in the past 5 years , hand - carried ultrasound ( hcus ) devices have joined the intensivist 's toolbox for guiding invasive procedures , for analyzing the pathophysiological basis for circulatory shock , and for predicting fluid responsiveness by revealing inferior vena caval collapsibility . in a recent paper , monge garca and colleagues demonstrated that respiratory - induced variation in brachial artery peak flow velocity ( vpeakba ) could be measured with hcus and the result predicted the cardiac output response to a fluid challenge . fluid therapy for critically ill , hemodynamically unstable patients presents clinicians with a dilemma . on the one hand , a fluid bolus may augment cardiac output , improve critical organ perfusion , and even save the patient 's life . on the other hand , fluid may confer no hemodynamic benefit , while adding to pulmonary edema , amongst other ills . how often is a fluid bolus harmful , as opposed to helpful ? when an intensivist judges that a fluid bolus is necessary , only one - half of patients respond with a meaningful boost in cardiac output . especially for patients most likely to be harmed ( for example , those with concomitant acute lung and kidney injury ) , knowing whether fluids will enhance perfusion should be clinically valuable . historically , clinicians have relied on static hemodynamic parameters , such as the central venous pressure , to judge whether fluids are likely to aid the circulation . a multitude of studies , accumulating for more than two decades , show that the central venous pressure and its more invasive cousin , the pulmonary artery occlusion ( or wedge ) pressure , are no more reliable than a coin toss in forecasting whether an individual subject will respond positively to a fluid bolus . when seen in subjects with sepsis , with acute respiratory failure , or following cardiac surgery , this lack of predictive accuracy was attributed to effects of surgery or positive end - expiratory pressure on , for example , ventricular compliance . how disturbing , then , to find that the central venous pressure and pulmonary artery occlusion pressure fail to correlate with ventricular volumes or fluid responsiveness even in healthy normal individuals ! in contrast to the failure of static measures , a novel set of predictors that rely on perturbing the circulation accurately foretell whether fluids will augment cardiac output . these dynamic measures generally employ controlled mechanical ventilation to raise the pleural pressure ( some alternatively depend on raising the legs , measuring the effect of spontaneous breathing , or altering the positive end - expiratory pressure ) and quite accurately predict fluid responsiveness . in the passive patient , the stroke volume varies with ventilation to a degree that reflects whether the ventricles are operating on the rising or flat portion of the starling function curve . patients whose circulations can respond to fluids will therefore demonstrate substantially greater cyclical variability in stroke volume . as the stroke volume changes , so vary the systolic pressure , the pulse pressure , and the aortic blood flow velocity . similar cardiopulmonary interactions explain that changes in the diameter of the inferior vena cava also predict fluid responsiveness . at the same time , dynamic predictors have limitations : a regular cardiac rhythm is required ; most predictors rely on a fully passive patient ventilated with a tidal volume higher than that usually accepted ( 8 to 10 cm3/kg ) ; and many of the measures are invasive , requiring , for example , an arterial catheter . we have previously shown that vpeakba measured by hcus mirrors the respiratory changes in arterial blood pressure transduced through a radial artery catheter , and suggested that this might serve as a non - invasive parameter for gauging fluid responsiveness . monge garca and colleagues have confirmed and extended this work by directly correlating vpeakba to the impact of a fluid challenge . subjects had a regular cardiac rhythm , lacked respiratory efforts ( assured by examination of ventilator waveforms and , if signs of effort were seen , by neuromuscular blockade ) , were ventilated with tidal volumes of 8 to 10 cm / kg ideal body weight , and were judged fluid responsive if the stroke volume index increased by at least 15% after 500 cmcolloid . the primary finding was that vpeakba > 10% predicted fluid responsiveness with a sensitivity of 74% and a specificity of 95% . first , as others have shown , radial artery pulse pressure variation quite accurately predicted the response to fluid , with a value > 10% being both sensitive and specific ( 95% and 95% ) . second , the mean arterial blood pressure increased 13 mmhg in the nonresponders , confirming that this simple vital sign can not serve as a surrogate for changes in perfusion . finally , the central venous pressure performed poorly ( area under the receiver operator characteristic curve only 0.64 ) . dynamic predictors ( especially pulse pressure variation ) are clearly superior to static pressures , but the role of vpeakba is less certain . first , a screening test demands high sensitivity ( not specificity ) . for the clinician to withhold a fluid challenge , the predictor must identify nearly all patients capable of responding , otherwise too many patients will be denied a potentially life - saving therapy . a sensitivity of 74% does not meet this test . second , a rapid , non - invasive monitor such as hcus might have greatest application in the field or very early in resuscitation , before invasive lines are placed . yet the risk - benefit dilemma posed by fluid bolus is rare in the field - renal failure is not established and the likelihood of responding to fluid is surely much higher than the 50% range typical of intensive care unit patients . while the study of monge garca and colleagues corroborates the view that fluid responsiveness is best predicted dynamically , further work is needed before vpeakba finds arole in clinical practice . vpeakba : variation in brachial artery peak flow velocity ; hcus : hand - carried ultrasound .
dynamic predictors are clearly superior to static pressures in predicting whether a patient will respond to a fluid bolus . 
 hand - carried ultrasound ( hcus ) can measure changes in blood flow velocity in the brachial artery that parallel arterial pulse pressure variation . 
 the potential for hcus to guide fluid therapy non - invasively must overcome problems of sensitivity and applicability .