Transthoracic echocardiography (TTE) is becoming increasingly essential in the diagnosis and management of critically ill patients, either as a comprehensive examination or a targeted tool for obtaining specific clinical information. Various parameters can be rapidly assessed, such as biventricular function, dimensions, and respiratory variability of the inferior vena cava, presence of pericardial effusion, transvalvular flows, estimation of cardiac output, and pressure in the left atrium and pulmonary artery. Integrating these findings with the clinical presentation allows for improved assessment of different shock etiologies and aids in fluid management, vasoactive therapy, and mechanical ventilation parameters.⁽¹⁾

Acute respiratory distress syndrome (ARDS) is an extreme form of hypoxemic acute respiratory failure associated with multiple etiologies, such as pneumonia (mainly bacterial and viral), gastric content aspiration, sepsis from other sources, non-cardiogenic shock, pancreatitis, severe trauma, and major surgeries.⁽²⁾ According to the Berlin criteria for ARDS,⁽³⁾ it is essential to rule out primary cardiac causes. In this context, TTE is a highly useful tool for clinicians owing to its availability, speed, and relatively low cost.

Given the high incidence of severe hypoxemia, patients with ARDS are often placed in the prone position, as this posture enhances ventilation in the dependent alveolar regions of the lung bases. However, prone positioning limits the acquisition of conventional echocardiographic windows. Recently, owing to the coronavirus disease 2019 (COVID-19) pandemic, the number of patients with ARDS ventilated in the prone position has significantly increased.

Several studies have evaluated the use of TTE in the prone position, either with a transesophageal transducer^(4,5) or a standard transducer^(6-14) (Table 1). When a standard transducer is used, techniques such as the swimmer's position with elevation of the left upper limb,^(6,7) partial deflation of the pneumatic mattress,⁽⁸⁾ and the placement of a pillow under the left upper limb to elevate the chest have been described.^(10-13) However, in most of these studies, the parasternal window could not be obtained using a transthoracic transducer, which is an important limiting factor in performing the examination.

More recently, in 2023, Nuzhny et al. performed cardiac examinations on 14 healthy male participants using an adapted medical bed with an access opening, which enabled the acquisition of parasternal windows.⁽¹⁴⁾ Their study focused on evaluating hemodynamic variables but did not include a comparative analysis of cardiac cavity dimensions and biventricular function parameters in the prone and conventional positions.

This study aimed to assess the feasibility of Transthoracic echocardiography in the prone position using a more accessible adaptation that expands the range of windows for analysis and enhances image acquisition.

Study population and setting

This single-center, cross-sectional, comparative pilot study was conducted at one of the authors’ private clinics in Mimoso do Sul, ES, Brazil. This study was approved by the Research Ethics Committee of Unimed Vitória (CAAE: 57816122.2.0000.5061; #6.276.364). All TTE examinations were performed by the same operator (NBC) between August and November 2023.

The sample was selected using a convenience sampling method, and healthy individuals were invited to participate voluntarily in the study. All participants understood the procedures, voluntarily agreed to participate, and signed an informed consent form in accordance with the Good Clinical Practice guidelines and national regulations.

Anthropometric and clinical data (sex, age, noninvasive blood pressure, weight, and height) were collected. Weight and height were measured using a Welmy R-110^® mechanical anthropometric scale. Blood pressure was measured using the Premium^® aneroid sphygmomanometer. Both devices were tested and calibrated before use. Weight and height measurements were used to calculate body mass index (BMI) and body surface area using the method described by DuBois.⁽¹⁵⁾

The study included 20 consecutive participants, half of whom were male, aged between 21 and 59 years. No participants were excluded because of acoustic window limitations, and none had chest deformities or breast implants. The clinical characteristics of the participants are presented in table 2.

All echocardiographic windows were successfully obtained in 100% of participants in both positions. Table 3 summarizes the findings related to technical feasibility and image acquisition quality in the prone and conventional positions.

The median number of successfully acquired variables was slightly lower in the prone position (16.0) than that in the conventional position (17.0); however, the difference was not statistically significant (p=0.412). No statistically significant differences were found between the positions for variables considered clinically relevant for ARDS.

The duration of the examination was significantly longer in the prone position (median, 19.0 min) than in the conventional position (median, 14.8 min; p = 0.037). No patient reported discomfort during the examination, and early termination was not requested by any participant.

Image quality assessment revealed a significant difference in the parasternal window (p=0.015), mainly due to a higher frequency of "excellent" scores in the conventional position (30% versus 10%) and a higher frequency of "good" scores in the prone position (80% versus 70%). No significant difference was observed in the apical window between the two positions. Examination duration and image quality were assessed in 10 participants.

Clinically relevant variables for ARDS management were obtained with reasonable success rates, except for PASP, which was estimated based on the peak velocity of tricuspid regurgitation. This measurement was feasible in only 10% of the patients in the prone position and in 25% in the conventional position. The primary reason for not obtaining measurements was the absence of tricuspid regurgitation. Additionally, no measurements were limited by the limitations of the acoustic window.

Figures 2 and 3 show a comparison of the images obtained at different positions in the same patient.

Tables 4 and 5 present a quantitative comparison of the two methods. After correcting for multiple comparisons, no statistically significant differences were observed between the prone and conventional positions for any of the echocardiographic parameters analyzed.

Our findings demonstrated that TTE was feasible in the prone position with adequate visualization of the apical, parasternal, and subcostal windows after low-cost adaptation. No statistically significant difference was observed in the number of variables successfully acquired in the prone and conventional positions.

A 2008 study reported that the prone position is associated with anterior displacement of the superior and lateral portions of the heart, as well as a reduction in lung volume between the heart and chest wall.⁽²¹⁾ In images obtained from the parasternal long-axis window in the prone position, the cardiac apex was laterally and superiorly displaced, making the longest cardiac axis more parallel to the intercostal spaces. This orientation allowed for a wider view of the apical region in the parasternal long-axis window, which is uncommon in examinations performed in the conventional position. Moreover, the proximity between the heart and chest wall may improve the acoustic window in selected patients, such as those with obesity or have lung diseases, potentially providing a benefit even in an outpatient setting.

Additionally, no differences were observed in clinically relevant variables for ARDS management, such as cardiac output estimation, LVEF assessment, TAPSE, and IVC assessment. These measurements were successfully obtained in the prone position for a large proportion of the participants (95%, 100%, 85%, and 100%, respectively).

Compared to other studies that used a transthoracic transducer without ultrasound table adaptation,^(6-13) we obtained similar success rates for the acquisition of biventricular function parameters in all participants, with higher success in obtaining parasternal long- and short-axis windows, which were not assessed in those studies. Specifically, the parasternal short-axis window is valuable for evaluating the paradoxical movement of the interventricular septum in cases of right ventricular pressure overload and for assessing segmental dysfunctions. This evaluation has also been performed using a transesophageal probe in transthoracic studies,^(4,5) providing good-quality images but at a higher cost.

Furthermore, the reported rates of IVC evaluation vary in previous studies (ranging from 33.3% to 92.6% of cases),^(6,9,12) reaching 100% only when using the partial deflation technique of the pneumatic mattress⁽⁸⁾ because of the difficulty in obtaining the subcostal window in patients ventilated in the prone position. In our study, the IVC was adequately visualized in all evaluated patients.

Consistent with previous studies^(6,7) we observed a low rate of PASP estimation using the peak tricuspid regurgitation flow velocity. However, most patients did not present with significant regurgitation in the conventional position, which could be attributed to the fact that the sample consisted of healthy individuals.

The quality of the images acquired in the prone position was satisfactory. Although a statistically significant difference in image quality was observed in the parasternal window (p=0.015), this difference was unlikely to have meaningful clinical implications. The distinction primarily resulted from a shift between "excellent" and "good" image quality scores. Both scores indicate adequate image quality for diagnostic purposes, suggesting that, despite statistical significance, the overall technical feasibility and clinical utility of echocardiography in the prone position remain unaffected.

However, although no statistically significant difference was observed in the quality of images obtained in the apical window, likely due to the small sample size, 50% of the participants evaluated in the prone position had poor or fair quality scores, whereas 80% assessed in the conventional position showed good or excellent quality. These findings suggest a reduction in apical window image quality in the prone position; however, it remains feasible to obtain important variables for the clinical management of patients.

Although a statistically significant difference was observed in the duration of the echocardiographic examination between positions, a 4.2-min difference in the median duration is unlikely to be clinically meaningful. In the studied sample, the longer examination time and the adopted position did not result in any reports of discomfort in patients. However, none of the patients had conditions that might have predisposed them to discomfort in the prone position, such as osteomuscular disorders or breast implants. Furthermore, critically ill patients are often intubated and sedated, which minimizes potential discomfort during image acquisition. These patients typically remain in the prone position for several hours, which further reduces the impact of a slightly longer examination. This modest increase in examination time may be minimized through technical improvements and increased examiner experience.

Quantitatively, no statistically significant differences were observed between the prone and conventional positions for any of the echocardiographic parameters analyzed. These included commonly used variables for hemodynamic evaluation, such as the IVC measurement, which is often used for estimating central venous pressure; the left ventricular outflow tract diameter and VTI of the left ventricular outflow tract, which are widely used for estimating stroke volume and cardiac output; and the E/e’ ratio, which is used to estimate PCWP.

Our study has some limitations. First, the small sample size and inclusion of healthy patients with spontaneous ventilation limit the applicability of the results to patients in other clinical settings. Mechanical ventilation, especially with high positive end-expiratory pressure levels, poses a challenge in echocardiographic assessment owing to alveolar distension, which limits the acoustic window, even in patients in the supine position. Transesophageal echocardiography may be an alternative in cases of limited acoustic windows and has been evaluated in patients with ARDS of other etiologies^(22,23) and in COVID-19,^(24,25) demonstrating feasibility and safety when performed by experienced operators. However, compared with TTE, transesophageal echocardiography is associated with higher costs, lower availability, greater risk of operator contamination, the need to interrupt enteral feeding, and an increased risk of accidental extubation and aspiration.⁽²⁶⁾ Therefore, it should be considered a targeted approach for selected cases rather than a first-choice examination for the hemodynamic evaluation of critically ill patients.

Additionally, the average BMI of the patients evaluated in this study was 26 kg/m². In patients with obesity, image acquisition may be compromised by the attenuation of the ultrasound signal due to adipose tissue. Obesity was identified as an important risk factor for requiring mechanical ventilation during the COVID-19 pandemic, and was present in 43% of patients requiring this type of support.⁽²⁷⁾ Finally, the proposed adaptation was performed on an ultrasound table intended for outpatient examination. Hospital beds, commonly used in intensive care units, have more complex structures with more advanced adjustments for height, inclination, and lateral safety rails. To implement the proposed adaptation in this setting, it is crucial to consult hospital engineering professionals to assess its feasibility and determine the most effective method of implementation.

The use of transthoracic echocardiography in the prone position was feasible following the proposed adaptation, with measurements comparable to those obtained in the conventional position, with good-quality image acquisition, and expansion of the assessed windows. Further studies are needed to validate the accuracy and applicability of this method in real-life scenarios, including critically ill patients in emergency and intensive care units, those receiving mechanical ventilation, and individuals with other comorbidities that may affect the proposed assessment.