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.
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:
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.
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.
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.
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).
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.
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).
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 | |
| Maldistributive | |
| Cardiogenic | |
| Obstructive |
The two most common causes of hypovolaemia in companion animals are haemorrhage and salt and water loss. Haemorrhagic hypovolaemia may cause some extravascular fluid deficits, while animals presenting with hypovolaemia due to salt and water loss (e.g. from vomiting and diarrhoea) will certainly have extravascular fluid deficits.
Clinical Tip
A number of physical examination parameters are used to assess perfusion:
The dorsal pedal artery is located just distal to the hock on the craniomedial aspect of the pelvic limb. Being confident in palpating the arterial pulse at this site is useful for a number of reasons in addition to assessing perfusion status.
In animals with uncomplicated hypovolaemia, the perfusion parameters listed above are reliable indicators of volume status and tend to follow predictable trends both as hypovolaemia progresses, from compensatory through to early and then late decompensatory phases, and as normal perfusion is restored.
In this phase, hypovolaemia is mild. A compensatory increase in sympathetic activity causes both an increase in heart rate and cardiac contractility, and peripheral vasconstriction. The corresponding clinical picture is hyperdynamic:
Note that most healthy adult dogs have a heart rate in the practice setting of 70–120 beats per minute. There is some variation in heart rate amongst dogs of different size (with bigger dogs usually having slower heart rates) and resting heart rate is also dependent on factors such as the individual animal’s level of fitness. It would be highly unusual to detect a resting heart rate in a calm healthy greyhound of 110 beats per minute; likewise a resting heart rate of 70 beats per minute in a Chihuahua. Thus the heart rate identified must be interpreted in the context of the patient in question as a heart rate within the normal range may in fact be an inappropriate finding.
In this phase, hypovolaemia is moderate and systemic perfusion has become compromised due to progressive failure of compensatory mechanisms:
In this phase, hypovolaemia is severe and systemic perfusion is severely compromised. The corresponding clinical picture is markedly hypodynamic:
Some dogs in this final stage of hypovolaemic shock present with a heart rate that is inappropriately within normal limits or bradycardic. This is because the heart can no longer sustain its compensatory chronotropic response.
Clinical Tip
There are a number of important differences with respect to perfusion assessment and hypovolaemia in cats that lend further support to the notion that ‘cats are not small dogs’.
Animals with hypovolaemia may have concurrent abnormalities that preclude their perfusion parameters changing in the predictable way described above. When assessing a patient’s perfusion status it is therefore important to adopt a holistic approach that takes changes in all the perfusion parameters into account and correlates them in the context of the patient in question. For example:
In addition to physical examination, other measures of perfusion, if available, include blood pressure measurement, venous lactate concentration and urine output. Again these findings must be interpreted in the context of the patient’s physical examination parameters.
Clinical Tip
The majority of causes of hypovolaemia will also result in pain that can interfere with cardiovascular and respiratory function, and confuse clinical assessment. Adequate analgesia is therefore extremely important. The use of opioids initially is the mainstay of pain management and nonsteroidal antiinflammatory agents should be avoided until hypovolaemia and dehydration have been corrected (see Ch. 5).
Early on when intravascular volume remains adequate, dogs in maldistributive shock typically have a hyperdynamic vasodilatory cardiovascular picture, i.e. tachycardia, hyperdynamic pulses, markedly hyperaemic mucous membranes and a fast capillary refill time. With the onset of concurrent severe hypovolaemia, tachycardia progresses but pulses become weaker and CRT more prolonged. However, unlike dogs with uncomplicated severe hypovolaemia, dogs in severe hypodynamic maldistributive shock are likely to retain colour in their mucous membranes that may be normal in appearance or hyperaemic. By cross-referencing perfusion findings it is often possible to identify this retained mucous membrane colour as a discrepant finding that suggests the presence of a distributive component to the hypoperfusion.
Cats in maldistributive shock are more likely to present with a hypodynamic cardiovascular picture.
For clinical purposes, dehydration can be defined as the process whereby an animal loses more salt and water from its body than it takes in. This loss of body fluid is predominantly extravascular and the effect on intravascular volume depends on both the degree of fluid loss and the tonicity (isotonic or hypotonic) of the fluid. The tonicity is related to the sodium concentration.
The physical examination parameters that are used to assess hydration are related to interstitial volume. They are the moistness of the mucous membranes, skin turgor (elasticity), and the presence and degree of globe retraction. The perfusion parameters described above also become relevant if dehydration is severe enough to cause hypovolaemia.
Guidelines similar to those in Table 2.2 are generally used to estimate dehydration on the basis of physical examination findings. However, these guidelines must only be used as an approximation of fluid loss in the dehydrated patient to facilitate the initial fluid plan (see Ch. 4) as there are a number of potential inherent inaccuracies. For example, skin turgor can be affected by the degree of subcutaneous fat present (with obese animals having increased turgor), and mucous membrane moistness can be affected by salivation (e.g. due to nausea).
Table 2.2 Guidelines for estimating dehydration on the basis of physical examination
| Severity of dehydration (estimated % of body weight) | Progression of physical examination findings |
|---|---|
| <5% | Normal |
| Mild (5–6%) | Skin turgor mildly reduced |
| Moderate (6–10%) | |
| Severe (10–15%) |
In addition to physical examination, a number of other parameters can readily be evaluated to determine the presence and severity of dehydration. Given the inaccuracies of assessing hydration status based on physical examination, it is recommended that these additional measures are utilized as much as possible especially for monitoring rehydration (see Ch. 3). They include:
Short-term changes in body weight may also provide useful information with respect to fluid balance in hospitalized patients.