Daniel A. Lichtenstein
The FALLS-protocol  is the main product of the BLUE-protocol, and is used in acute circulatory failure, since it provides a direct parameter of clinical volemia.
The FALLS-protocol is included in a synthetic approach called Limited Investigation (considering hemodynamic therapy), which sequentially rules out the main causes of shock*. After a first approach which uses simple cardiac sonography**, basic diagnoses such as pericardial tamponade and pulmonary embolism can be ruled out***. Lung ultrasound rules out compressive pneumothorax.
At this point, obstructive shock is ruled out.
The absence of B-profile rules out cardiogenic shock ****.
Facing an A-profile*****, the FALLS-protocol begins; fluid therapy is initiated, with narrow monitoring of lung artifacts and clinical signs of circulatory function. Hypovolemic shock *** *** resolves under fluid therapy before any lung fluid overload.
The FALLS-protocol considers the apparition of B-lines, schematically, as the endpoint for fluid therapy, and at the appropriate time for introducing vasopressors in a patient who has not been improved by fluid therapy.
Several blood cultures (plus usual blood tests) are done first for relieving the left heart from this slight, infra-clinical pressure, and positioning it at the ideal point of the cardiac curve, second for detecting the micro-organism. At this point indeed, FALLS-protocol has ruled out obstructive shock, cardiogenic shock and hypovolemic shock. Septic shock is the only remaining cause.
This example of use shows that critical ultrasound is a holistic discipline.
*Anaphyllactic shock, spinal shock, etc, are rare causes, of usually obvious diagnosis.
** Using a simple, two-dimensional unit with a microconvex probe, allowing major simplification of traditional “Echo” (echocardiography-Doppler)
*** Pulmonary embolism yields dilated right ventricle, a suggestive feature when associated with circulatory failure. If cardiac window is poor, the BLUE-protocol  can be used with no drawback for a same result (lung A-profile plus calf DVT provides 99% specificity).
**** Assessment of left heart contractility is done independently from the Limited Investigation (and FALLS-protocol). B-profile associated with well-contractile heart and particular clinical context may indicate noncardiogenic pulmonary edema. Here, assessment of inferior and superior caval veins (using microconvex probe in our protocol) is useful for guiding fluid therapy.
***** Or equivalents (A/B profile, C-profile)
*** *** Or equivalents (hemorragic shock, acute adrenal failure, etc).
1. Chest 136:1014-1020
2. Chest 134:117-125
For knowing more:
Whole-Body Ultrasonography in the Critically Ill (Springer-Verlag, 2010), Chapter 23
Fluid Administration Limited by Lung Sonography: place of lung ultrasound in assessment of acute circulatory failure (the FALLS-protocol). Expert Rev Respir Med 2012; 6(2),1-xxx (in press)