This research study will compare two ultrasound methods for assessing the inferior vena cava (IVC), a major vein that reflects intravascular fluid status and cardiac function before and after surgery. The standard method uses a subcostal ultrasound view obtained below the breastbone, but this approach may be limited in patients with obesity, surgical dressings, or postoperative discomfort. An alternative approach, the transhepatic view, uses the liver as an acoustic window and may provide improved feasibility in these situations.

The study will evaluate whether the transhepatic view provides measurements comparable to the standard subcostal view and whether operators with different levels of ultrasound experience obtain consistent results using both methods. Adult patients who are awake and scheduled for cardiac surgery at Sunnybrook Health Sciences Centre will undergo a brief ultrasound examination before surgery. The scan takes less than 10 minutes, involves no discomfort, and does not alter clinical care. This is a minimal-risk observational study with no therapeutic interventions. Participation is voluntary, and all personal health information will remain confidential. Findings may inform future approaches to ultrasound-guided assessment and training in perioperative care.

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)

The study population consists of adult patients scheduled to undergo elective cardiac surgery at Sunnybrook Health Sciences Centre. Eligible participants will be spontaneously breathing and able to lie supine during ultrasound assessment. The study will include male and female patients aged eighteen years or older, with no upper age limit. Exclusion criteria focus on conditions that impair accurate visualization of the inferior vena cava, including right heart failure, significant tricuspid regurgitation, portal hypertension, end-stage renal disease, obstructive dressings, or inadequate acoustic windows despite optimization maneuvers. This population reflects typical perioperative cardiac surgery patients in whom reliable non-invasive assessment of fluid status is clinically relevant.

De-identified individual participant data (IPD) underlying published results (including ultrasound measurements and basic demographic variables) will be made available to qualified researchers upon reasonable request to the Principal Investigator, following publication of the primary manuscript. Data will be stored on secure institutional servers at Sunnybrook Research Institute.

De-identified individual participant data (IPD) and supporting documents (study protocol, statistical analysis plan, and analytic code) will be made available beginning 12 months after publication of the primary manuscript. Data will remain available for a minimum of 5 years following publication, through secure institutional servers at Sunnybrook Research Institute. Access will be granted to qualified researchers upon reasonable request to the Principal Investigator, subject to a data sharing agreement that ensures privacy and appropriate use.

De-identified individual participant data (IPD) and supporting documents (study protocol, statistical analysis plan, and analytic code) will be accessible to qualified researchers affiliated with academic or healthcare institutions. Requests must include a brief research proposal outlining objectives, methods, and intended use of the data. Access will be granted upon approval by the Principal Investigator and execution of a data-sharing agreement that ensures compliance with privacy, confidentiality, and ethical use standards. Approved researchers will receive access to the data through secure institutional servers at Sunnybrook Research Institute.