Section Four: Pulmonary Artery Waveform Interpretation

Transcription

1 Section Four: Pulmonary Artery Waveform Interpretation All hemodynamic pressures and waveforms are generated by pressure changes in the heart caused by myocardial contraction (systole) and relaxation/filling (diastole) phases of the cardiac cycle. This mechanical activity of the heart is generated in response to the electrical activity, that is, the depolarization and repolarization of myocardial cells. Interpretation of the hemodynamic waveforms is dependent, therefore, on the correlation of mechanical to electrical activities using the ECG. There are three categories of hemodynamic waveforms: atrial, which includes RA, LA, and PA wedge; ventricular, which includes left and right ventricular; and arterial, which includes PA and systemic aortic. The waveforms in each category have similar characteristics because they result from the same cardiac events. The differences are in the pressure measurements because of the different amount of pressure that is generated in the right heart and the left side of the heart. Right Atrial Pressure The RA is a low pressure chamber, receiving blood volume passively from the vena cava. The normal pressure is 2 6 mm Hg, which reflects the mean RA pressure (RAP). Atrial waveforms have three positive waves: a, c, and v (Fig 8). The a wave is caused by the increase in atrial pressure during atrial contraction with systole. The c wave, often not seen, reflects the small increase in RAP associated with closure of the tricuspid valve and early atrial diastole. The v wave represents atrial diastole and the increase In pressure caused by filling of the atrium with blood. The v wave of RA diastole is also affected by right ventricular contraction and the bulging of the tricuspid valve up into the RA after RA systole. Accurate identification of the a, c, and v waves requires correlation of the waveform with the ECG. On the ECG, the P wave represents discharge of the SA node and atrial depolarization, which causes right atrial contraction. Therefore, the a wave occurs after the P wave and usually in the PR interval. Measure the mean a wave to obtain the RA pressure. The QRS complex represents ventricular depolarization and causes ventricular contraction. Simultaneously the RA relaxes and fills with blood. The v wave generated by these events thus falls after the QRS complex and in the T to P interval. The c wave is not always visible because of the small increase in pressure caused in early diastole and with tricuspid valve closure. If it is visible, the c wave occurs between the a and v waves.. Figure 8. Normal components of RA (CVP) waveform Section Four PA Waveform Interpretation 27

2 Reading RA Waveforms Method 1 Mean A Wave Most reliable, but must have a P wave in order to have normal a waves: Method 2 End QRS Useful when a wave is not present or abnormal, but not as accurate: Locate the beginning of the a wave (correlates with PR interval) Identify the top and the bottom of the a wave and average the 2 values to obtain the RA pressure. Method 1 Mean A Wave Find the end of the QRS complex Draw a line straight down to RA wave Where line intersects RA wave, that is the RA pressure Examine examples below. Method 2 End QRS RA pressure = 12 mm Hg Section Four PA Waveform Interpretation 28

3 It is important to obtain the RA pressure by examining the RA waveform and correlating it with the ECG in order to ensure the accuracy of the values. Obtaining values solely by taking the digital value on the monitor may result in inaccurate determinations. Significance of the Right Atrial Pressure The normal right atrial pressure is 2-6 mm Hg. It reflects the filling pressure of the right ventricle, or right ventricular preload. Causes of abnormal RA pressures include: Low Right Atrial Pressure Hypovolemia Vasodilatation High Right Atrial Pressure Fluid overload Right ventricular failure Pulmonary hypertension Left-ventricular failure Right Ventricular Pressure As the PA catheter is advanced from the RA into the right ventricle, the waveform generated has a distinctive square root configuration (Fig.9). RV Systole An initial rapid rise in pressure represents isovolumetric contraction. Follows the QRS complex, before the end of the T wave of the ECG. RV Diastole Pulmonic valve closes, and the rightventricular pressure rapidly decreases creating a diastolic dip. Falls near the end of the QRS on the ECG. The point on the waveform just prior to the rapid rise in pressure represents rightventricular end-diastole. The right-ventricular end diastolic pressure should be close to or equal to the RA, CVP. Figure 9. Normal RV pressure waveform. Section Four PA Waveform Interpretation 29

4 Significance of Right Ventricular Pressure The right ventricle is considered a lowpressure chamber. Right-ventricular systolic pressure is normally 20 to 30 mm Hg because the right ventricle needs to generate only enough pressure to open the pulmonic valve and move blood through the low-pressure pulmonary vasculature Low Right Ventricular Pressure Hypovolemia Vasodilatation High Right Ventricular Pressure Right ventricular failure Pulmonary hypertension Pulmonic stenosis Right-ventricular end-diastolic pressure is usually 0-6 mm Hg, equal to the RA pressure as the tricuspid valve is open. Causes of abnormal RV pressure includes: nnnnnnnnnnnnnnnnnnnnnn nnnnnnnnnnnnnnnnnnnnnn Pulmonary Artery Pressure The pulmonary vasculature is a relatively low-resistance and low-pressure system in normal individuals. PA systolic pressure and the peak of the PA waveform are generated by rightventricular systolic ejection; therefore, the PA and right-ventricular systolic pressure are the same. The PA waveform characteristics are similar to the systemic arterial waveform (see Figure 10). PA Systole Measure the peak of the systolic waveform to obtain the PA systolic pressure. Systole is found after the QRS complex, but before the end of the T wave. Normal PA systole is mm Hg. Dicrotic Notch Found in the downward slope of the PA waveform. Occurs after systole but before diastole, usually after the T wave. Corresponds with pulmonic valve closure at the beginning of right-ventricular diastole and is the beginning of the PA diastolic phase. PA Diastole PA diastolic pressure theoretically and in absolutely normal conditions is an indirect measure of LV pressure because the pulmonary vasculature, left atrium, and open mitral valve allow equalization of pressure from the left ventricle back to the tip of the PA catheter. Measure the PA diastolic pressure just before the upstroke of the systolic waveform. PA diastole corresponds with the end of the QRS. Normal PA diastolic pressure is ~ 8 15 mm Hg. Section Four PA Waveform Interpretation 30

5 Figure 10. PA pressure waveform showing phases of systole, dicrotic notch (pulmonic valve closure), and end-diastole. Significance of Pulmonary Artery Pressures Normal PA systolic pressure is 20 to 30 mm Hg and is equal to right-ventricular systolic pressure. Normal PA diastolic pressure is 8 to 15 mm Hg and is usually about the same as the left atrial pressure because there is no valve between the left atrium and the pulmonary circulation. The normal PA mean pressure is 10 to 20 mm Hg. Abnormal PA pressures may be due to: Low Pulmonary Artery Pressure Hypovolemia High Pulmonary Artery Pressure Pulmonary hypertension Left ventricular failure Pulmonary Capillary Wedge Pressure When the PA catheter is properly positioned, the pulmonary capillary wedge pressure (PCWP) is obtained by inflating the balloon at the catheter tip. The balloon occludes forward flow in the branch of the PA, creating a static column of blood from that portion of the PA through the left atrium, an open mitral valve during diastole, and the left ventricle. In this way the PCWP reflects left ventricular end-diastolic pressure. (See Figure 11A below.) Inflation of the PA balloon causes the PA waveform on the monitor to become a PCWP tracing. No more than 1.5 ml of air is used to inflate the balloon. If less than 1 ml of air generates a PCWP tracing, the PA catheter has migrated distally and needs to be withdrawn slightly by the physician or RN (check policy), for proper placement. If spontaneous wedge occurs the physician should be notified and measures taken immediately to reposition. (See table on page 44.) Section Four PA Waveform Interpretation 31

6 Figure 11A. PA catheter, chamber pressures, relationship to left heart. PCWP Waveform The PCWP tracing has a, c, and v waves (Fig 11A). The electrical and mechanical events of the heart generating these waves are identical to those of the RA waveform, except that the PCWP waveform reflects activity from the left side of the heart. The a wave is caused by LA contraction, and the v wave corresponds with LV contraction and left atrial filling. The c wave is rarely visible on the PCWP tracing. As with the RA, RV and PA pressures, it is important to obtain PCWP measurements from a graphic waveform strip with simultaneous ECG in order to ensure utmost accuracy. Two methods for accurate interpretation will be reviewed. The a wave occurs during LA contraction, often the same size as the v wave, but may be larger or smaller. It is the mean a wave that best reflects the LVEDP and should be used to interpret the PCWP reading. The a wave is found near the end or after the QRS. Giant a waves may be due to arrhythmias such as AV block, AV asynchrony found in paced rhythms or mitral stenosis. The v wave is due to blood filling the LA with a closed mitral valve. The v wave is located after the T wave, in the T-P interval. Giant v waves may occur due to mitral regurgitation or in problems of ventricular wall compliance. Methods for Reading PCWPs Method 1 Mean of the A Wave Most reliable method (see Figure 11B). The a wave is usually easy to identify and is present unless atrial contraction is absent, e.g. atrial fibrillation. The a wave is correlated with the end of the QRS complex. Locate the top and bottom of the a wave and average the 2 values to obtain the PCWP. Method 2 Z Point Useful when the a wave is absent or abnormal. Assumes that the left ventricular end diastolic pressure (LVEDP) is seconds after initiation of the QRS, end QRS. At the end of the QRS, draw a line straight down and take the reading. (See Figure 11c on the next page.) Section Four PA Waveform Interpretation 32

7 Differences in CVP and PCWP ECG Correlation Wave RA, CVP PCWP A In the PR interval End of QRS V Near end of T wave In the TP interval Method 1 Mean of the A Wave Figure 11 B. Normal pulmonary capillary wedge pressure waveform showing a and v waves and x and y descents. The top of the a wave (15) and bottom (8) are averaged to get the mean PCWP (15+8=23, 23 2=11.5) Method 2 Z Point (End QRS) Figure 11 C. There are no clear P waves, therefore read the PCWP at the end of the QRS complex. The v waves are larger than the a waves in the tracing. Section Four PA Waveform Interpretation 33

8 Significance of PCWP The PCWP more closely measures left atrial and ventricular end- diastolic pressure than the PA diastolic pressure because balloon inflation halts blood flow past the catheter tip and thereby decreases the influence of pulmonary vascular resistance on the pressure reading. Normal PCWP is 8 to 12 mm Hg. Abnormal PCWP may be due to: Low PCWP Hypovolemia Vasodilatation Elevated PCWP (A Waves and V Waves) Hypervolemia Left heart failure Cardiac tamponade Elevated V waves Mitral valve regurgitation, insufficiency Acute ischemia PAD/PCWP Relationship Normally PA diastole (PAD) closely represents left ventricular end-diastolic pressure, although when pulmonary hypertension is present the PA diastolic pressure may be higher than the PCWP. Elevated PCWP frequently is due to LV dysfunction or hypervolemia. In some cases, such as acute respiratory distress syndrome or mechanical ventilator settings that generate extremely high intrathoracic pressure, PCWP will be elevated because of noncardiogenic causes. In these cases, however, the PA diastolic pressure will be greater than the PCWP. When the PA catheter is inserted, the relationship of the PAD and PCWP should be noted. If the PAD correlates with the PCWP (as it should), a sudden increase in the PAD would alert the RN to check the PCWP. When the PAD is higher than the PCWP, the LVEDP can only be obtained by wedging the catheter. See Figure 11d and 11e below to see the usual PAD/PCWP relationship and the PAD/PCWP in the presence of conditions that cause pulmonary hypertension. Note: The PCWP cannot be higher than the PAD. Figure 11e. The PAD is 50, the PCWP is 20, much less than the PAD. Figure 11d. The PAD is 20, the PCWP is ~18, similar to the PAD. Review material from this section by completing the Self-Test that follows, and then compare your answers with those given. Section Four PA Waveform Interpretation 34

9 Section Four: Pulmonary Artery Waveform Interpretation Self-Test Fill in the blanks with one of the following: CVP, RA RV systolic PAD Normally, the right ventricular diastolic pressure is ~ the same as the ; the PA systolic pressure is ~ the same as the ; and the PCWP is ~ the same as the. It is important to correlate the hemodynamic waveform with the ECG and to know where the reading should be taken in order to get accurate data. Fill in the following: RA, CVP is best read at the wave, which correlates with the on the ECG. RV end-diastole is read just prior to ejection, which correlates with the on the ECG. PA diastole is read just prior to ejection, which correlates with the on the ECG. PCWP is best read at the wave, which correlates with on the ECG. Match the anatomic location with its normal pressure: Aortic Pulmonary artery a) 2-8 mm Hg b) 8-12 mm Hg PCWP c) 20/8 30/12 Right atrial, CVP d) 20/0 30/5 Right ventricular e) 100/60 140/80 Match the pressures with the possible corresponding conditions: PA 50/30, PCWP 10 PA 20/10, PCWP 10 heart function PA 35/22, PCWP 28 PA 32/25, PCWP 23 PA 18/3, PCWP 2 a) Left ventricular failure b) Pulmonary hypertension, normal left c) Hypovolemia d) Normal e) Impossible reading Section Four PA Waveform Interpretation 35

10 Identify the following waveforms and document the pressures: 1) Waveform: Pressure: 2) Waveform: Pressure: Section Four PA Waveform Interpretation 36

11 3) Waveform: Pressure: 4) Waveform: Pressure: Section Four PA Waveform Interpretation 37

12 Section Four: Waveform Interpretation Self-Test Answers Fill in the blanks with one of the following: CVP, RA RV systolic PAD Normally, the right ventricular diastolic pressure is ~ the same as the CVP ; the PA systolic pressure is ~ the same as the RV systolic ; and the PCWP is ~ the same as the PAD. It is important to correlate the hemodynamic waveform with the ECG and to know where the reading should be taken in order to get accurate data. Fill in the following: RA, CVP is best read at the a wave, which correlates with the PR interval on the ECG. RV end-diastole is read just prior to systolic ejection, which correlates with the end of the QRS on the ECG. PA diastole is read just prior to systolic ejection, which correlates with the end of the QRS on the ECG. PCWP is best read at the a wave, which correlates with end of QRS on the ECG. Match the anatomic location with its normal pressure: E Aortic C Pulmonary artery a) 2-6 mm Hg b) 8-12 mm Hg B PCWP c) 20/8 30/12 A Right atrial, CVP d) 20/0 30/5 D Right ventricular e) 100/60 140/80 Match the pressures with the possible corresponding conditions: B PA 50/30, PCWP 10 D PA 20/10, PCWP 10 function E PA 35/22, PCWP 28 A PA 32/25, PCWP 23 C PA 18/3, PCWP 2 a) Left ventricular failure b) Pulmonary hypertension, normal left heart c) Hypovolemia d) Normal e) Impossible reading Section Four PA Waveform Interpretation 38

13 ANSWERS TO WAVEFORMS: 1) Waveform: PA Pressure: 63/25 mm Hg 2) Waveform: RA (A wave in PR) Pressure: mm Hg Section Four PA Waveform Interpretation 39

14 3) Waveform: RV Pressure: 78/28 mm Hg 4) Waveform: PCWP Pressure: 10 mm Hg Section Four PA Waveform Interpretation 40

15 Section Four PA Waveform Interpretation 41