Shock Syndromes I

conditions (e.g., use of vasopressors), and assessment method or device. In a systematic review,

thresholds to predict fluid responsiveness were PPV greater than 12.5% and SVV greater than

11.6%.

IVC variation (also termed IVC collapsibility or IVC distensibility) uses echocardiography to

visualize the diameter of the IVC during positive pressure ventilation. With a positive pressure

breath from mechanical ventilation, venous return is impaired, and the diameter of the IVC

increases (IVC distensibility). The opposite change in IVC diameter occurs in patients breathing

spontaneously (IVC collapsibility). The change in IVC diameter during inspiration is higher in

patients who are fluid responsive than in those who are not fluid responsive.

In one study, the AUC ROC for predicting fluid responsiveness for IVC distensibility was

0.84 (95% confidence interval [CI], 0.63–1.0), and the best cutoff was 16% (sensitivity 67%,

specificity 100%).

ii.

In a larger, multicenter study of mechanically ventilated patients with varied causes of

circulatory shock, IVC distensibility had an AUC ROC for predicting fluid responsiveness

of 0.64 for a threshold of 8% (sensitivity 55%, specificity 70%). In the patient subset with

all assessed dynamic markers of fluid responsiveness available, the AUC ROC for predicting

fluid responsiveness was significantly higher for respiratory variations in superior vena cava

diameter (0.74, threshold of 21% or greater) than PPV (0.66, threshold of 11% or greater,

p=0.01) and IVC distensibility (0.65, threshold of 13% or greater, p=0.02). Of note, superior

vena cava diameter changes must be assessed by transesophageal echocardiography, which is

not commonly performed for this purpose in the United States. IVC variation may be assessed

by transesophageal or transthoracic echocardiography.

The passive leg raise (PLR) test measures the hemodynamic effects of a positional change in the patient’s

legs. Changing the position of the patient’s bed such that the patient’s legs are lifted to a 45-degree angle

with the patient’s head placed horizontally leads to a transfer of blood from the abdominal compartment

and lower extremities to the intrathoracic compartment.

This increase in venous return may subsequently increase SV and CO if the patient is preload

responsive.

An increase in CO by 10%–15% after PLR is considered a positive test.

The benefit of the PLR test is that it can be used in spontaneously breathing or nonintubated patients.

In addition, it does not require the administration of fluid (which may be harmful if the patient is

not fluid responsive) and can easily be reversed by returning patients to their previous position.

(or lack thereof). A change in blood pressure is not an adequate surrogate marker for CO, as noted

earlier. The CO measurement may be obtained from an arterial pulse pressure waveform analysis

monitor (minimally invasive approach) or from a bioimpedance device, bioreactance device, or

echocardiographic LVOT VTI (noninvasive approach).

Several caveats exist for using dynamic markers of fluid responsiveness.

PPV and SVV assume the following: sinus cardiac rhythm, the absence of significant valvular

dysfunction, intubation and mechanical ventilation without spontaneous breaths, and tidal volume

of 8 mL/kg or more of predicted body weight. Values for PPV and SVV above the noted thresholds

do not reliably predict fluid responsiveness in the setting of arrhythmias (e.g., atrial fibrillation). If

these assumptions are not fulfilled, PPV and SVV are not reliable in predicting fluid responsiveness.

IVC distensibility also requires intubation and mechanical ventilation without spontaneous breaths

and is not conducive to continuous monitoring.

The real-time response of CO (or lack thereof) with PLR must be assessed using a CO monitoring

device. In addition, intra-abdominal hypertension reduces the ability of PLR to detect fluid

responsiveness.