Respiratory Variation of the Inferior Vena Cava

The increase in intrathoracic pressure with positive-pressure mechanical ventilation can induce changes in the diameter of the inferior vena cava (IVC). Due to the IVC’s high compliance, the changes are more likely to be observed in the case of hypovolemia than in normo- or hypervolemia. In patients receiving positive- pressure ventilation, respiratory variation of the diameter of the IVC has been demonstrated to be a reliable predictor of fluid responsiveness [59]. As with other dynamic measures based on heart-lung interactions, IVC variation has only limited predictive value in the case of spontaneous breathing activity [60]. Additionally, low lung compliance and mechanical ventilation with a low tidal volume should theoretically minimize the effect of ventilation on the vena cava diameter and may thus invalidate the method. By contrast, IVC collapsibility to assess fluid responsiveness can be used in the setting of cardiac arrhythmias. The respiratory variation of the inferior vena cava is simple to measure by transthoracic echocardiography, which represents an important advantage. In fact, the subcostal ultrasonic window may prove to be the only window providing an acceptable image in patients with ARDS due to the increased echogenicity of the lungs. Ultrasonic assessment of the IVC may be particularly useful for assessing volume responsiveness in the early phase of care, before arterial cannulation has been performed.