From the Guidelines
Ultrasound assessment of the inferior vena cava (IVC) under US is a recommended tool to evaluate fluid responsiveness in critically ill patients, as it provides valuable information on volume status and can guide fluid management decisions. To perform this assessment, measure the IVC diameter and its respiratory variation using a low-frequency probe in the subxiphoid view 1. A collapsible IVC (>50% decrease in diameter during inspiration in spontaneously breathing patients or during expiration in mechanically ventilated patients) suggests fluid responsiveness, while a distended, non-collapsible IVC (diameter >2.1 cm with <50% respiratory variation) indicates fluid overload.
Some key points to consider when using IVC assessment include:
- Obtain measurements 2-3 cm from the IVC-right atrial junction during normal respiration
- Calculate the IVC collapsibility index as: [(maximum diameter - minimum diameter)/maximum diameter] × 100%
- A collapsibility index >40-50% in spontaneously breathing patients suggests fluid responsiveness
- Be aware of limitations, including obesity, increased abdominal pressure, right heart failure, and severe pulmonary hypertension, which may affect accuracy
The use of IVC assessment is supported by recent guidelines, which recommend its use as a basic skill for intensivists in the general and neuro intensive care unit population 1. Additionally, other studies have demonstrated the usefulness of IVC assessment in predicting fluid responsiveness, including a study that found a sensitivity of 0.72 and a specificity of 0.91 for pulse pressure variation to predict fluid responsiveness in patients with sepsis or septic shock 1.
However, it is essential to integrate IVC assessment with clinical context and other hemodynamic parameters for optimal fluid management decisions, as the use of IVC assessment alone may not be sufficient to guide fluid management in all patients 1. Furthermore, the assessment of severe hypovolemia using IVC diameter and chamber sizes should be considered a basic skill, while the use of US for determination of fluid responsiveness in patients with persistent shock in the absence of features of hypovolemia is not recommended 1.
In summary, IVC assessment under US is a valuable tool for evaluating fluid responsiveness in critically ill patients, and its use should be integrated with clinical context and other hemodynamic parameters to guide fluid management decisions.
From the Research
IVC Under US to Evaluate Fluid Responsiveness
- The use of ultrasound (US) to evaluate the inferior vena cava (IVC) for fluid responsiveness has been studied in various patient populations 2, 3, 4, 5, 6.
- A systematic review and meta-analysis found that the pooled area under the curve, logarithmic diagnostic odds ratio, sensitivity, and specificity of IVC diameter or its respiratory variations measured by US in predicting fluid responsiveness were 0.71,2.02,0.71, and 0.75, respectively 2.
- However, another study found that IVC collapsibility, as measured by point-of-care ultrasound (POCUS), performed well in distinguishing fluid responders from non-responders among spontaneously breathing critically-ill patients, with an area under the curve (AUC) of 0.84 3.
- The IVC collapsibility index (CI) has been found to have a pooled sensitivity of 71% and specificity of 81% for predicting volume responsiveness in spontaneously breathing patients, and a pooled sensitivity of 75% and specificity of 82% in mechanically ventilated patients 4.
- The value of the IVC area distensibility index and its diameter ratio for predicting fluid responsiveness in mechanically ventilated patients has also been evaluated, with an area under the receiver operating characteristic curve of 0.749 and 0.829, respectively 5.
- A systematic review and meta-analysis found that respiratory variation in IVC diameter had limited ability to predict fluid responsiveness, particularly in spontaneously ventilating patients, with a pooled sensitivity and specificity of 0.63 and 0.73, respectively 6.
Key Findings
- IVC diameter or its respiratory variations measured by US may not be a reliable method to predict fluid responsiveness 2.
- IVC collapsibility, as measured by POCUS, may be useful in distinguishing fluid responders from non-responders among spontaneously breathing critically-ill patients 3.
- The IVC CI may be a useful tool for predicting volume responsiveness in both spontaneously breathing and mechanically ventilated patients 4.
- The IVC area distensibility index and its diameter ratio may be useful for predicting fluid responsiveness in mechanically ventilated patients 5.
- Respiratory variation in IVC diameter has limited ability to predict fluid responsiveness, particularly in spontaneously ventilating patients 6.