Focused assessment with sonography for trauma (FAST) is the most common application of sonography for the initial evaluation of trauma patients ( ). The purpose for using FAST is to search for any fluid collection in the intraabdominal and pericardial spaces, which suggests organ injury near the fluid. The main reason for assessment of pericardial space in these patients is determination of possible pericardial hematoma or effusion from blunt or penetrating trauma to the chest or upper abdominal region, which may subsequently lead to cardiac tamponade and circulatory collapse. In some patients however, visualization of pericardium is not possible because of natural body build of the patient, or interposition of gas-distended stomach causing distortion of sonographic images.
Evaluation of inferior vena cava (IVC) and monitoring the diameter of IVC is an alternative way to determine the presence of cardiac tamponade. The normal IVC diminishes in caliber with inspiration and increases in caliber during expiration ( ). Changes in the caliber of IVC are attributed to variations in blood flowing through the inferior vena cava in accordance with the respiratory and cardiac cycles. In inspiration, blood is literally sucked into the chest by negative pressure, causing the vessel to collapse, and these changes are reversed in expiration causing ballooning of the inferior vena cava. In cardiac tamponade, external pressure over the right side of the heart increases, which in turn leads to impaired emptying of the IVC during inspiration; hence normal fluctuation of the IVC diameter seen in the respiratory cycle of a normal individual is lost. Monitoring of IVC diameter and its changes is best done by M-mode sonographic examination of the IVC. Examinations can be performed in the supine position with the ultrasound transducer placed in a subxiphoid location imaging the sagittal sections of the IVC behind the liver to measure the maximal diameter of the IVC during expiration (IVCe) and the minimal diameter of the IVC during inspiration (IVCi). The difference between the diameters of IVCe and IVCi then will be regarded collapsibility, and collapsibility index will be defined as IVCe-IVCi/IVCe ( Fig 1 ). Minutiello revealed that collapsibility index of ≥20% is associated with normal cardiac function ( ). Therefore, finding a collapsibility index of <20% in a trauma patient can be suggestive of cardiac tamponade. Other potential causes of decreased collapsibility index of IVC are right-sided heart failure and severe tricuspid insufficiency; however, most trauma patients are young and unlikely to have these conditions. Therefore measuring the collapsibility index can be very helpful in diagnosing patients with cardiac tamponade and can guide us to perform more specific examinations such as echocardiography or chest CT scan in suspicious patients. This measurement can be added to FAST sonography of trauma patient with minimum additional time.
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Figure 1
Measurement of inferior vena cava diameter in a normal patient using M-mode sonography. The diameters of inferior vena cava during expiration and during inspiration are 1.27 and 0.58 cm, respectively; therefore, the collapsibility index will be 54%.
References
1. Scalea T.M., Rodriguez A., Chiu W.C., et. al.: Focused assessment with sonography for trauma (FAST): results from an international consensus conference. J Trauma 1999; 46: pp. 466-472.
2. Grant E., Rendano F., Sevine E., et. al.: Normal inferior vena cava: caliber changes observed by dynamic ultrasound. Am J Roentgenol 1980; 135: pp. 335-338.
3. Rein A.J., Lewis N., Forst L., et. al.: Echocardiography of the inferior vena cava in healthy subjects and in patients with cardiac disease. Isr J Med Sci 1982; 18: pp. 581-585.
4. Minutiello L.: Non-invasive evaluation of central venous pressure derived from respiratory variations in the diameter of the inferior vena cava. Minerva Cardioangiol 1993; 41: pp. 433-437.