CLINICAL STUDY: ECHOCARDIOGRAPHY
The deceleration time of pulmonary venous diastolic flow is more accurate than the pulmonary artery occlusion pressure in predicting left atrial pressure
Tim D. Kinnaird, MB, BCh ,
Christopher R. Thompson, MD, FACC* and
Bradley I. Munt, MD, FACC*
* Cardiac Echo Laboratory, St. Paul’s Hospital, Vancouver, British Columbia, Canada
Department of Cardiology, London Chest Hospital, London, United Kingdom
Manuscript received July 31, 2000;
revised manuscript received February 16, 2001,
accepted March 1, 2001.
Reprint requests and correspondence: Dr. Tim Kinnaird, Department of Cardiology, London Chest Hospital, Bonner Road, London, United Kingdom, E2 9JX
tkinnaird{at}113offord.freeserve.co.uk
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Abstract
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OBJECTIVES
This study compared a prediction of mean left atrial pressure (PLA) ascertained by Doppler echocardiography of pulmonary venous flow (PVF), with predicted PLA using the pulmonary artery occlusion pressure (PPAO).
BACKGROUND
In select patient groups, PVF variables correlate with PPAO, an indirect measure of PLA.
METHODS
In 93 patients undergoing cardiac surgery, we recorded with transesophageal echocardiography mitral valve early (E) and late (A) wave velocities, deceleration time (DT) of E (DTE), and pulmonary vein systolic (S) and diastolic (D) wave velocities, DT of D (DTD) and systolic fraction. The PPAO was measured using a pulmonary artery catheter zeroed to midaxillary level. A further catheter was held at midatrial level to zero a transducer and was then inserted into the left atrium. A prediction rule for PLA from DTD was developed in 50 patients and applied prospectively to estimate PLA in 43 patients.
RESULTS
A close correlation (r = –0.92) was found between PLA and DTD. Systolic fraction (r = –0.63), DTE (r = –0.61), D wave (r = 0.57), E wave (r = 0.52), and E/A ratio (r = 0.13) correlated less closely with PLA. The mean difference between predicted and measured PLA was 0.58 mm Hg for DTD method and 1.72 mm Hg for PPAO, with limits of agreement (mean ± 2 SE) of –2.94 to 4.10 mm Hg and –2.48 to 5.92 mm Hg, respectively. A DTD of <175 ms had 100% sensitivity and 94% specificity for a PLA of >17 mm Hg.
CONCLUSIONS
Deceleration time of pulmonary vein diastolic wave is more accurate than PPAO in estimating left atrial pressure in cardiac surgical patients.
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Abbreviations and Acronyms
| | A | = late mitral inflow | | CI | = confidence interval | | D | = diastolic pulmonary vein flow | | DTD | = deceleration time of diastolic pulmonary vein flow | | DTE | = deceleration time of early mitral inflow | | E | = early mitral inflow | | PLA | = mean left atrial pressure | | PPAO | = pulmonary artery occlusion pressure | | PVF | = pulmonary venous flow | | S | = systolic pulmonary vein flow | | SE | = standard error |
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Pulmonary artery occlusion pressure (PPAO) is considered the clinical gold standard for estimation of mean left atrial pressure (PLA), an indirect indicator of left ventricular intracavity filling pressures (1,2). However, insertion of a pulmonary artery catheter is not a risk-free procedure, and a reliable, less-invasive alternative has been sought (3). Both pulsed-wave Doppler echocardiography of mitral inflow and, subsequently, pulmonary vein flow (PVF) have been extensively studied, and a clear relationship between selected variables and PPAO was found (4–12). However, mitral inflow and PVF patterns are influenced by multiple factors including left atrial pressure, left ventricular relaxation (4,13), compliance and afterload (14,15), ventricular interaction (16,17), heart rate (18,19), cardiac output (20) and age (21). These confounding factors preclude routine clinical use of mitral inflow or PVF patterns to predict PLA.
Two recent studies have found a close relationship between the deceleration time of the diastolic wave (DTD) of PVF and PPAO in selected patient groups (22,23). Therefore, this study set out to investigate the relationship between the DTD and directly measured PLA in a more general group of cardiac surgical patients. We then attempted to predict PLA in a test group using a regression equation developed from the correlation between DTD and PLA in the study group. Finally, we compared the accuracy of this method of estimating PLA with PPAO estimation of PLA.
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Methods
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Patients.
Ninety-three patients scheduled for coronary artery bypass surgery and/or aortic valve replacement were studied in the operating room. Patients were divided into two groups: Patients in group 1 (50 patients)—the derivation group—were used to develop the prediction rule for PLA, and group 2 patients (43 patients)—the test group—were used to test the prediction rule. All patients had undergone cardiac catheterization ± transthoracic echocardiography prior to surgery. Patients with any degree of mitral stenosis, moderate or severe mitral regurgitation (24,25), or a history of prior cardiac surgery were excluded. The study protocol was approved by the St. Paul’s Hospital Research Ethics Board. All patients gave written, informed consent in a preadmission clinic or on the cardiac ward after full explanation of the study protocol.
Echocardiographic data.
Following induction of anesthesia, endotracheal intubation and placement of a pulmonary artery catheter (Model 131 F7, Baxter, Deerfield, Illinois), a multiplane 5-MHz transesophageal probe (Hewlett-Packard, Palo Alto, California) was placed in the esophagus. Data were obtained using a Hewlett-Packard Sonos 1500 ultrasound unit and recorded on videotape for later analysis. All measurements were obtained after pericardotomy, with the patient in a stable hemodynamic state, and ventilation briefly suspended at end expiration. Pulmonary venous flow was obtained by placing the pulsed-wave Doppler sample volume approximately 1 cm beyond the orifice of a superior pulmonary vein. Color flow Doppler was used when necessary to assist with optimal sample volume placement. Mitral flow was obtained in a four-chamber view with the pulsed-wave Doppler sample volume placed at the tips of the mitral leaflets. All Doppler tracings were recorded at 100 mm/s sweep speed.
Hemodynamic data.
After induction of anesthesia, a transducer for the pulmonary artery catheter was zeroed visually at the midaxillary level by the anesthesiologist and then fixed in relation to the chest. The PPAO measurements were taken by the anesthesiologist immediately after the echocardiographic data were acquired.
Following PPAO measurement, a fluid-filled catheter attached to a 21-gauge needle was held by the surgeon adjacent to the mid-right atrial wall to rezero the pressure transducer. The left atrium was then cannulated to directly record PLA. All measurements were obtained in a steady hemodynamic state with ventilation briefly suspended at end expiration. The maximum time to acquire all echocardiographic and hemodynamic data was 10 min.
Echocardiographic analysis.
Analysis of the echocardiographic data was performed offline by an interpreter (T.K.) blinded to the hemodynamic data. For all measurements, five consecutive beats were traced and the results averaged.
Pulmonary venous flow was analyzed for peak systolic (S) and diastolic (D) wave velocities, their ratio, and velocity time integrals. The DTD, and the peak velocity and duration of the atrial reversal wave were also measured. In the presence of a bimodal D wave deceleration slope, the initial, steeper part was extrapolated to zero to obtain the deceleration time (Fig. 1) (22). The systolic fraction of PVF was calculated as the ratio of the velocity-time integral of the S wave to that of the combined velocity-time integral of the S and D waves.
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Figure 1 Transesophageal pulsed-wave Doppler showing a biphasic slope of deceleration of the diastolic wave (D) of pulmonary venous flow. The deceleration time of the D wave is measured as the time interval between peak velocity and the upper deceleration slope extrapolated to zero. The deceleration time in this case is 223 ms.
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From mitral flow recordings, the velocities of peak early (E) and late (A) waves and their ratio, E wave deceleration time (DTE) and A wave duration were measured. The initial and steeper part of the E deceleration slope was also extrapolated to baseline where necessary to measure DTE. The difference between the duration of the A wave and the duration of the atrial reversal wave was also calculated.
Statistical analysis.
Using Statistical Analysis Software (SAS Institute, Cary, North Carolina), quadratic regression analysis was performed to examine the correlation between Doppler variables and PLA and the correlation between PPAO and PLA. A PLA prediction rule was developed based on the correlation between DTD and PLA in the first 50 patients (group 1) and then applied prospectively to the subsequent 43 patients (group 2). To evaluate the agreement between predicted and actual PLA, and between PPAO and PLA (in the same 43 patients), the data were processed by the Bland-Altman method, and the 95% confidence intervals (CI) expressed (26). Sensitivity and specificity were calculated with standard formulae.
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Results
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The baseline clinical and hemodynamic characteristics of the study group are described in Table 1. Group 2 patients were slightly older and had a higher incidence of aortic stenosis than patients in group 1. Ejection fraction was measured during cardiac catheterization.
Correlation of mitral and PVF variables with PLA.
A close correlation (r = –0.92) was found between DTD and PLA for the entire patient group (Fig. 2), whereas correlation of the other echocardiographic parameters was less close (Table 2). Among the PVF variables, DTD, systolic fraction and D wave peak velocity correlated most closely with PLA. Of the mitral inflow variables, DTE and E wave peak velocity correlated most closely with PLA. A DTD of <175 ms had 100% sensitivity and 94% specificity for a PLA of  17 mm Hg in the entire group, and 100% sensitivity and 90% specificity for a PLA of > 17 mm Hg in the test group. A DTD >275 ms predicted a PLA of  6 mm Hg , with 88% sensitivity and 95% specificity. There was no correlation between ejection fraction and DTD.
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Figure 2 Scatterplot of the correlation between deceleration time of the diastolic wave of pulmonary venous flow (DTD) and mean left atrial pressure (PLA). Data are plotted for group 1 combined with group 2. The horizontal dashed line indicates a DTD of 175 ms, which predicted a PLA of >17 mm Hg, with 100% sensitivity and 98% specificity.
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Estimation of PLA from DTD in the test group.
Using the DTD and PLA plot from group 1, the following regression equation was developed:
This formula was then applied prospectively to group 2 to predict PLA. The correlation between the estimated PLA using DTD and actual PLA is shown in Figure 3. Figure 4 displays a Bland-Altman plot of the difference between estimated and actual PLA versus actual PLA. The mean difference between predicted and measured PLA was 0.58 mm Hg, with 95% CI (mean ± 2 SE) of –2.94 to 4.10 mm Hg.
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Figure 3 Scatterplot of the correlation between estimated mean left atrial pressure (PLA) (calculated from the deceleration time of diastolic pulmonary venous flow [DTD] using the derived quadratic regression equation) and the directly measured PLA in group 2.
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Figure 4 Bland-Altman plot of the differences between estimated mean left atrial pressure (PLA) using the deceleration time of diastolic wave and actual PLA versus the actual PLA. The 95% confidence intervals for PLA estimation are –2.94 to 4.10 mm Hg and are shown by dashed lines.
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Estimation of PLA from PPAO in the test group.
There was also a close relationship between PPAO and PLA (r = 0.93, Fig. 5) although a systematic error was introduced, in part, by the visual estimation of the midaxillary line. When the zero point from this level was referenced to the surgeon’s visual zero at midatrial level, the midaxillary estimation was, in general, consistently lower than midatrial level. Thus, the mean difference between predicted PLA from PPAO and measured PLA was 1.72 mm Hg, with 95% CI (mean ± 2 standard error) of –2.48 to 5.92 mm Hg.
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Figure 5 Scatterplot of the correlation between pulmonary artery occlusion pressure (PPAO) and mean left atrial pressure (PLA). The tendency for the PPAO to overestimate the PLA is apparent from this plot.
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Thus, although both PPAO and DTD predict the PLA with a similar SE, the DTD method is not influenced by the systematic error introduced by visual estimation of the midaxillary line.
Intraobserver and interobserver variability.
This was assessed from 20 random Doppler recordings. In measuring the DTD, the mean percentage of variation between observers was 6% and for repeated measurement was 4%.
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Discussion
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We have shown that the DTD correlates strongly with the PLA in a group of general cardiac surgical patients. The other mitral inflow and PVF variables measured correlate less well with PLA.
Comparison with previous studies.
Two previous studies have directly examined the DTD and PLA relationship. Chirillo et al. (22) studied the correlation between the two variables in patients with atrial fibrillation in whom more traditional measures of diastolic function such as the E/A ratio or systolic fraction cannot be used. They found a very close correlation between DTD and PPAO, (r = –0.91), and they concluded that in patients with atrial fibrillation, DTD could be used to estimate PPAO. More recently, Yamamuro et al. (23) studied the relationship in patients within one week of an acute myocardial infraction and also found a close relationship between PPAO and DTD (r = –0.89). The correlation between DTD and PPAO or PLA is remarkably similar in all three studies, as are the regression lines. To our knowledge, our study is the first to show the strong correlation between mean PLA and DTD in a more general group of cardiac patients and to compare directly the prediction of PLA from PPAO and DTD.
Mechanism of relationship between DTD and PLA.
Controversy exists as to whether the left atrium is a passive structure through early diastole and ventricular systole. Little et al. (14), in an experimental model, found that DTE depended strictly on left ventricular chamber stiffness and assumed that, in early diastole, the left atrium and left ventricle act as a common conduit. However, Henein et al. (27) believe that the left atrium is active throughout most of the cardiac cycle. The discrepancy between DTD and DTE found in the Henein et al. (27) study, and by other investigators, suggests that the left atrium in the patient group studied behaves as a receiving chamber in its own right (22,23). Differing left ventricular and left atrial compliances may have an important role in modulating both PVF and mitral inflow patterns. If this is the case, then the driving pressure between the pulmonary veins and the left atrium and the compliance of the left atrium itself might be the most important determinants of the deceleration time of the DTD. This would explain the much closer correlation between DTD and PLA than between DTE and PLA found in the present study.
Thus, in early diastole, blood flowing into the left ventricle will cause a rapid pressure drop in a poorly compliant left atrium (with volume loss), resulting in blood accelerating in from the pulmonary veins (28). Rapid pulmonary vein inflow associated with low left atrial compliance will result in a rapid rise in left atrial pressure, an early abolition of the driving pressure gradient and a short deceleration time of early diastolic pulmonary flow. We did not examine the relationship between DTD and left atrial volumes in this study because of the inherent difficulties in accurate measurement of left atrial diameters from the transesophageal route, and because of time constraints. Future studies investigating left atrial compliance and PVF are needed.
Comparison between echocardiography and PPAO estimation of PLA.
To our knowledge, this is the first study in which left atrial pressure was measured directly rather than estimated using PPAO. Kuecherer et al. (9) used direct left atrial pressure measurement in a third of the periods studied in his series and measured PPAO in the remainder. Cannulation of the left atrium enables a comparison between echocardiographic estimation and PPAO estimation of PLA to be made.
We validated the regression equation developed from the initial patient data in the test group and were able to predict PLA within limits that would make it clinically useful. The 95% CIs for the estimate are narrower than in a previous study, which may reflect comparison with direct measurement, rather than estimation of PLA (22). The SE of the estimate of PLA using DTD was similar to the SE of the estimate using PPAO. However, there was a tendency for the PPAO to consistently overestimate the PLA as reflected by a mean difference of 1.72 mm Hg. The explanations for this consistent error are threefold: 1) the tendency for the estimate of midatrial level (as referenced to the midaxillary level) to be too low; 2) as found in the original study relating PPAO to left atrial pressure, the PPAO does overestimate the PLA because of the contribution of pulmonary venous resistance (29); and (3) the contribution of the right ventricular systolic pressure wave to PPAO (30). Our findings suggest that the prediction of PLA from DTD is more accurate than the prediction from PPAO—the current clinical practice.
Risks of pulmonary artery catheters.
Recent controversy has centered on whether pulmonary artery catheters improve or worsen survival in critically ill patients (31). Irrespective of this controversy, there are well-recognized risks of pulmonary artery catheter placement including pneumothorax, pulmonary artery rupture and sepsis (3,32). Alternative and less invasive techniques to obtain hemodynamic data such as esophageal Doppler echocardiography and thoracic bioimpedance are emerging technologies (33,34).
Study limitations.
Patients were studied after pericardotomy to allow left atrial cannulation to take place immediately after echocardiography and PPAO measurements. It is possible that the relationship found between DTD and PLA would be different with a closed pericardium. However, in nine patients we measured DTD immediately before and after pericardotomy and found no significant difference in the predicted PLA.
A further study limitation is that only three patients were in atrial fibrillation and, therefore, it is not possible to conclude from this study alone that the DTD can be used to estimate PLA in patients in atrial fibrillation. However, in a previous study examining only patients in atrial fibrillation (22), there was a similar correlation between DTD and PPAO as found in the present study. Therefore, the combined evidence suggests that the DTD can be routinely applied to predict PLA in patients with atrial fibrillation as well as to patients in sinus rhythm. The present study considered only the relationship between DTD and PLA in a steady hemodynamic state. If echocardiography is to replace the pulmonary artery catheter in certain situations, further work is needed to investigate whether changes in hemodynamic parameters and PLA are reflected by appropriate changes in the DTD.
Measurements of PVF were made using transesophageal ultrasound because the study was conducted during cardiac surgery. Routine clinical application would be facilitated if transthoracic measurements were feasible. Pulmonary vein flow can be recorded in over 80% of patients from the transthoracic approach, and measurements taken correlate closely with simultaneous transesophageal recordings (35,36). Previous studies (22,23) showing a similar correlation between PPAO and DTD as found in our study were conducted using transthoracic ultrasound. Therefore, the use of the transesophageal rather than the transthoracic approach should not prevent extrapolation of the study results to wider clinical practice.
Conclusions.
Finally, we conclude that, in cardiac surgical patients, measurement of the DTD using echocardiography can reliably estimate PLA, and it may obviate the need for invasive hemodynamic measurement with its attendant risks.
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Abstract
Methods