From the Guidelines
The diameter of the Inferior Vena Cava (IVC) decreases in response to inspiration due to the negative intrathoracic pressure leading to an increase in right ventricular filling from the systemic veins. This physiological response is a result of the changes in intrathoracic pressure during inhalation, where the diaphragm contracts and moves downward, creating negative pressure in the thoracic cavity, which helps draw blood into the right atrium of the heart from the IVC 1. The increased abdominal pressure from the descending diaphragm also partially compresses the IVC, causing it to narrow slightly.
Key Points
- The IVC diameter should be measured in the subcostal view with the patient in the supine position at 1.0 to 2.0 cm from the junction with the right atrium, using the long-axis view 1.
- The diameter of the IVC and the percentage decrease in the diameter during inspiration correlate with right atrial pressure, which can be quantified as the collapsibility index 1.
- A brief sniff is often required to evaluate the inspiratory response, as normal inspiration may not elicit this response 1.
- The degree of IVC collapse during respiration can be used clinically to assess volume status in patients, indicating hydration status or right heart function.
Clinical Implications
The changes in IVC diameter during inhalation are a normal physiological response that helps regulate venous return to the heart during the respiratory cycle. Understanding these changes is essential for clinicians to accurately assess patient volume status and right heart function. By measuring the IVC diameter and its response to inspiration, clinicians can gain valuable insights into a patient's hemodynamic status, guiding treatment decisions and improving patient outcomes.
From the Research
Effects of Inhalation on the Inferior Vena Cava (IVC)
- During inhalation, the Inferior Vena Cava (IVC) may experience a compressive effect due to the descent of the diaphragm, leading to a decrease in its diameter 2.
- The IVC 'collapsibility index' is calculated as (IVC(max)-IVC(min))/IVC(max), and diaphragmatic breathing has been shown to result in a higher collapsibility index compared to chest wall breathing 2.
- Real-time flow MRI studies have demonstrated that inspiration during deep breathing can elicit brief periods of flow reversal in the IVC, with mean peak velocities of -21 cm/s 3.
- The flow pattern in the IVC can be classified as "cardiac", "respiratory", or "mixed" type, and the subject-specific IVC flow pattern persists during deep breathing although the enhanced respiratory influence may shift subjects from "cardiac" to "mixed" or from "mixed" to "respiratory" type 3.
Relationship between IVC Flow and Respiration
- IVC flow is dominated by the acquired respiratory behavior of individual subjects, and its pressure-induced alterations relative to cardiac pulsation 3.
- Inspiration facilitates IVC flow at rest, but this effect diminishes with exercise, when the peripheral pump seems to be more important 4.
- The waterfall phenomenon normally occurs in the IVC during inspiration just before it enters the thorax, and this phenomenon disintegrates with elevation of the central venous pressure 5.
Clinical Implications
- Respiratory movements have a major influence on IVC dynamics, and the increase in intracaval pressure during Valsalva maneuver results in a significant decrease in the IVC cross-sectional area 6.
- The collapsibility indices of the IVC can be calculated and correlated with venous pressures and cross-sectional areas, providing valuable information for clinical diagnosis and management 6.