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    PA catheters: What the waveforms reveal

    RN/DREXEL Home Study Program
    CE CENTER

    CE credit is no longer available for this article. Expired July 2005


    Originally posted September 2003

    PA catheters: What the waveforms reveal

    SALLY BEATTIE DULAK, RN, MS, CNS, GNP

    SALLY BEATTIE DULAK is the principal of CV Health Promotion, a cardiovascular consultancy in Columbia, Mo., and a member of the RN editorial board.

    KEY WORDS: pulmonary artery catheter (PA catheter), pulmonary artery wedge pressure (PAWP), right atrial pressure (RAP), left atrial pressure (LAP), a waves, cannon waves, v waves, pulmonary artery pressure (PAP), pulmonary systolic pressure (PAS), pulmonary artery end-diastolic pressure (PAEDP)

    Monitoring pulmonary artery pressure with a catheter provides a snapshot of the patient's cardiac activity at particular moments—giving vital data and early warning of a potential problem. With careful study, you can learn to master this complex, but critical tool.

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    The pulmonary artery (PA) catheter provides precise hemodynamic data that enables clinicians to identify problems and treat critically ill patients effectively. In a previous article, "A PA catheter refresher course" (RN, Apr. 2003), we covered the basic concepts and skills required of the nurse who may be called on to work with a PA catheter.1 Here, I'll focus on accurate interpretation and analysis of hemodynamic pressures and waves provided by the PA catheter.

    Competence in this area is crucial since, in most institutions, it's the nurse's responsibility to interpret this data initially and notify the physician if a change has occurred or if the patient is not responding to treatment.2

    PA catheters, typically, are used in critically ill patients in hemodynamic imbalance who have not responded to treatments such as diuretics, inotropic agents, or vasodilating agents. And patients undergoing open-heart surgery may have PA catheters inserted so their fluid status can be monitored in the immediate postoperative period. You may also encounter PA catheters in patients with trauma, those in shock or renal failure, and those who have an unstable blood pressure (BP), heart rate, or heart rhythm.

    Ultimately, the choice of appropriate interventions and their outcomes depends upon correct interpretation of the information provided by the PA catheter. Misinterpretation, such as relying solely on the numerical data displayed on the monitor, can lead to diagnostic and therapeutic errors, and thus, put patients at risk.

    First, a brief review of PA catheter basics

    As shown in Figure 1, recognizable waves indicating changes in pressure appear on the hemodynamic monitor as the catheter passes through the right atrium (RA) and right ventricle (RV) and into the pulmonary artery.

    When the balloon at the tip of the catheter is inflated, the PA catheter may be "floated" and "wedged" into a branch of the PA. A waveform is displayed, indicating the pulmonary artery wedge pressure (PAWP), also known as the pulmonary capillary wedge pressure (PCWP) or pulmonary artery occlusion pressure (PAOP).1

    The mechanical actions that produce the characteristic rises and falls in hemodynamic pressure waves are due to two factors: (1) changes in blood volume as blood either enters or exits a chamber or vessel, and (2) changes in myocardial fiber tension. Atrial and ventricular systole, or contraction, causes pressures to rise, while diastole, or relaxation in these chambers, causes pressures to fall.3 These mechanical activities, however, are always preceded by corresponding electrical events that occur during the cardiac cycle of depolarization and repolarization, as evidenced on the EKG.

    In order to accurately interpret the waves of each hemodynamic tracing, you must correlate them with the preceding electrical event. This correlation requires measurement and assessment of hemodynamic pressures from a two-channel paper recorder that provides a simultaneous tracing of the EKG and the specific waves under scrutiny. A clear grasp of the relationship between these electrical and mechanical events is essential for understanding where to measure and record the accurate pressure of each monitored hemodynamic wave.3,4

    And, although the monitors give digital readings, the waves must be measured because the readings may not always be accurate. The monitors are programmed to read only the "high" and "low" waves and calculate the mean. They cannot differentiate between the different waves.

    Read the pressure at end-expiration

    The changes in surrounding pleural or intrathoracic pressure that naturally occur during respiration also affect hemodynamic pressures, as intrathoracic pressures are transmitted to the cardiovascular structures in the chest. As a result, (see Figure 2) the waves displayed on the monitor rise and fall in a rhythmic fashion corresponding to the respiratory cycle of inspiration and expiration.

    During spontaneous inspiration, negative pleural pressures cause hemodynamic pressures to fall, whereas a rise in hemodynamic pressures will be observed during expiration. The reverse is true for patients on mechanical ventilation: Hemodynamic pressures rise during inspiration and fall during expiration.

    Regardless of whether the patient is breathing spontaneously or with the aid of a ventilator, always read the PA catheter pressures at the end of expiration, the point at which intrathoracic pressure equals atmospheric pressure. Observing the patient's chest during breathing while you read the dual-channel graphic recording will help you to identify when end-expiration occurs. This is an important practice to master to ensure accurate hemodynamic pressure measurements and interpretation.

    You'll find, however, that identifying end-expiration poses a greater challenge when positive end-expiratory pressure (PEEP) and intermittent assisted mechanical ventilation (where the patient spontaneously initiates some breaths) are being used. Some institutions use respiratory tracings and airway pressure monitoring to aid in detecting end-expiration in these instances.

    Capnography, which measures the level of carbon dioxide present in the airway at the end of expiration, can also be used to locate the end-expiration point for PA catheter pressure measurements.5,6 And, in some critically ill patients who are mechanically ventilated, muscle relaxants and temporary paralysis may be necessary in order to eliminate the confounding effects of variations in intrathoracic pressures.3,4

    Analyzing atrial pressure waves

    Once you understand how the PA catheter does its job, and you're aware of the necessity of reading pressure at end-expiration, you're ready to learn how to read and understand pressure waves.

    The right atrial pressure (RAP) reflects pressure in the right atrium during right ventricular filling and is essentially the same as central venous pressure (CVP). Indirectly, the RAP is an indicator of the pressure in the right ventricle at the end of the diastolic period of the cardiac cycle. This right ventricular end-diastolic pressure (RVEDP) is referred to as the RV preload.

    Preload is the distending force (partially determined by the volume of blood pushing against the walls of the ventricular chamber) stretching myocardial fibers at the end of diastole, just before ventricular contraction (systole) begins. The RAP is, therefore, a reflection of RV function and is used in the differential diagnosis of fluid overload (i.e., hypervolemia), RV failure or infarction, pulmonary embolism, pericardial tamponade, tricuspid insufficiency, and tricuspid stenosis. In each of these instances, the RAP will be elevated. It's decreased, however, in hypovolemia.3,4,7

    As shown in Figure 3a, the RAP tracing is represented by a series of three positive deflections (in order of observation, the a, c, and v waves) and two negative deflections (the x and y descent). Each deflection, or wave, represents a mechanical event in the heart as described in the accompanying table. The figure itself illustrates the normal RAP and the timing of its waves relative to the electrical events reflected by the EKG. The RAP is expressed as the mean value of the a wave, which represents atrial contraction and reflects RV filling at end-diastole.

    To obtain the RAP (and all subsequent PA catheter pressures), run a paper strip of the simultaneous graphic recording of the waveform and EKG. As seen in Figure 3a, align the a wave (the first positive deflection) on the RAP waveform with the PR interval on the EKG. On at least two complexes at the point of end-expiration, draw a vertical line from the beginning of the P wave down to the RAP waveform. The a wave is observed approximately 80 to 100 milliseconds after the P wave. Calculate the average between the lowest and highest numerical pressure of the a wave to obtain the mean RAP.3,4,8 This is done by adding the two values and dividing by two.

    The left atrial pressure (LAP) is a reflection of the left ventricular (LV) pressure at the end of diastole, in other words, the LV preload. The LAP provides a reflection of LV function, just as the RAP provides a reflection of RV function.

    The LAP is used in the differential diagnosis of fluid overload and the risk of developing pulmonary edema, LV failure, pericardial tamponade, constrictive pericarditis, and mitral valve insufficiency or stenosis. In these situations, the LAP will be elevated, but it will be decreased in the presence of hypovolemia. However, while RAP is measured directly from the proximal port of the PA catheter, the LAP is an indirect measurement obtained by observing the PAWP.

    To obtain the PAWP, the balloon at the tip of the PA catheter is inflated; blood flow "pushes" the PA catheter into a branch of the PA where the balloon "wedges" and sits until it's deflated. This maneuver allows the tip of the PA catheter to essentially "look" at the left atrium (LA). As blood flows from the LA to the LV during diastole, the pressure within the LA can be obtained.3,7

    The LAP or PAWP waveform has the same characteristics as the RAP waveform, with three positive and two negative deflections. The c wave, however, is often not visible as it may be "hidden" in the x descent of the a wave. If only two positively deflected waves are seen, the first is the a wave, the second, the v wave. Unlike the timing of the RAP, however, the timing of the PAWP waveform is slightly delayed relative to the EKG. This is because the LA normally contracts about 20 milliseconds after RA contraction, and also because the PAWP is a reflected LA pressure. It takes longer to traverse back through the pulmonary vasculature to the tip of the PA catheter.

    Keep in mind that being able to recognize this time delay in relation to the EKG will help you differentiate the RAP from the PAWP waveform. Figure 3b illustrates this difference in timing of the RAP and PAWP waveforms relative to the EKG.3,4

    PAWP is also read as a mean pressure value of the a wave. Align the end of at least two QRS complexes at the point of end-expiration with the PAWP waveform. Draw a vertical line from the end of the QRS down to the PAWP waveform, which identifies the a wave. (See Figure 3c and its accompanying table.) Then, calculate the mean pressure of the a wave to obtain the PAWP.3, 4, 8

    Recognizing altered or abnormal waves

    Once you know what normal RAP and LAP waveforms look like, you will be able to identify abnormal waves. (See Figures 4a, 4b, and 4c.) Abnormalities in the atrial waves may indicate a pre-existing or new pathologic condition such as an arrhythmia or valvular dysfunction. These abnormalities may be characterized by either an abnormal wave value (i.e., a low or elevated pressure), or an alteration in the normal contour of a specific wave. If you detect any change in the patient's baseline waveform, as discussed below, notify the treating physician or clinician.

    Altered a waves. Large a waves (Figure 4a) typically occur when the atria contract against partially closed, or stenotic, mitral or tricuspid valves. Cannon waves (sometimes referred to as giant a waves) are transmitted when there is a loss of atrial-ventricular (AV) synchrony, as in the case of junctional and AV dissociative rhythms. In these instances, the a wave produced by the simultaneous contraction of the atria and ventricles is enlarged and occurs later in the cardiac cycle, usually at the timing of the normal v wave. The combination of the a and v waves creates the cannon wave. Cannon waves will also appear if there are premature ventricular contractions (PVC), or re-entrant ventricular tachycardia.4

    In the presence of cannon waves, the accurate measurement of mean atrial pressures that coincide with the EKG is also affected. The mean RAP is measured at the end of the QRS complex, which is the time when ventricular end-diastole occurs (see Figure 4a again); the PAWP is measured approximately one to two boxes (40 – 80 milliseconds) after the QRS complex.3,4

    Large v waves. This is the most frequent right atrial wave abnormality, most commonly caused by tricuspid and mitral insufficiency.3 Other causes include ventricular ischemia/failure, decreased atrial compliance, increased pulmonary or systemic resistance, and ventricular septal defect.

    In large v waves (see Figure 4b), the exaggerated and elevated v wave (sometimes referred to as a broad cv wave) is produced by the increased blood volume entering the atria during the period of rapid atrial filling in the cardiac cycle. On the monitor, the v wave will be taller than the a wave and will be followed by an exaggerated y descent that reflects relief of the increased atrial pressure with the opening of the tricuspid or mitral valve.

    Obtaining an accurate PAWP in the presence of large v waves can pose a challenge, especially when attempting to differentiate it from the PA pressure (which we'll discuss later). The difficulty is that the large v wave may be mistaken for the systolic PA pressure wave. This complication can be avoided, however, if pressures and waves are consistently measured in relation to the patient's EKG.

    The RAP and PAWP waveforms with elevated v waves are measured in the usual way, by correct alignment with the EKG. Note that in the PAWP, the dominant v wave occurs after the T wave, whereas the peak of the systolic PA pressure occurs within the T wave.3,4 (See Figure 4b.)

    As mentioned earlier, clinical scenarios that can produce high RAPs and abnormal waves include hypervolemia, RV failure, tricuspid stenosis or insufficiency, cardiac tamponade, constrictive pericarditis, and chronic LV failure.3 Likewise, an elevated or abnormal PAWP can result from hypervolemia, LV failure, mitral stenosis or insufficiency, cardiac tamponade, and constrictive pericarditis.3 Hypovolemia can cause low RAP or PAWP.3

    Overwedged wave. Overinflation of the balloon tip during PAWP measurement is a potential cause of life-threatening complications such as pulmonary artery infarction or rupture. Inflation should not last for more than 8 – 15 seconds and should be stopped when the wave changes from that of the PA to the PAWP. Overwedging of the balloon is indicated by a progressive increase in pressure off the screen and loss of the a and v wave components, as seen in Figure 4c. In this situation, immediately stop inflation, deflate the balloon, and reposition the PA catheter or notify a clinician who is qualified to do so.1,4

    Taking a look at the normal PAP

    In addition to being aware of atrial waves, you need a solid understanding of the normal pulmonary artery waves.

    Pulmonary artery pressure (PAP) assesses the state of resistance in the pulmonary vasculature as well as ventricular function.7 PAP is continuously monitored. A normal PAP reading indicates that the catheter is located in the correct position, as the catheter can migrate either to the wedge position or slip back to the RV. You will need to interpret the systolic, end-diastolic, and mean PAP.4

    Pulmonary systolic pressure (PAS) indicates the pressure generated by RV ejection into the pulmonary vasculature. It is also equal to the RV systolic pressure observed during insertion of the PA catheter. Elevated PAS may be present with RV or LV failure, cardiac tamponade, or pulmonary hypertension. As with other hemodynamic pressures, it may be lower in hypovolemic states.

    Pulmonary artery end-diastolic pressure (PAEDP) indicates the pressure in the PA as blood moves from there into the pulmonary capillaries. It is also an indirect indicator of left ventricular end-diastolic pressure (LVEDP) in the absence of any obstruction "downstream," such as pulmonary hypertension or mitral stenosis. In addition, the PAEDP is approximately 1 – 3 mm Hg above the PAWP and, if there is no downstream occlusion, can be used to estimate this parameter.

    Mean pulmonary pressure (PAM) represents the average PAP throughout the cardiac cycle.3,4,7

    Now let's look at the normal PA waveform in Figure 5. It resembles the familiar systemic arterial waveform characterized by a rapid upstroke, followed by a rounded peak with a gradual downslope. This downslope is marked by a small hump, called the dicrotic notch, which is produced by the mechanical event of pulmonic and aortic valve closure and which separates systole from diastole.3

    To find the PAS, align the QT interval of the EKG with a tall peak of the PAP waveform. For PAEDP pressure, use the end of the QRS as a marker of where the PA diastolic phase should occur and record the reading just before the upstroke of the systolic wave.8

    It's important to note that in order to perform safe hemodynamic monitoring, you must be able to distinguish the PAP and RVP waveforms. These can be seen in Figure 1.

    The most apparent difference between the RVP and PAP waveforms occurs in the diastolic pressure, which increases from a near baseline level in the RVP to a higher value in the PAP. However, as Figure 1 indicates, the systolic level of the two waves, and, therefore, the pressures, are nearly identical. Note that the RVP waveform doesn't have a dicrotic notch present on its downslope.3,4

    If you see an RVP waveform on the monitor after PA catheter insertion, you should quickly reposition the catheter to prevent the occurrence of ventricular dysrhythmias.1 To reposition the catheter, either inflate the balloon to allow the PA catheter to float forward into the PA or, alternatively, with the balloon deflated, withdraw the catheter tip back into the RA.3 If you do not have the clinical privilege required to reposition the catheter, immediately call a physician or another individual who is qualified to do this.

    Keep in mind that accurate assessment and interpretation of hemodynamic waves and pressures must always be made in association with the patient's clinical signs and other hemodynamic data that can be obtained from the PA catheter.1 Absolute values have little meaning unless you continuously observe the patient's clinical status and trends in his response to therapy and interventions.

    Competence in hemodynamic waveform and pressure analysis represents one of the most challenging skills for nurses working with PA catheters. With continuous education, practice, and mentoring, you can gain mastery of this advanced form of assessment.

    REFERENCES

    1. Dulak, S. B. (2003). A PA catheter refresher course. RN, 66(4), 28.

    2. Antle, D. E. (2000). Ensuring competency in nurse repositioning of the pulmonary artery catheter. Dimens Crit Care Nurs, 19(2), 44.

    3. Daily, E. K. (2001). Hemodynamic waveform analysis. J Cardiovasc Nurs, 15(2), 6.

    4. Bridges, E. J. (2000). Monitoring pulmonary artery pressures: Just the facts. Crit Care Nurse, 20(6), 59.

    5. Schallom, L., & Ahrens, T. (2001). Hemodynamic applications of capnography. J Cardiovasc Nurs, 15(2), 56.

    6. Frakes, M. A. (2001). Measuring end-tidal carbon dioxide: Clinical applications and usefulness. Crit Care Nurse, 21(5), 23.

    7. Dvorak-King, C. (1997). PA catheter numbers made easy. RN, 60(11), 45.

    8. Lynn-McHale, D. J., &. Carlson, K. K. (Eds.). 2001. AACN procedure manual for critical care (4th ed.). Philadelphia: W. B. Saunders.



    Click here to view full-size graphic


    Click here to view full-size graphic


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    Electrical event (EKG) Mechanical event Right atrial pressure (normal RAP is 2 – 6 mm Hg)
    80 – 100 milliseconds after P wave RA systole a wave
      RA diastole x descent
    After QRS Tricuspid valve closure c wave
    After peak of T wave RA filling/tricuspid valve closed v wave
      RA emptying at opening of tricuspid valve/onset of right ventricle diastole y descent


    Click here to view full-size graphic


    Click here to view full-size graphic

     

    Electrical event (EKG) Mechanical event Pulmonary artery wedge pressure (normal PAWP is 6 – 12 mm Hg)
    Approximately 20 milliseconds after P wave Left atrial (LA) systole a wave
      LA diastole x descent
    T-P interval LA filling/mitral valve closed v wave
      LA emptying at opening of mitral valve/onset of left ventricle diastole y descent

    Sources: 1. Daily, E. K. (2001). Hemodynamic waveform analysis. J Cardiovasc Nurs, 15(2), 6. 2. Lynn-McHale, D. J., & Carlson, K. K. (Eds.). (2001). AACN procedure manual for critical care (4th ed.). Philadelphia: W. B. Saunders.


    Click here to view full-size graphic


    Click here to view full-size graphic

     

    Electrical event (EKG) Mechanical event Pulmonary artery pressure
    T wave Right ventricle ejection of blood into pulmonary vasculature Systolic
    (PAS 15 – 30 mm Hg)
    80 milliseconds after onset of QRS Indirect indicator of LVEDP End-diastolic
    (PAEDP 8 – 12 mm Hg); Mean (9 – 18 mm Hg)
    PAS: pulmonary artery systolic; LVEDP: left ventricular end-diastolic pressure; PAEDP: pulmonary artery end-diastolic pressure.

    Source: Lynn-McHale, D. J., & Carlson, K. K. (Eds.). (2001). AACN procedure manual for critical care (4th ed.). Philadelphia: W. B. Saunders.


     

    Emil Vernarec, ed. Sally Beattie Dulak. PA catheters: What the waveforms reveal. RN Sep. 1, 2003;66:56.

    Published in RN Magazine.

    Sally Beattie, RN, MS, CNS, GNP
    Sally Beattie, is an RN editorial board member and an advanced practice nurse at the University of Missouri Hospital and Clinics, ...