Study Of Ultrasound Windows For Measuring Inferior Vena Cava Changes Before Cardiac Surgery

Respiratory variation in inferior vena cava (IVC) diameter is a widely accepted non-invasive marker of intravascular volume status and fluid responsiveness, particularly in spontaneously breathing patients. Among transthoracic echocardiographic windows, this variation is most commonly assessed using the subcostal (SC) view, which is considered the clinical reference standard due to its direct acoustic pathway and extensive validation in research and clinical practice. The physiologic basis for this measure is strongest in the context of spontaneous respiration, where negative intrathoracic pressure significantly influences venous return dynamics. For this reason, the study population is limited to spontaneously breathing patients to ensure physiologic consistency and interpretive validity.

The SC view, however, may be limited by factors such as body habitus, postoperative dressings, or an obstructed subxiphoid window. The transhepatic (TH) view has been proposed as a complementary or alternative approach, providing an oblique acoustic window through the liver that facilitates visualization of the IVC long axis. Several observational studies have reported strong correlation and agreement between TH and SC measurements of IVC diameter and collapsibility, including in both spontaneously breathing and mechanically ventilated patients. Despite these promising findings, prior studies have been constrained by small sample sizes, heterogeneous methodologies, and limited assessment of reproducibility-particularly across operators with different levels of ultrasound experience. These gaps underscore the need for a larger, methodologically rigorous validation study.

The primary objective of this prospective, single-center observational study is to evaluate agreement between the SC and TH views using methodological standards aligned with diagnostic accuracy frameworks such as QUADAS-2, in a larger cohort of patients scheduled for cardiac surgery. The secondary objective is to assess interrater reliability of TH and SC IVC measurements obtained by novice and expert sonographers. The results are intended to strengthen the evidence base supporting the use of the transhepatic view in perioperative care and to inform future training, competency assessment, and quality-assurance processes in point-of-care ultrasound.

Inclusion criteria

• Adults aged eighteen years or older
• Scheduled to undergo cardiac surgery at Sunnybrook Health Sciences Centre
• Spontaneously breathing at the time of ultrasound assessment
• Able to lie supine for image acquisition
• Able to provide written informed consent

Exclusion criteria

• Inability to lie supine
• Known right heart failure
• Moderate to severe tricuspid regurgitation
• Portal hypertension
• End-stage renal disease
• Presence of abdominal dressings or conditions that preclude transhepatic imaging
• Poor acoustic windows that prevent adequate measurement from either the subcostal or transhepatic view, despite standard optimization techniques (positioning, breathing coaching, depth and gain adjustment, probe angulation)