Normovolaemia defined by central blood volume and venous oxygen saturation

Stroke volume optimisation: is the fairy tale over?

“…Their study confirms previous work by Riley and colleagues [2], who performed cephalometric analysis showing that a long hyo-mandibular distance is associated with obstructive sleep apnoea because if the hyoid is unusually low in the neck, a large portion of the tongue mass lies in the hypopharynx rather than in the oral cavity [3,4]. Similarly, difficult laryngoscopy has also been shown to occur in patients with an increased submandible angle [5], probably due to difficulty in displacing a large hypopharygngeal tongue.…”

“…It is believed that individuals are 'normovolaemic' when their stroke volume is at the plateau of the Frank-Starling curve in the supine position [1]. This definition by Truijen et al appears reasonable since when stroke volume is thus located in awake subjects, cardiac output and oxygen delivery are such that maximal venous oxygen saturation is established [2]. Since the curve is flat here, an increase in preload will not increase stroke volume.…”

Detecting Change in Left Ventricular Ejection Time During Head-Up Tilt-Induced Progressive Central Hypovolemia Using a Finger Photoplethysmographic Pulse Oximetry Wave Form

“…3-5,14 -16 Although tilt-induced central hypovolemia is not identical to actual blood loss because the blood volume is merely redistributed to the lower body rather than actually lost from the circulatory system, the initial cardiovascular response to hemorrhage is essentially the same as that elicited by a reduction in central blood volume, for example by head-up tilt or by LBNP. 3,5,6,14 Twenty-four degrees head-up tilt produces a similar cardiovascular response to 15 mm Hg LBNP, 26 which approximates mild hemorrhage (loss of 400 mL-550 mL or ϳ10% of total blood volume), 6 whereas 60 degrees head-up tilt produces a similar central cardiovascular response to 20 mm Hg to 40 mm Hg LBNP, 27 which approximates moderate hemorrhage (loss of 550 mL-1,000 mL or ϳ10%-20% of total blood volume). 6 However, a limitation of using head-up tilt as a model of blood loss is that regional blood volume changes and the associated vascular responses induced by gravitational fluid shift to the lower body may be different to that in actual hemorrhage.…”

“…A detectable drop in BP is a late sign of hypovolemia, which signifies the loss of 30% or more of total blood volume, by which time patients are at high risk of cardiovascular collapse as a result of severe hemorrhagic shock. 1,[3][4][5][6] Delayed control of hemorrhage has been recognized as a major contributor to preventable trauma deaths and has often been related to delays in the assessment or diagnosis of hemorrhage. 7,8 There are potentially large benefits to the critical care clinician if small volume losses could be diagnosed early, accurately, and reproducibly simply by the assessment of a physiologic variable that may be conveniently derived from existing patient monitoring equipment.…”

“…It has been suggested by a number of investigators that the initial hemodynamic response to hemorrhage is essentially the same as that caused by reduction in central blood volume in experimental conditions, such as head-up tilt and LBNP. [3][4][5][6]14 Although the physiology associated with graded head-up tilt is not truly equivalent to that associated with actual blood loss, because the blood volume is merely redistributed to the lower body under gravitational influence rather than actually lost from the circulatory system, it may at least simulate most of the cardiovascular responses to a progressive decline in central blood volume similar to those occurring during hemorrhage. In this study, graded head-up tilt was used as a model to simulate progressive central hypovolemia.…”