a cursor.

Effects of positive end-expiratory pressure — Alveolar pressure will not return to atmospheric pressure at end-expiration in the presence of positive end-expiratory pressure (PEEP), a change that can affect the measurement of intravascular pressures. PEEP can be applied therapeutically or can occur via incomplete expiration of alveolar gas, leading to air trapping ("auto PEEP"). The effects of either form of PEEP on the PAWP are variable and depend largely upon the compliance of the lungs. The effects of PEEP are generally felt not to be clinically significant for the following reasons:

An estimate of the true transmural filling pressures can be made in the presence of PEEP by subtracting one-half of the PEEP level from the PAWP if lung compliance is normal, or one-quarter of the PEEP level if lung compliance is reduced [14]. Since 10 cmH2O pressure is approximately 7.7 mmHg, the effects of PEEP on PAWP are usually small, and often do not affect clinical management. If the respiratory variation seen in the PAWP tracing exceeds that seen in the pulmonary artery tracing, then the PAWP may be unreliable due to non-zone 3 conditions.

Even though there may be a small effect of PEEP on intravascular pressure measurements, it is not advisable to eliminate (turn off) PEEP temporarily while pressure measurements are being made, as this may induce hemodynamic instability due to changes in venous return and PO2. These measurements off PEEP will therefore not accurately reflect the patient's hemodynamic status when PEEP is being used.

DETECTION OF LEFT TO RIGHT SHUNTS — Arterial blood sampling from the RA, RV, and PA provides helpful information when evaluating a suspected intracardiac (left to right) shunt. Detection of an oxygen saturation "step-up" allows confirmation of the shunt and determination of its location [15,16]. If a left-to-right shunt is suspected, the oxygen saturation of the above chambers as well as the superior and inferior vena cava should be measured while inserting the catheter under fluoroscopic guidance to ensure proper sampling sites. A step-up is defined as a greater than 10 percent rise in oxygen saturation when comparing the calculated saturation of mixed venous blood to the saturation of blood in the RA, RV or PA. Mixed venous saturation is calculated as:

The site of the step-up defines the level of the shunt. The degree of left to right shunting can then be quantitated by calculating a ratio of pulmonary flow (Qp) to systemic flow (Qs) (show table 3).

REFERENCES

1.  Swan, HJC, Ganz, W, Forrester, J, et al. Catheterization of the heart in man with the use of a flow-directed balloon-tipped catheter. N Engl J Med 1970; 283:447.

2.  Bridges, EJ, and Woods, SL. Pulmonary artery pressure measurement: State of the art. Heart Lung 1993; 22:99.

3.  Winsor, T, Burch, GE. Phlebostatic axis and phlebostatic level, reference levels for venous pressure measurements in man. Proc Soc Exp Biol Med 1945; 58:165.

4.  Weed, HG. Pulmonary "capillary" wedge pressure not the pressure in the pulmonary capillaries. Chest 1991; 100:1138.

5.  Mirini, JJ. Pulmonary artery occlusion pressure: Clinical physiology, measurement and interpretation. Am Rev Respir Dis 1983; 128:319.

6.  Putterman, C. The Swan-Ganz catheter: A decade of hemodynamic monitoring. J Crit Care 1989; 4:127.

7.  Sharkey, SW. Beyond the wedge: Clinical physiology and the Swan-Ganz catheter. Am J Med 1987; 83:111.

8.  Raper, R, Sibbald, WJ. Misled by the wedge? The Swan-Ganz catheter and left ventricular preload. Chest 1986; 89:427.

9.  Nemens, EJ, Woods, SL. Normal Fluctuations in pulmonary artery and pulmonary capillary wedge pressures in acutely ill patients. Heart Lung 1982; 11:393.

10.  Armaganidis, A, Dhainaut, JF, Billard, JL, et al. Accuracy assessment for three fiberoptic pulmonary artery catheters for SvO2 monitoring. Intens Care Med 1994; 20:484.

11.  Yeldermen, ML, Ramsay, MA, Quinn, MD, et al. Continuous thermodilution cardiac output measurements in intensive care unit patients. J Cardiothorac Vasc Anesth 1992; 6:270.

12.  Snyder, RW, Glamann, B, Lange, RA, et al. Predictive value of prominent pulmonary arterial wedge v waves in assessing the presence and severity of mitral regurgitation. Am J Cardiol 1994; 73:568.

13.  West, JB, Dollery, CT, Naimark, A. Distribution of blood flow in isolated lung: Relation to vascular and alveolar pressures. J Appl Physiol 1964; 19:713.

14.  Teboul, JL, Besbes, M, Andrivet, P, et al. A bedside index assessing the reliability of pulmonary artery occlusion pressure measurements during mechanical ventilation with positive end-expiratory pressure. J Crit Care 1992; 7:22.

15.  Gore, JM, Alpert, JS, Benotti, JR, et al. Handbook of Hemodynamic Monitoring, 1st ed, Little Brown, Boston, 1985.

16.  Grossman, W (Ed). Cardiac Catheterization and Angiography. 4th ed, Lea Febiger, Philadelphia, 1996.

17.  Flamm, MD, Cohn, KE, Hancock, EW. Measurement of systemic cardiac output at rest and exercise in patients with atrial septal defect. Am J Cardiol 1969; 23:258.

GRAPHICS

PA catheter leveling

 

Patient position and catheter

 

Calculation Fick cardiac output

 

Calculation vascular resistance

 

Right atrial pressure tracing

 

Swan Ganz catheter tracings

 

Ventricular pacemaker tracing

 

Pressure tracings pericarditis

 

Right ventricular tracing

 

Pulmonary artery tracing

 

Pulmonary artery wedge tracing

 

Swan Ganz PAWP tracing

 

Swan Ganz lung zones

 

Calculation of pulmonary flow