Interrogation of the inferior vena cava (IVC) plays an important role in the performance of critical care echocardiography and provides insight into the systemic hemodynamics of patients with critical illness. In conjunction with clinical examination, IVC assessment can be used as a surrogate marker of right atrial pressure and fluid responsiveness (1–3). Non-invasive measurements of IVC size and collapsibility may allow one to grossly infer central venous pressure (1–3). Sonographic capture of the IVC can be performed readily with good inter-rater reliability after a brief training period (4). The arguments for and against the use of IVC as a surrogate marker for volume status and fluid responsiveness are well described in the recent published literature (5–8), Although the IVC can imaged readily after a brief, focused training period,(4) clinical integration requires thoughtful consideration of contextual factors.
Important considerations include:
- Clinical assessment and pre-test probability of hypovolemia, hypervolemia or euvolemia?
- Spontaneously-breathing or mechanically-ventilated?
- Risk of error in integration or sonographic acquisition (9)
- Confounding factors, which may invalidate conventional principles of IVC assessment including pericardial tamponade, pre-existing pulmonary hypertension, abdominal compartment syndrome, severe tricuspid regurgitation, right ventricular myocardial infarction, others (10)
In the following section, we will outline key principles of IVC assessment including image acquisition, assessment, and integration.
Image acquisition for IVC assessment
With the patient in the supine position and the probe placed in the sub-xyphoid position with marker cephalad, the IVC lies to the anatomical right of midline. Three components of the IVC must be identified; an anechoic tubular structure that traverses through the liver, right atrium-IVC junction, and hepatic vein (HV).
Differentiating the IVC from the abdominal aorta
It is crucial to recognize that the aorta, which lies to the anatomic left of midline, may mimic the appearance of the IVC. The aorta is covered in echogenic adipose tissue and does not join the right atrium. These structure are easily visualized, except in cases of intervening air in the abdominal viscera.
Measurement of IVC size and collapsibility can be performed in the long axis view, perpendicular to direction of the IVC and approximately 0.5 to 3.0 cm proximal to RA entrance. Measurement should be performed at end-expiration in the spontaneously breathing patient. An additional measurement can be performed by asking the patient to perform a gentle sniff or quiet inspiration for measurement of collapsibility. M-mode imaging is generally used to perform the measurement but does introduce an additional concern. If the beam is off the central axis of the IVC the widest diameter may not be displayed.
The IVC dynamically responds to respiro-phasic cycling and is different depending on whether the patient is breathing spontaneously or is mechanically ventilated. In a patient with spontaneous breathing, the IVC will decrease in size (diameter) secondary to negative intra-thoracic pressure during inspiration. By analyzing the IVC size and collapsibility, we can derive a gross right atrial pressure (as shown in the table to the left). Alternatively, positive pressure ventilation leads to IVC distension with inspiration (commonly called a 'distensibility index'). IVC measurements on mechanical ventilation will be covered in the video below.
The following video tutorial describes sonographic assessment of the IVC in volume status. Also discusses considerations and confounding factors in assessment of the critically ill patient.
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