A multidisciplinary consensus on dehydration: definitions, diagnostic methods and clinical implications

Figures & data

Figure 1. Schematic representation of the body fluid compartments in humans and their relative sizes. The approximate absolute volumes of the compartments (in litres) are based on a 70 kg adult. TBW: total body water; ICF: intracellular fluid; ECF: extracellular fluid; ISF: interstitial fluid; IVF: intravascular fluid.

Table 1. Distribution of the fifteen consensus statements across the three subject areas.

Subject area	Consensus statements produced within subject area
Dehydration: description and definition	Statements 1–5
Objective markers of dehydration	Statements 6–12
Physiological impact of dehydration and patient outcome	Statements 13–15

Table 2. Dehydration research recommendations generated from all the working groups.

Research recommendations
1.	A large scale “landscaping” study that looks at the prevalence of abnormal hydration, as measured by biochemical measures. Further, the relationship between these surrogate measures of dehydration, their changes during admission and the relationship with coded clinical outcomes (e.g. mortality, ICU admission, hospital length of stay, specific conditions) should be sought
2.	A study looking at the relationship between markers of abnormal hydration (e.g. pOsm) with markers of tissue perfusion (e.g. subcutaneous oxygen tension, microvascular flow) and circulating volume (e.g. direct co-extensive plethysmography)
3.	A prospective, interventional study that targets parametres of normal hydration (e.g. plasma osmolarities 280–300 mOsm/kg) and determines whether this translates to health and health economics co-benefits
4.	The causality of the association between pOsm thresholds and adverse outcomes needs to be tested through interventional studies
5.	Further work looking at the correlation of TBW (and/or pOsm) with bioresistance readings and its use in various patient populations
6.	The development of a suitable device for the routine, bedside measurement of plasma osmolality
7.	The development of quantitative measures of volume status, including the validation of peripheral venous waveform analysis and biomarkers in hypovolaemic humans

Figure 2. The homeostatic responses of the two major forms of dehydration: hypertonic (primarily osmotic-dependent response) and isotonic (primarily volume-dependent response). The osmotic response is more sensitive and acts as the principal determinant of water balance. Note that both responses can co-exist. TBW: total body water; pOsm: plasma osmolality; SNS: sympathetic nervous system; RAAS: renin–angiotensin–aldosterone system; AVP: arginine vasopressin.

Figure 3. The relationship between osmotic state and total body water (TBW). It is possible for patients to exist in any one of the nine panels. The graded colouration reflects severity of condition. Possible treatment options for different parts of each panel are written in italics, with the arrows representing intended effects of intervention to normalize physiology. IV: intravenous (Image reproduced with permission from the Perioperative Quality Initiative, POQI).