dehydration

Japanese: 脱水

Concept 60% of the body is made up of water. Therefore, if the balance between water going in and out is disrupted, it can lead to dehydration or overhydration. Basically, water in comes mainly from oral intake, but there is also a component called metabolic water (water produced by the metabolism of nutrients), which is estimated to be approximately 5 mL per kg of body weight. On the other hand, water out comes mainly from urine, but we must not forget about the component called insensible perspiration (about half from evaporation from the skin and half from exhalation), which is estimated to be approximately 15 mL per kg of body weight. It is also said to increase by about 15% with a 1°C rise in body temperature.
The normal daily water balance for an adult weighing 60 kg is as shown in Table 2-41-1. Therefore, to measure the water balance of a patient who cannot take in fluids orally, it is sufficient to infuse the amount of fluid (from the previous day) in excess of 600 mL (insensible perspiration - metabolic water = body weight x 10 mL). If there are abnormal excretions such as excessive sweating, aspiration of digestive juices, vomiting, diarrhea, or bleeding, this must also be taken into consideration.
Physical findings and test results Dehydration is a state in which extracellular fluid is reduced. To diagnose dehydration at the bedside, signs such as dry mouth, xerostomia, dry tongue, the appearance of tongue grooves, decreased skin turgor, decreased urinary output, weight loss, tachycardia, and orthostatic hypotension are useful, but making a diagnosis can often be difficult. The capillary refill time measured using the nail bed of the finger (dehydration if it is greater than 2 seconds) has low sensitivity but high specificity (Table 2-41-2).
Test values can often provide useful information, some of which are listed in Table 2-41-2. In actual clinical settings, a comprehensive judgment will be made based on this and physical findings. Dehydration usually involves both water and sodium deficiency, but depending on which is more deficient, it can be broadly divided into three types: isotonic dehydration, hypertonic dehydration, and hypotonic dehydration (Figure 2-41-1). The physical and test findings differ for each type of dehydration (Table 2-41-3).
Pathophysiology
1) Isotonic depletion (volume depletion):
When an amount of body fluid equal to the osmotic pressure of the extracellular fluid is lost, plasma osmotic pressure does not change. Water does not move from inside the cell to outside the cell, so a drop in blood pressure due to a decrease in circulating blood volume occurs. This is the most common clinical occurrence when large amounts of extracellular fluid are rapidly lost due to bleeding, vomiting, diarrhea, burns, etc.
2) Hypertonic dehydration:
Although sodium is also lost, if more water is lost, plasma osmolality rises. As a result, water moves out of the cells to compensate for the lack of extracellular fluid, the circulating plasma volume is corrected to a certain extent, and symptoms of peripheral circulatory failure are less likely to appear than in isotonic or hypotonic dehydration. However, intracellular dehydration is highly persistent, and is characterized by stronger thirst than other dehydration conditions. Therefore, if consciousness is normal, the patient will attempt to drink water, so the actual occurrence is less frequent than with isotonic dehydration. It is seen in patients with high fever, young children and elderly people with reduced renal concentrating ability, patients with impaired consciousness, and patients with impaired thirst centers. Intracellular dehydration also becomes hyperosmolar in response to the hyperosmolality of the extracellular fluid (hypernatremia), but in brain cells and other cells, some osmotic substances accumulate in an attempt to maintain a constant cell volume. If plasma osmolality is lowered by rapidly administering fluids to improve hypertonicity, the osmotic substances that have accumulated within the cells will then cause water to move into the cells, which can lead to cerebral edema in brain cells, which is extremely dangerous.
3) Hypotonic dehydration:
When more Na is lost than water, the osmotic pressure of the extracellular fluid decreases. As a result, water moves into the cells, further reducing the volume of extracellular fluid, making peripheral circulatory failure (low blood pressure, pale face, cold extremities) more likely to occur. However, since most of the body fluid lost from the body is hypotonic, this type of dehydration is rare (adrenal insufficiency and salt-losing nephropathy are exceptions). Therefore, most hypotonic dehydration is iatrogenic, occurring when electrolytes are not properly replenished to treat dehydration. Hyponatremia and hypoosmolality are seen, and patients rarely complain of dry mouth due to intracellular flooding. [Uchida Toshiya]

Table 2-41-1
Daily water balance for adults

Table 2-41-1

Table 2-41-2
How to assess body fluid volume (differences due to dehydration type are shown in Table 2-41-3)

Table 2-41-2

Table 2-41-3
Differences in physical and laboratory findings according to type of dehydration

Table 2-41-3

Figure 2-41-1
Three types of dehydration (ICF: Intracellular fluid, ECF: Extracellular fluid)

Figure 2-41-1

Source : Internal Medicine, 10th Edition About Internal Medicine, 10th Edition Information

Japanese:

概念
　体の60％は水でできている．そのため水の出入りのバランスがくずれると，脱水になったり溢水になったりする．基本的に水の入り（in）は経口摂取が主たる成分であるが，代謝水（栄養素の代謝による水生成）とよばれる成分がありおよそ体重 kgあたり5 mLとされている．一方水の出（out）は尿が主体であるが，不感蒸泄（皮膚からの蒸散と呼気で約半々）とよばれる成分も忘れてはならない．およそ体重 kgあたり15 mLとされている．また体温1 ℃の上昇で約15％増加するとされる．
　通常体重60 kgの成人の1日の水出納は表2-41-1のようになる．したがって経口摂取できない患者の水バランスをはかるためには，（前日の）尿量＋600 mL（不感蒸泄－代謝水＝体重×10 mL）を輸液すればよいことがわかる．これに発汗過多，消化液吸引，嘔吐，下痢，出血などの異常排泄量があればこれを考慮することになる．
身体所見・検査成績
　脱水症とは細胞外液が減少した状態である．ベッドサイドで脱水症を診断するには，口渇感，口腔乾燥，舌乾燥，舌溝出現，皮膚ツルゴールの低下，尿量減少，体重減少，頻拍，起立性低血圧などが参考になるが，判断に苦慮することも多い．指の爪床を用いて行うcapillary refill time（毛細管再充満時間；2秒以上で脱水）は感度は低いものの特異度が高い（表2-41-2）．
　検査値からも参考になることが多く，その一部を表2-41-2にあげる．実際の臨床の場では，身体所見とあわせて総合的に判断することになる．　脱水は水の欠乏とNaの欠乏の両方を伴うのが普通であるが，いずれがより多く欠乏するかによって等張性脱水，高張性脱水および低張性脱水の3型に大別される（図2-41-1）．そして各々の脱水で身体所見や検査所見も異なってくる（表2-41-3）．
病態生理
1）等張性脱水（volume depletion）：
細胞外液の浸透圧と等しい体液が失われる場合で，血漿浸透圧は変化しない．細胞内から外への水の移動は起こらないため循環血液量の減少による血圧低下などがみられる．出血，嘔吐・下痢，熱傷など大量の細胞外液が急速に失われる場合で，臨床的に遭遇することが最も多い．
2）高張性脱水（dehydration）：
Naも失われるが，水の方がより多く失われる場合で血漿浸透圧は上昇する．その結果，細胞内から水が移動し，細胞外液の欠乏を補うため，循環血漿量はある程度補正され，等張性脱水や低張性脱水の場合に比し末梢循環不全の症状が出現しにくい．しかしながら，細胞内脱水は高度に持続するため，ほかの脱水症に比較し，口渇が強いのが特徴である．したがって意識が正常であれば，飲水行動に出るため，等張性脱水と比べて実際の出現頻度は少ない．高熱患者，腎濃縮力が低下している幼少者や高齢者，意識障害患者，口渇中枢が障害されている患者などでみられる．細胞外液の高浸透圧（高ナトリウム血症）に適応して，細胞内も高浸透圧になるが，脳細胞などでは細胞の容積を一定に保とうとして，何らかの浸透圧物質が蓄積してくる．高浸透圧を改善するために急速に輸液をして血漿浸透圧を下げた場合，細胞内に蓄積しているその浸透圧物質のため，今度は水が細胞内に移行し，脳細胞においては脳浮腫を招ききわめて危険である．
3）低張性脱水（hypotonic dehydrationとも）：
Naが水よりも多く失われる場合で，細胞外液の浸透圧は減少する．その結果，細胞内に水が移行し細胞外液量はさらに減少するため，末梢循環不全（血圧低下，顔面蒼白，四肢冷感）を起こしやすくなる．しかしながら体外に喪失する体液のほとんどは低張液であるため，このタイプの脱水症になることは少ない（例外的に副腎皮質機能不全症や塩類喪失性腎症がある）．したがって低張性脱水の原因は，脱水症に対し電解質の補給が適切でなかった場合に起こる医原性のものがほとんどである．低ナトリウム血症，低浸透圧血症がみられ，細胞内溢水のため口渇を訴えることは少ない．［内田俊也］

表2-41-1
成人の１日あたりの水バランス">

表2-41-1

表2-41-2
体液量の評価の仕方（脱水型による差異は表2-41-３）">

表2-41-2

表2-41-3
脱水のタイプ別の身体所見と検査所見の差異">

表2-41-3

図2-41-1
脱水の３つのタイプ（ICF：細胞内液，ECF：細胞外液）">

図2-41-1

出典　内科学第10版内科学第10版について　情報

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