Follow ASNKidney. Read Later Share. Read the Up-To-Date entry « Dehydration is not synonymous with hypovolemia » by Burton Rose if there is any confusion about this. It can be produced by either salt and water loss e. Next Post Renal Pharmacokinetics of Dilantin. August 12, at am. A competent fluid assessment requires sound knowledge of the dynamic interaction between body fluid compartments, as well as a skilful examination and careful biochemical analysis of serum and urine.
In addition, a command of medical language and terminology is essential to precisely describe and categorize body-fluid status. Mange et al [ 1 ] highlighted the importance of recognizing dehydration and volume depletion as two completely different clinical entities.
Considering the magnitude of the problem, in the International Classification of Diseases coordination and maintenance Committee made recommendations to modify the coding for body fluid disorders to uniquely identify dehydration and volume depletion[ 2 ].
Another subcategory of ECF, albeit small, is transcellular fluid not shown in the figure that resides in pleural, pericardial, peritoneal, synovial, ocular and cerebrospinal spaces, although in some cases, its chemical composition and physical properties may differ from that of intravascular or interstitial fluid[ 6 ].
Fluid input and output from the body proceeds via the intravascular compartment. Intravascular and interstitial compartments are separated solely by highly permeable capillary membranes. Consequently, the osmolality of the ECF is largely dependent on sodium and chloride whereas the osmolality of the ICF is derived from potassium along with other intracellular osmoles. Water moves freely between all fluid compartments through highly water permeable cell membranes; therefore, the osmolality of the plasma is equal to the osmolality of other compartments.
Considering the differing permeability of the membranes that separate fluid compartments in the body, administration of different IV fluids will result in differing distribution amongst these compartments.
On the other hand, infusion of one litre of normal saline 0. Though the water content of both D5W and 0. Hence, 0. Conversely, if the aim is to correct dehydration pure water loss then a fluid that flows to all the compartments, such as D5W is the preferred solution. Giving D5W is equivalent to giving free water because glucose is rapidly metabolized. Because cell membranes are freely permeable to water, this results in osmotic movement of water from the larger intracellular compartment to the extracellular compartment.
There is a contraction of all body water compartments proportional to their share of TBW[ 12 ]. Since the intracellular compartment is the largest reservoir of body water, it suffers the largest water deficit. For instance, for each litre of water lost from the body, the intracellular compartment contributes mL. In contrast, the intravascular compartment suffers a loss of only 80 mL; hence pure water loss rarely compromises the effective circulating volume or haemodynamic stability.
This induces shrinkage of osmoreceptor cells in the anterior hypothalamus, stimulating the release of antidiuretic hormone ADH from the posterior pituitary gland.
ADH promotes incorporation of water channels aquaporin 2 in the distal nephron segments allowing increased water reabsorption. At the same time, the thirst mechanism is triggered leading to increased water ingestion. Renal conservation of water along with increased water intake act to reverse the osmolal changes brought about by the initial water loss by restoring normonatremia Figure 2 ; 2 Volume depletion, which implies an ECF volume deficit secondary to the loss of both sodium and water.
Sodium is confined into the extracellular compartment by the Na-K-ATPase pumps in the cell membranes, which helps to hold water extracellularly[ 13 ]. Sodium and water loss lead to a reduction in the effective circulating volume. The human body orchestrates a number of homeostatic responses to combat hypovolemia that include activation of the renin-angiotensin-aldosterone system receptors in renal afferent arterioles , stimulation of the sympathetic nervous system aortic arch and carotid sinus receptors , suppression of ANP atrial receptors and stimulation of ADH release.
All these lead to renal conservation of both salt and water, thereby restoring normovolemia. It is noteworthy that ADH release is stimulated in both dehydration due to hypertonicity , and ECF volume depletion due to decreased effective circulating volume. Though uncommon, some physicians have insufficient knowledge of body fluids due to a lack of factual information about body fluid compartments and differences in their composition.
Suppose an elderly patient is admitted with community-acquired pneumonia. He has been rather drowsy for two days before admission with poor oral intake. He is tachypneic and pyrexial, but his blood pressure is normal with no postural change.
He is receiving antibiotics and D5W infusion. It also appears that although some students have knowledge of the different fluid compartments, they fail to apply their knowledge to real life cases. The vast majority of doctors appreciate that patients who present with profuse diarrhoea and vomiting and are consequently hypotensive and tachycardic are intravascularly depleted.
They also very appropriately resuscitate these patients with 0. So, although they correctly identify and treat the clinical syndrome of intravascular volume depletion, they use imprecise terminology. Another common misbelief among students is that dehydration can be reliably diagnosed by physical signs such as sunken eyes, decreased skin turgor and dry mucous membranes.
Contrarily, the predictive value of these individual clinical signs in diagnosing dehydration is limited in adult populations. Studies endorsing these physical signs were mostly carried out on paediatric and elderly patient populations[ 14 - 18 ]. Many of these patients in fact had ECF volume depletion rather than dehydration, as evidenced by haemodynamic compromise and normal serum sodium levels.
However, exophthalmometry, the standard objective technique for measuring enophthalmos, is not used in general medicine leaving substantial variation in inter-observer agreement for this physical sign.
Reduced skin turgor means reduced elastic recoil of the skin to its normal contour after being pinched in a fold. As pointed out by Laron et al [ 19 ], it reflects contraction of the interstitial and intravascular space both are subcategories of the extracellular compartment rather than the loss of intracellular water. Skin turgor also correlates directly with the elastin content of the skin, which decreases significantly with ageing[ 16 , 20 ].
Though it had long been known that primary loss or deprivation of water produces biological disturbances thirst dissimilar to those seen in primary loss or deprivation of salt circulatory instability , both types of deficits were considered to be subcategories of dehydration in the early 20 th century[ 21 , 22 ].
In fact, it is volume depletion that has isonatraemic, hyponatraemic and hypernatraemic subtypes determined by the tonicity of the fluid lost and the type of fluid ingested[ 28 - 31 ]. If the losses are isotonic, i. However, if more sodium relative to water is lost or the patient takes plenty of salt-free fluids, for example tap water , hyponatraemic volume depletion ensues. Finally, if less sodium is lost relative to water or if the patient does not drink water, or takes hypertonic soup , hypernatraemic volume depletion follows.
Some patients can present with features of both dehydration and intravascular volume depletion. The co-existence of these two different entities is partly responsible for some physicians misjudging them as a single disorder. Indeed, many patients in paediatric clinical studies with diarrhoeal illnesses were both dehydrated and ECF volume depleted[ 14 , 32 ].
Clinically, it is not possible to establish whether hypernatremia in an intravascularly depleted patient is secondary to hypernatraemic intravascular depletion water loss greater than sodium loss , severe dehydration profuse water loss alone , or a combination of the two. In clinical situations, there is hardly any need for this differentiation. As a first step, intravascular volume depletion is treated with 0. Once adequate haemodynamic stability is achieved, hyperosmolality is corrected with D5W.
Usually, dehydration does not lead to intravascular volume depletion as the intravascular space contributes only a small percentage to the TBW loss; the major bulk is lost from the intracellular space, the largest reservoir of body water. As discussed earlier, a loss of 1 L from TBW removes only 80 ml from the intravascular space 2. Development of signs and symptoms of intravascular volume depletion usually require more than 0.
The drawback of this is that these brief forms can lead to varied interpretations and thus confound medical personnel. It neither clarifies whether the loss of fluid is from intracellular or extracellular space, nor indicates the type of fluid lost hypotonic or isotonic. Hence, for some it may imply depletion of TBW i.
An organized approach is imperative in correcting robust misconceptions related to body fluid deficit disorders. First, it is crucial that all faculty members develop a critical understanding of the body-fluids, as misconceptions acquired from faculty members and textbooks are very difficult to eliminate from the minds of young doctors later in their professional lives.
In the following section we present our approach to overcoming misconceptions in a manner that will create a lasting effect on students and prevent them from reverting to their preconceptions.
Although misconceptions about body fluids disorders are widespread, students are generally unaware that the knowledge they possess is faulty. We actively bring up the subject when encountering patients with body fluid deficits in order to probe students for the presence of misconceptions. Once identified, we try to make students discontent with their misconceptions. This provides a strong stimulus for refinement or replacement of the flawed concepts with intelligible and plausible ones.
Utmost care is given to maintain a favourable learning environment where the students are not ridiculed for holding incorrect preconceptions. We split teaching into short modules, each with a clear framework and objectives. Introductory presentation: We start with a min introductory presentation using visual aids such as a white-board or PowerPoint presentation to orient the students to body fluid compartments.
Classifying the body fluid deficit disorder based on the nature of the fluid deficit water alone vs water with salt and the main body fluid compartment affected intracellular vs extracellular in each disorder generates uneasiness in the minds of those students who misconceive dehydration and volume depletion as one entity. We make a conscious effort to avoid ambiguous linguistic expressions. This also encourages students to abandon the habit of using misleading abbreviations.
Clinical encounter: The newly implanted concepts must be supplemented with real life applications within a patient care context ensuring the students do not merely learn the new rote information. One group evaluates a pre-selected patient with dehydration while the other group assesses a patient with ECF volume depletion.
The instructor facilitates the learning process and highlights the contrasting features of the two patients. Updated: October 13, Accessed: April 14, Last updated: November 3, The treatment of diarrhoea: a manual for physicians and other senior health workers.
Diagnosis and management of dehydration in children.. Am Fam Physician. Principles and Practice of Oral Rehydration. Curr Gastroenterol Rep. Managing acute gastroenteritis among children: oral rehydration, maintenance, and nutritional therapy..
Polymer-based oral rehydration solution for treating acute watery diarrhoea. Cochrane Database Syst Rev. The use of oral rehydration solutions in children and adults.
Subcutaneous hydration and medications infusions effectiveness, safety, acceptability : A systematic review of systematic reviews. J Clin Med. Hypernatremia in critically ill patients. J Crit Care. Clinical practice guideline: Maintenance intravenous fluids in children. N Engl J Med. Am J Gastroenterol. Diarrhoea in adults acute.. BMJ clin evid. Physical findings of volume depletion in infants and children. Last updated: January 1, Sweating Decreased water intake Diuresis Osmotic diarrhea.
Vomiting Inflammatory diarrhea Secretory diarrhea. Hypotonic i. Activation of thirst center ADH release Creation of additional intracellular osmoles [3] [4]. Symptoms Thirst Lethargy Neurological symptoms caused by extracellular hypertonicity: e. Rehydration regimens and multicompartmental fluid replacement Enteral fluid therapy : e. IV fluid resuscitation to restore intravascular fluid volume. Lethargic Disoriented Children: may also have marked irritability when touched. Extreme but may be too lethargic to drink.
HR: elevated BP: Normal or slightly reduced; orthostatic hypotension may be present. Palpated pulses weaker than normal RR: may be elevated with deep inspirations. HR: Tachycardia or bradycardia BP: Hypotensive Peripheral pulses: thready, difficult to palpate RR: Kussmaul breathing ; Bradypnea may indicate impending respiratory failure and arrest. Sunken orbits Decreased tear production.
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