This is a prospective observational study conducted in the adult post-cardiac surgical unit of a tertiary care center. The center is one of the largest cardiac centers in India which specializes in minimally invasive coronary bypass, complex valve surgeries, pulmonary thromboendarterectomy and cardiac transplantation. All adult consecutive post-cardiac surgery patients were included for the study. Patients with inadequate window for USG, in respiratory distress (respiratory rate > 35/min, accessory muscles of respiration in use), liver cirrhosis, deep vein thrombosis of lower limb, pregnant women were excluded from the study. The study was approved by the ethics committee (NH/AEC-CL-2022-833) and waiver of consent was obtained. Baseline demographic details were obtained from medical records. USG examination for VExUS score, femoral vein pulsatility were done by an trained and intensivist with more than 5 years’ experience in bedside ultrasound. All images were reviewed and scores were confirmed by another intensivist with extensive bedside ultrasound experience. Any discrepancy in score were discussed and resolved.

Ultrasound assessment was performed bedside using a Sonosite M Turbo machine using a cardiac probe (1–5 MHz) Patients were positioned in the dorsal decubitus position with the head of bed elevated at 30°. In the same position, CVP was recorded using a central venous catheter with the transducer positioned at mid thoracic level. Hepatic venous Doppler was done, visualizing either the middle hepatic vein in the subxiphoid area or the right hepatic vein from a lateral angle. Similarly, the portal vein Doppler was interrogated from a lateral approach. All Doppler examinations were recorded and measured at end-expiration during respiratory pause.

The VExUS score was obtained using the published guidelines detailed in Additional file 1.

All the patients had CVP line inserted through right internal jugular vein, none were in the femoral vein. All patients were monitored with CVP in which the transducer was zeroed at the mid-axillary point. Mean CVP value read on display was recorded. The mean CVP values were noted simultaneously to the femoral venous Doppler examination.

The FVD was obtained with the patient in supine position with a linear array probe. Common femoral vein was identified just 2–3 cm below the inguinal ligament and it was examined in both short and long axis with angle correction within 60°. Normal FVD was defined as antegrade mildly pulsatile uninterrupted pattern with respiratory variation and retrograde flow of less than 1/3rd of antegrade flow (Fig. 1).

Fig. 1

Normal FVD waveform antegrade flow more than retrograde flow with respiratory variation and the flow can be described as antegrade mildly pulsatile uninterrupted pattern

FVD was considered significant or suggestive of venous congestion if either of the criteria was fulfilled:

1.

Pulsatile in nature

2.

Retrograde flow velocity of more than 10 cm/s

3.

Retrograde flow velocity being more than 1/3rd of antegrade flow velocity (flow reversal) (Figs. 2, 3).

Fig. 2

FVD suggestive of venous congestion. Retrograde velocity of more than 10 cm/s (pulsatile with flow reversal)

Fig. 3

FVD suggestive of venous congestion. Retrograde flow 1/3rd more than antegrade flow (pulsatile with flow reversal)

Pulmonary artery pressure was estimated using continuous wave (CW) Doppler of the tricuspid regurgitation (TR) tracing. This method allows measurement of the peak regurgitant velocity which represent the pressure difference between the right ventricle and right atrium. The simplified Bernoulli equation [P = 4(TRmax)2] was used to calculate this pressure difference using peak TR velocity. This method was used to calculate systolic pulmonary artery pressures. Mean PAP were approximated from the systolic PAP (SPAP) using the following formula: mPAP = 0.61 × SPAP + 2 mmHg [6].