Sonography has traditionally been used to assess anatomic abnormalities.
However, its value in evaluating physiologic characteristics has recently been
recognized, particularly in the care of patients in shock. As the use of
point-of-care sonography grows in critical care and emergency medicine,
noninvasive assessment of intravascular volume status is increasingly being
used to guide therapy of the critically ill.
Although intravenous fluid is often the initial treatment in
hypotensive patients, aggressive volume resuscitation may be detrimental in
some patients and in certain types of shock. Accurate diagnosis of shock state
can be challenging because physical findings of hypovolemic, distributive,
cardiogenic, and obstructive shock often overlap. Pulmonary artery and central
venous pressure catheters, which provide physiologic data such as cardiac
output and right atrial pressure, are time-consuming, invasive, and carry
considerable risks. Central venous pressure has long been used to guide fluid management;
however, data suggest that in critically ill patients, central venous pressure
may not correlate with the effective intravascular volume. Furthermore, invasive hemodynamic monitoring has not been
shown to benefit patients.
Given the importance of determining intravascular volume in shock,
a rapid bedside sonographic examination can be instrumental in guiding medical
management of critically ill patients. Multiple sonographic protocols now exist
for the evaluation of shock, dyspnea, and cardiac arrest. This article will describe the use of sonography of the inferior vena cava (IVC) in the evaluation of
patients in shock.
Physiology: IVC
Parameters
The IVC is a compliant vessel that distends and collapses with pressure
and volume changes. Although the absolute IVC size varies widely among healthy
individuals and may not by itself be diagnostic, the maximal IVC diameter has
been shown to be lower in patients with hypovolemia.5
A better indicator of intravascular volume is collapsibility of the
IVC. As intrathoracic pressure decreases with inspiration, venous blood is
pulled from the lower half of the body into the right atrium. This action
causes a transient, but normal, decrease in the IVC diameter. With expiration,
the IVC diameter increases and returns to its baseline. These changes are known
as respirophasic variability. The IVC collapsibility index, also known as the
caval index, is defined as the difference between the maximal (expiratory) and
minimal (inspiratory) IVC diameters divided by the maximal diameter. The caval
index is used in spontaneously breathing patients to estimate right atrial
pressure.6,7 In patients with
minimal respirophasic collapse, having the patient inspire forcefully, or
sniff, will differentiate between patients with poor inspiratory effort and
those with elevated right atrial pressure. The sniff method may provide more
accurate estimation of volume status; however, measurements taken during normal
respiration are reasonably accurate as well.8
Recent guidelines from the American Society of Echocardiography support
the general use of IVC size and collapsibility in assessment of volume status.9 Studies have
suggested the use of specific parameters for maximal IVC diameter and caval
index to predict volume status.6,8 In one of these
studies, using 2 cm as the cutoff for the maximal IVC diameter resulted in good
sensitivity and specificity for predicting elevated right atrial pressure.8 A caval index
greater than 50% suggests a low volume state,6 especially in
combination with a small IVC diameter. Conversely, a low caval index with a
large IVC diameter suggests a high volume state.
Inferior vena cava size does not predict right atrial pressure in
patients receiving mechanical ventilation.10 Mechanical
ventilation reverses the hemodynamics of venous return during the respiratory
cycle. During positive pressure inspiration, intrathoracic pressure is
increased, impeding blood flow from the IVC to the right atrium. During
expiration, intrathoracic pressure is lower, and venous return increases. In a
patient with normal right atrial pressure, this cyclic venous return produces
minimal variation of the IVC size during the respiratory cycle. When a patient
is volume depleted, however, the right atrium and IVC become more compliant,
and the IVC size increases with positive pressure inspiration. Assessment of
the IVC has been used in mechanically ventilated patients to predict whether
fluid expansion is expected to increase the stroke volume and cardiac output.
The variation of the IVC in positive pressure ventilation, known as the IVC
distensibility index, is the difference between the maximum and minimum IVC
diameters divided by the minimum diameter. In contrast to IVC collapsibility,
which indicates volume status, the distensibility index has been used to assess
preload dependence and predict fluid responsiveness such that the absence of
respiratory variation suggests that volume expansion is unlikely to be
effective.11,12 Fluid
responsiveness is an emerging and important concept in critical care that seeks
to avoid unnecessary fluid administration, which may expose the patient to
risks of volume overload, when a fluid challenge is not expected to improve
hemodynamics and organ perfusion.
Anatomy and Scanning
Technique
A low-frequency phased array transducer (3.5–5 MHz) is used to evaluate
the IVC, which lies in the retroperitoneum to the right of the aorta. It is
differentiated by its thinner walls and respiratory flow variation. The IVC
passes posterior to the liver and is joined by the hepatic veins before it
enters the thoracic cavity and drains into the right atrium. There exists
considerable variability in the literature regarding the location at which the
IVC diameter should be measured. Most studies agree that the measurement should
be distal to the junction with the right atrium and within 3 cm of that point.6,8,12–14 Other studies
measure the IVC at or near the junction with the hepatic veins.11,15–20 A study
comparing commonly measured locations found that respiratory variation of the
IVC at the junction with the right atrium did not correlate with variation at
sites distal to the hepatic veins.21 Guidelines from
the American Society of Echocardiography recommend an assessment of the IVC
just proximal to the hepatic veins, which lie approximately 0.5 to 3 cm from
the right atrium.9
To image the IVC, the probe is placed in the subxiphoid 4-chamber
position with the probe marker oriented laterally to identify the right
ventricle and right atrium. As the probe is progressively aimed toward the
spine, the convergence of the IVC with the right atrium will be seen. The IVC
should be followed inferiorly, specifically looking for the confluence of the
hepatic veins with the IVC (Figure 1). The IVC
can also be evaluated in the long-axis plane. For this view, the probe is
turned from a 4-chamber subxiphoid to a 2-chamber subxiphoid orientation, with
the probe now in a longitudinal orientation (Figure 2). Although
this view allows visualization of the IVC throughout the length of the hepatic
segment, the true size of the IVC may be underestimated in the long axis due to
a common error known as the cylinder tangent effect. This effect occurs when
the ultrasound beam travels through the vessel longitudinally in an
off-centered plane. One way to avoid underestimating the size of the IVC is to
angle the probe laterally and medially until the greatest dimension is
identified.
The diameter of the IVC should be measured perpendicular to the long
axis of the IVC at end-expiration and end-inspiration. The finding of a
small-diameter IVC with large inspiratory collapse (high caval index)
correlates with low volume states. This phenomenon may be observed in
hypovolemic and distributive shock states (Figures 3 and 4 and Videos 1 and
2). Conversely, a large IVC with minimal collapse (low caval index) suggests a
high volume state such as cardiogenic or obstructive shock (Figures 5 and 6 and Videos 3 and
4). Movement of the diaphragm, especially during forceful inspiration or sniffing,
may displace the IVC relative to the probe, making it difficult to obtain
comparative measurements at the same location. In the short axis, the probe may
need to be angled inferiorly during inspiration to locate the site measured at
expiration. In the long axis, displacement of the IVC may require angling
inferiorly and/or laterally (to avoid tangential measurement). In either
orientation, it is recommended to observe the changes of the IVC through
several respiratory cycles.
M-mode Doppler sonography of the IVC can be used to graphically
document the absolute size and dynamic changes in the caliber of the vessel
during the patient's respiratory cycle in both short and long axes (Figures 7–10). It should be
noted, however, that M-mode sonography may introduce inaccurate measurements
due to the displacement of the IVC relative to the probe during inspiration.
Movement of the IVC out of the plane of the M-mode cursor may appear as vessel
collapse on the M-mode tracing. It is therefore recommended that M-mode
sonography be used after adequately visualizing IVC variability in the B-mode
to avoid inaccurate estimation of vessel size and collapse.
Further studies are needed to define normal IVC parameters such as
size, collapsibility, and distensibility (in mechanically ventilated patients).
Until then, assessment of IVC collapsibility is useful in the critically ill
patient whose caval index approaches the extremes. Additionally, caval
sonography can be repeated during resuscitation to evaluate improvement of
these parameters.
Evidence
Incorporation of a goal-directed sonographic protocol including
assessment of the IVC has been shown to improve the accuracy of physician
diagnosis in patients with undifferentiated hypotension.22 In a recent
prospective study, point-of-care sonography evaluating cardiac contractility
and IVC collapsibility in patients with suspected sepsis was shown to increase
physician certainty and alter more than 50% of treatment plans.23 Inadequate
dilatation of the IVC after a fluid challenge was more sensitive than blood
pressure for identification of hypovolemia in trauma patients.24 Another study in
trauma patients showed the value of bedside caval sonography in evaluation of
fluid status and resuscitation of critically ill patients.25 A study in
acutely dyspneic patients presenting to the emergency department showed that
IVC sonography rapidly identifies patients with congestive heart failure and
volume overload.26
Rather than relying on a single measurement of the IVC, it may be more
useful to follow changes in vessel size and collapsibility over time in
response to an intervention. Studies have shown a decrease in the IVC diameter
and increased collapsibility after blood loss15 and fluid
removal during hemodialysis.27 In hypotensive
emergency patients, volume resuscitation was associated with increases in the
IVC diameter and less inspiratory collapsibility.14 Just as a single
blood pressure measurement is an incomplete representation of the hemodynamic
status of a patient, sonography of the IVC should be repeated after
interventions or changes in clinical parameters. Monitoring of the IVC diameter
during resuscitation is an emerging area of research, and further studies are
necessary to determine the exact parameters to interpret IVC size and
collapsibility.
Pitfalls
The IVC should be followed to the junction with the right atrium to
avoid misidentification with the aorta. Because a single long-axis view may be
inaccurate, it is recommended to assess the IVC in both short and long axes.
Inferior vena cava determinations should be made at or near the confluence with
the hepatic veins. Measurements elsewhere may not reflect intravascular volume.
A dynamic evaluation of the degree of IVC collapse with inspiration may
correlate better with the intravascular volume than a single static measurement
of the vessel size. Inferior vena cava size does not predict right atrial
pressure in patients receiving mechanical ventilation. Care should be taken to
maintain adequate visualization of the IVC throughout the respiratory cycle
because the probe and IVC may be displaced by diaphragmatic and abdominal wall
movements. Overestimation of intravascular volume may occur in conditions that
impede flow to the right heart, including valvular abnormalities, pulmonary
hypertension, and heart failure.
Interpretation of caval physiology is hindered by conditions that
restrict the physiologic variability of the IVC, such as liver cirrhosis and
fibrosis,28 masses causing
external compression, and elevated intra-abdominal pressure. Interpretation of
the physiologic characteristics of the IVC should be done in context with the
patient's clinical scenario and adjunctive data.
Conclusions
Determination of shock state in critically ill patients is challenging,
but caval sonography may be a substitute for invasive hemodynamic monitoring.
Assessment of the physiologic characteristics of the IVC provides a rapid
distinction between low and high volume states and offers the clinician a
rapid, noninvasive way to guide resuscitation in critically ill patients. In
addition to caval sonography, focused echocardiography and lung sonography have
been suggested by an increasing number of resuscitation sonography protocols to
further evaluate patients in shock.
