Volume assessment - Saving Lives With Sound

Assessment of volume status

Divining hydration status with the power of ultrasound

Assessment of volume status is a question that plagues those professionals who care for acutely unwell patients. While for some the answer may be obvious, there are a broad cross section of patients who need supplementary data to answer this question; this is precisely where ultrasound fits in.

The following section will breakdown key concepts in ultrasound guided volume status assessment and will conclude with a video tutorial.

The concept of volume status is broken down into 2 key concepts. First, will this patient respond to fluids? This may manifest in an increase in blood pressure or reduction in heart rate, at least in the short term. For many patients, however, the impact can be short-lived or the effect nuanced, such as an improvement in kidney function 24 hours later. This, however, does not take into account whether fluids themselves may actually harm the patient.

Second, how much fluid to give? While we may recognize the patient who slips into frank pulmonary edema, this is clearly not an acceptable end-point. While the real answer may be “just enough and not a drop more”, knowing the limit is where the rubber hits the road. Volume tolerance has emerged as a key concept in defining this literature.

Section contributors: Cristian Vadeanu MD, Brian Buchanan MD FRCPC

Inferior vena cava

The inferior vena cava enters the right atrum from the cephalad position. The diagram below demonstrates key numbers in estimating the right atrial pressure. While such tables can help estimate RAP in a spontaneously breathing patient, it is unlikely that such estimates correlate to states of volume responsiveness. One exception to this, however, is the patient with a collapsing, flat IVC, which is *likely to suggest they are fluid responsive.

IVC in mechanical ventilation

On positive pressure ventilation the IVC distends during inspiration (rather than collapsing). In this image, we demonstrate the “distensibility index”.

Superior vena cava collapsibility index

The superior vena cava can be imaged with the trans-esophageal approach. M-mode can be applied 90 degrees to the SVC to measure the collapsibility index.

LVOT VTI variation

Stroke volume measurement can be measured with PW Doppler at the left ventricular outflow tract. If you wish to find out more about measurement of this variable, press the button below.

LVOT VTI variation can also be used with passive leg raise.

Carotid blood flow

With a linear frequency probe applied in transverse position just below the carotid bulb, PW Doppler can be used to characterize the carotid blood flow waveform. Below are the 3 techniques that can be used to measure for change in carotid flow, in response to a fluid challenge.

Volume tolerance

B-lines and interstitial findings

B-lines are a marker of interstitial thickening. The finding of diffuse B-lines over multiple segments raises the suspicion of fluid overload. Nonetheless, it is wise to be mindful of the differential diagnosis of this interstitial pattern.

Hepatic vein flow

The hepatic vein can be interrogated with PW Doppler, just prior to where it enters the IVC. The typical IVC is tetra-inflectional; two waves above the baseline (A, V-waves) and two waves below (S, D-waves). The typical apprance is the S is greater than the D-wave. In volume overload, the D-wave becomes progressively more dominant (sinus rhythm). A major confounder to this is severe tricuspid regurgitation, which results in systolic flow reversal (S-wave goes above baseline).

Portal vein

From the right mid-axillary line the portal vein can be found. The typical blood flow is antegrade towards the probe (into the liver). Normal flow is continuous with little pulsatility. As a patient becomes more overloaded, this portal venous flow can become progressively more pulsatile (See video for more details). Be aware that there are a number of caveats to this pulsatility or flow pattern.

Inter-lobar renal vein flow

With PW applied to the inter-lobar renal veins (junction of medulla and cortext). A combination of arterial flow (above the baseline) and venous flow (below the baseline). For the purposes of this guide, we are strictly examining venous flow. Normal venous flow is below the baseline and continuous. As a patient becomes overloaded, this pattern becomes discontinuous and biphasic (two waves, D & D). Finally it becomes monophasic (only D wave).

Volume status video tutorial

In this tutorial, Dr. Cristian Vadeanu reviews the concepts of volume responsiveness and volume tolerance.