Shock and dehydration

2 Shock and dehydration


Shock and dehydration are both commonly seen in emergency practice and both require fluid therapy as a crucial part of their management (see Ch. 4). However, they represent different pathophysiological processes and have different clinicopathological findings. Understanding these differences is essential to ensuring that the most appropriate management is implemented.



Body Fluid Compartments


Total body water typically constitutes approximately 60% of a healthy non-obese adult dog or cat’s body weight. A basic understanding of the way in which the total body water is distributed is central to appreciating the concepts of perfusion, volume status, shock and hydration. The total body water is distributed as follows:


• Intracellular compartment (40% body weight)

• Extracellular compartment (20% body weight):
– Interstitial space including lymph (15% body weight)

– Intravascular space (4% body weight)

– Transcellular compartment (e.g. cerebrospinal and synovial fluid; approximately 1%).

Active and/or passive movement of fluid occurs continuously between the intravascular and interstitial compartments at the capillary membrane. The most important factor that determines the size of each fluid compartment is the amount of solutes it contains.



Hypoperfusion and Shock


Organs and peripheral tissues rely on an adequate intravascular fluid space (adequate perfusion) for the delivery of oxygen and nutrients and the removal of by-products of tissue metabolism. ‘Hypoperfusion’ describes a situation in which there is a decrease in the blood supply to an organ or tissue. Although this can occur solely on a local level, it is seen most commonly in the context of a global (systemic) reduction in blood supply.


The term ‘shock’ is used clinically to describe the condition of a patient in which this global hypoperfusion has reached a certain level of severity, sufficient for the patient to manifest a number of characteristic clinical findings. This usually occurs when the reduction in blood supply is severe enough to cause inadequate oxygen delivery to the cells and thereby inadequate energy production. If the degree of shock is sufficiently severe or prolonged, irreversible cell damage can occur and treatment is invariably unsuccessful.



Clinical Tip



• Systemic hypoperfusion is common in emergency patients and being able to recognize and treat this abnormality is crucial for successful stabilization.


Causes of hypoperfusion


The mechanisms of hypoperfusion and shock can be divided into four categories. However, it is important to remember that more than one type can exist in the same patient at the same time, and that they share some common changes and derangements at a cellular level.



Hypovolaemic shock


Hypovolaemia is a reduction in the effective circulating intravascular volume and is the most common cause of hypoperfusion in companion animals. It can be subdivided into absolute and relative hypovolaemia. Absolute hypovolaemia is the most common type and occurs as a result of loss of fluid from the intravascular space. This fluid loss may affect just the intravascular space (e.g. haemorrhage) or the extravascular space as well (e.g. salt and water loss, for example in vomiting and diarrhoea).



Maldistributive shock


With maldistributive shock, the fluid in the intravascular space (the absolute volume of which may or may not be reduced) is distributed across the body in an abnormal way. This occurs as a result of inappropriate generalized vasodilation and can potentially be viewed as a form of relative hypovolaemia. Maldistributive shock is seen in animals with systemic inflammatory response syndrome (SIRS) which may occur for example in sepsis, severe pancreatitis or major tissue injury. Maldistribution also occurs in anaphylactic/anaphylactoid shock.



Cardiogenic shock


Primary cardiac dysfunction (e.g. from organic heart disease or severe dysrhythmia) results in failure of the heart to pump adequately. This compromises cardiac output causing hypoperfusion (see Ch. 31).



Obstructive shock


Systemic hypoperfusion occurs due to obstruction of blood flow from the heart or venous return to the heart; the most common example of this in small animals is pericardial tamponade.


Causes of systemic hypoperfusion in companion animals are listed in Table 2.1.


Table 2.1 Causes of systemic hypoperfusion in dogs and cats


















Type of hypoperfusion Causes
Hypovolaemia
Blood loss (trauma, coagulopathy, ruptured neoplasm)

Vomiting and diarrhoea

Severe dehydration

Third space fluid loss (peritoneal, pleural, interstitial, gastrointestinal)

Neoplasia

Burn injury

Severe polyuria
Maldistributive
Systemic inflammatory response syndrome (SIRS):
• Sepsis

• Severe pancreatitis

• Major tissue trauma

• Neoplasia

• Burn injury

Anaphylaxis/anaphylactoid reaction
Cardiogenic
Cardiomyopathy (especially dilated and hypertrophic)

Valvular disease

Severe dysrhythmia
Obstructive
Pericardial tamponade

Related posts:

  1. Jaundice
  2. Sudden loss of vision
  3. Pelvic limb paresis and paralysis
  4. Ocular emergencies

Stay updated, free articles. Join our Telegram channel
Tags:
Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Shock and dehydration

Full access? Get Clinical Tree

 Get Clinical Tree app for offline access