“…Hypovolemic shock is the hemodynamic response to a critically reduced central blood volume (CBV) and its diagnosis has challenged clinicians since the Second World War (Grant and Reeve, 1941 ; Secher and Van Lieshout, 2016 ). The main treatment consists of intravenous volume administration (Secher and Van Lieshout, 2005 ) to raise cardiac output (CO) and improve microvascular blood flow (Vincent and De Backer, 2013 ; Perner and De Backer, 2014 ; Secher and Van Lieshout, 2016 ) and tissue oxygen delivery (Zollei et al, 2013 ; Simon et al, 2015 ). However, detection of a clinically relevant blood volume deficit remains difficult (Marik et al, 2011 ; Vincent and De Backer, 2013 ; Bronzwaer et al, 2015 ; Secher and Van Lieshout, 2016 ) because the blood volume is not only characterized by its magnitude but also by its function as preload to the heart (Marik et al, 2011 ; Bronzwaer et al, 2015 ; Secher and Van Lieshout, 2016 ).…”