Because echocardiographic measurement of cardiac output based on LVOT VTI is unfeasible in some critically ill patients, we aimed to determine whether measurements based on RVOT VTI would be reliable. We found that it is possible to monitor cardiac output through at least one echographic window in near all critically ill patients. In our critically ill population in which 44% of patients were under mechanical ventilation with moderate pressure, measuring LVOT VTI through the gold-standard apical window was feasible in 81% of patients and measuring RVOT VTI through the modified subcostal window was feasible in 100%. Importantly, measurements of RVOT VTI through the parasternal and the modified subcostal window were moderately consistent with measurements of LVOT VTI through the gold-standard apical window.
Maizel et al. (10) reported lower feasibility for subxiphoid view, with no echographic image in 14% of patients. However, they aimed to analyze the quality of two-dimension images through the subxiphoid view for hemodynamic assessment. In our study, 100% of the patients had echographic window of quality enough for Doppler measurements.
Regardless of the window used to measure RVOT VTI, mean RVOT VTI was lower than mean LVOT VTI, although the difference was greater when the modified subcostal window was used. These differences are mainly due to the greater ease of positioning the transducer to obtain a Doppler angle near 0º in the parasternal window, suggesting that the parasternal and apical windows enable better alignment with the ventricular outflow tracts (10). However, as RVOT VTI was higher measured through the modified subcostal window than through the parasternal window in 4 (4%) patients, anatomical peculiarities might enable a better Doppler angle optimal through the subcostal window in some patients. Our results corroborate Lightly et al.’s (14) findings in nonventilated patients, where RVOT TVI was higher in the parasternal window than in the subcostal window; to our knowledge, this is the only other study that compared measurements of right ventricular cardiac output obtained through different windows.
The clinical importance of positive and negative deviations from 0 in a correlation analysis is secondary to the risk of both overestimation and underestimation of the gold standard. It is noteworthy that although right-ventricular and left-ventricular cardiac output are similar when the complete respiratory cycle is considered, the two parameters differ when measured for any given heartbeat. In spontaneously breathing patients, right-ventricular output is greater than left-ventricular output during inspiration, and left-ventricular output is greater than right-ventricular output during expiration; however, in patients under positive-pressure mechanical ventilation, the opposite relations are observed (20, 21, 22). Echocardiographic monitoring studies select the best measurements, but when transthoracic windows are used, most of the best measurements are obtained during exhalation, as the increased lung volume during inspiration negatively affects the quality of the echographic images. Mechanical ventilation further limits measurements of left ventricular output through the transthoracic window during inspiration, but its effects on measurements of right ventricular output are not as severe. These differences are lost when populations including both ventilated and nonventilated patients are analyzed.
Measuring RVOT VTI through the subcostal window enables echocardiographic monitoring in patients with clinical contraindications for LVOT VTI measurements. Measuring cardiac output through the subcostal window rather than the transthoracic window has several advantages. RVOT VTI measurements are more feasible and more consistent in patients under mechanical ventilation. Measurements deviate from the gold standard in only one direction (i.e., sometimes underestimating, but scarcely overestimating cardiac output) and are possibly less dispersed in patients with lower VTI.
Interobserver reproducibility is an important issue in echocardiographic monitoring of cardiac output because this operator-dependent technique is used for point-of-care hemodynamic monitoring, where different professionals measure the same parameters on different occasions. For this reason, we analyzed interobserver reproducibility by having two experienced echocardiographers measure cardiac output parameters in the same studies to reduce the risk of observer-related bias in our main results. The interobserver correlation was excellent (ICC > 0.8). One study reported 6% interobserver variability for systolic volume (7). The most important limitation for the reproducibility of measurements of cardiac output is variability in measurements of aortic outflow tract diameter; for this reason, we used VTI as a surrogate for cardiac output to reduce the impact of this variability.
Limitations of the study:
This study did not consider causes of RVOT obstruction such as severe pulmonary stenosis or severe pulmonary hypertension. However, severe pulmonary stenosis is rare and is therefore unlikely to affect our results. Severe pulmonary hypertension modifies the shape of the VTI curve, mainly by shortening the acceleration time. Nevertheless, the area under the VTI curve has not been related to pulmonary hypertension, even in severe cases. Another possible limitation is that we did not measure the diameter of the RVOT. Our reasons for not measuring this parameter are the same as our reasons for not measuring the LVOT diameter. Moreover, the protocols for measuring RVOT diameter are far from being standardized, and VTI is the best surrogate parameter for clinical decision making (15).
Technical limitations did not allow us to store respiratory cycle information along with the echocardiographic images and electrocardiogram waveforms for offline measurements. However, cyclical deterioration of the acoustic window secondary to lung inflation facilitates the recognition of respiratory cycle during echocardiographic monitoring. In addition, the results of the post hoc analysis support the use of right-sided measurements of cardiac output in patients under mechanical ventilation.
None of the patients were under dobutamine perfusion when they underwent echocardiography, because most studies were indicated mainly to diagnose shock. However, VTI was lower than 20 cm in 18%, so we consider that our results can be extrapolated to patients with limited cardiac output.