ResUS Pearl: IVC Ultrasound & Volume Assessment

                 

Background:
Determining when to give intravascular volume to hypotensive patients is an imperfect science. Commonly used methods to assess volume status include urine output, passive leg raise, arterial pulse pressure variation, jugular venous pressure, fluid challenge as well as IVC collapsibility.  US-guided assessment of the IVC is an attractive option because it is rapid, reproducible, noninvasive and does not require specialized monitoring equipment. Of note, studies have varying results about the accuracy of IVC ultrasound to predict fluid responsiveness.

Key Physiology:
The IVC serves as a high-capacity intravascular reservoir for the circulatory system.
In spontaneous ventilation the ambient pressure in the thoracic cavity is slightly negative, becoming more negative during inspiration.  With inspiration blood is driven from the IVC into the right heart, which reduces IVC volume and can result in collapse.With positive pressure mechanical ventilation the ambient pressure is positive (i.e., PEEP) and becomes more positive during inspiration. During expiration this pressure is reduced and the IVC can collapse.
The theory behind IVC ultrasound is that if the IVC collapses more or if its baseline diameter is smaller, this represents lower intravascular volume and may predict volume responsiveness. There are several conditions that will enlarge the IVC diameter, confounding such measurements, including:

Cardiac: tamponade, right heart failure
Pulmonary: tension pneumothorax, pulmonary embolism, pulmonary hypertension, status asthmaticus

The bottom line:
Despite the sweet sweet physiology behind IVC ultrasound, the myriad studies don’t give us reason to rely on it to titrate resuscitation of our hypotensive patients, although the data is promising at the extremes of the IVC diameter. LifeInTheFastLane has a great summary.

A very collapsible or very small IVC is likely fluid responsive
Spontaneously breathing patients: IVC diameter <0.9cm
Ventilated patients: IVC diameter <1.2 cm or 18%+ collapse

A very large or non-collapsible IVC isn’t fluid responsive
Ventilated patients: IVC diameter >2.5cm

References

Submitted by: K Tiemeier, MD