The measurement of cardiac output is pivotal in the management of critically ill patients. Although pulmonary artery catheter monitoring has long been the gold standard for determining cardiac output,1 noninvasive methods have been increasingly used in recent years; in particular, echocardiography has proven feasible and accurate.1,2 Some authors have proposed that echocardiography should be the preferred modality for measuring cardiac output in the initial evaluation of shock.3–5

Various approaches to assess cardiac output with echocardiography in critically ill patients have been described, but determining the stroke volume at the left ventricular outflow tract (LVOT) through the apical window is considered the gold standard. This parameter is calculated by multiplying the cross-sectional area of the LVOT by the velocity-time integral (VTI) of the LVOT. However, errors in measuring the LVOT area are common, resulting in inaccurate calculations of stroke volume. This limitation is especially relevant in emergency settings.6

Since the LVOT area remains constant, any changes in the stroke volume must be due to changes in the LVOT VTI. Therefore, LVOT VTI can serve as a surrogate for stroke volume. This approach could be used not only for the initial assessment of stroke volume, but also for monitoring the response to treatment.7

However, using LVOT VTI as a surrogate of stroke volume is problematic in certain clinical situations (e.g., severe aortic regurgitation or dynamic LVOT obstruction), and this parameter is difficult to measure in patients under mechanical ventilation, where the transthoracic acoustic window may not be feasible. This limitation affect up to 40–50% of patients.8,9

To circumvent these limitations, in patients with severe aortic regurgitation or dynamic LVOT obstruction, VTI could be measured in the right ventricular outflow tract (RVOT) through the parasternal short-axis or modified subcostal windows. The parasternal window is more susceptible to echo air-mediated artifacts in mechanically ventilated patients. By contrast, the short-axis plane through the modified subcostal window approaches the heart from the abdomen, avoiding the lungs, so measurements of RVOT VTI are not affected by mechanical ventilation. Although the subxiphoid view presented some limitations for a different hemodynamic assessment,10 to date, the feasibility of this approach has not been tested.

As in estimations of stroke volume from LVOT VTI, the rationale for estimations from RVOT VTI is based on the principle that the RVOT area remains constant, so changes in stroke volume reflect changes in VTI where the flow is measured.

This prospective study aimed first to compare the feasibility to measure RVOT VTI through the parasternal and the modified subcostal window. Secondly, we aimed to study the correlation of RVOT VTI, measured through the parasternal window or through the modified subcostal window, with LVOT VTI measured through the apical window (gold standard)

We analyzed 100 consecutive patients who met the inclusion criteria (Fig. 1), 44 (44%) under mechanical ventilation, mean PEEP during echocardiography, 6.4cmH2O; 28 (28%) on norepinephrine, mean dose, 0.27mcg/kg/min). Table 1 reports the baseline characteristics of the entire population as well of the subgroups of ventilated and nonventilated patients.

Figure 1.

Flowchart of Participants. Abbreviations: LVOT, left ventricle outflow tract; TTE, transthoracic echocardiography.

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Good-quality Doppler images were obtained in the parasternal short-axis and/or apical views in 81 patients. In the modified subcostal view, good quality Doppler images in all patients.

Primary outcome

The mean difference between LVOT VTI and RVOT VTI was −2±4.5cm when RVOT VTI was measured in the parasternal window (Fig. 2) and −5±3.8cm when RVOT VTI was measured through the modified subcostal window (Fig. 3). The median (IQR) results for VTI obtained with the three windows for the entire group as well as for the subgroups of ventilated and nonventilated patients are presented in Table 2. RVOT VTI was greater than LVOT VTI in only four patients when RVOT VTI was measured through the modified subcostal window and in 22 patients when RVOT VTI was measured through the parasternal window. RVOT VTI was less than 20cm in 14 patients when measured through the parasternal window and in 18 patients when measured through the subcostal windows; all these patients underwent echocardiography for a primary diagnosis of shock, and none were on inotropic drugs. In patients with VTI<20cm, the mean difference between LVOT VTI and RVOT VTI was −1±4.1cm when RVOT VTI was measured through the parasternal window and −3±2.4cm when RVOT VTI was measured through the modified subcostal window.

Figure 2.

Intraclass correlation between LVOT VTI and RVOT VTI through the transthoracic window. The y-axis represents the difference derived from subtracting LVOT VTI from RVOT VTI.

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Figure 3.

Intraclass correlation between LVOT VTI and RVOT VTI through the modified subcostal window. The y-axis represents the derived from subtracting LVOT VTI from RVOT VTI.

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Table 2.

Outcome measurements.

	Without MV(n=56)	With MV(n=44)	Total population(n=100)
Apical LVOT VTI, cm, median (IQR)	22.1 (19.8–24.7)	25.9 (21.6–29.4)	24.2 (20.9–28.5)
Pes RVOT VTI, cm, median (IQR)	24.4 (18.8–26.4)	21.9 (16.5–23.3)	23.1 (17.1–25.7)
Msv RVOT VTI, cm, median (IQR)	19.2 (16.6–23.5)	17.6 (14.5–20.9)	18.9 (15.7–22.6)

IQR=interquartile range; LVOT=left ventricular outflow tract; MSV=modified subcostal view; MV=mechanical ventilation; PES=parasternal view; RVOT=right ventricular outflow tract; VTI=velocity-time integral.

The ICC between LVOT VTI and RVOT VTI was 0.727 (95%CI 0.62–0.808) when RVOT VTI was measured through the modified subcostal window (Fig. 3) and 0.715 (95%CI 0.59–0.807) when RVOT VTI was measured through the parasternal window (Fig. 2).

Error characterization analyzed for RVOT from the modified subcostal view was as follows: MSE 44.27 (95%CI 34.4–54.9), RMSE 6.65 (95%CI 5.8–7.7), BIAS −5.4 (95%CI −6.2 to −4.7), and MAPE 15.34 (95%CI 12.8–17.8). When MSE was broken down into its parts, the result was as follows: for the RVOT VTI from the modified subcostal view and parasternal view the bias proportion was 0.67 and 0.25 respectively; the variance proportion 0.007 and 0 respectively; the covariance proportion 0.31 and 0.74 respectively.

The interobserver correlation between the two investigators was excellent (ICC=0.93).

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. Interestingly, consistency between measurements of RVOT VTI through the parasternal and subcostal windows were only moderate and better in nonventilated patients. This result suggests that cyclic cardiac movements according to the respiratory cycle are different when negative or positive pressure drives the breath, as lung expansion is greater at basal portions in spontaneously breathing patients and upper portions are better aerated while on mechanical ventilation.

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.

Mean RVOT VTI was lower than mean LVOT VTI, and the difference was greater when the modified subcostal window was used, mainly because RVOT VTI through the parasternal window was higher than LVOT VTI when measured in nonventilated patients. This results can be explained by the opposite deterioration of parasternal and subcostal window along the respiratory cycle: while parasternal window deteriorates during inspiration, subcostal window deteriorates during expiration. These differences are mainly due to the greater ease of positioning the transducer to obtain a Doppler angle near 0° 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.’s14 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.20–22 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 limitations 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.

The best approach is to attempt measurements through all windows in each patient; when more than one window is available, the best measurement should be used for clinical decisions. Given the moderate consistency between measurements made in different windows, once a window is chosen for clinical decision making, it should be used for subsequent measurements for hemodynamic monitoring; changing the window will make comparisons with the initial examination less reliable.

Adding the modified subcostal window to echocardiographic monitoring of hemodynamics enables monitoring RVOT VTI, and hence cardiac output monitoring, in all the patients of this selected cohort.

Dr. Colinas had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Concept and design: Colinas, Vicho. Acquisition, analysis, or interpretation of data: Colinas, Hernandez, Vicho. Drafting of the manuscript: Colinas, Hernandez, Vicho, Serna, Tuero, Cuesta-Montero. Critical revision of the manuscript for important intellectual content: Colinas, Hernandez, Vicho, Serna, Tuero, Cuesta-Montero, Cuena. Statistical analysis: Cuena. Administrative, technical, or material support: Colinas. Study supervision: Colinas, Hernandez, Vicho.

The study received no external funding.

No potential conflict of interest relevant to this article was reported.