Anaesthesia for cats whether healthy or critically ill always presents a challenge to the clinician. In the vomiting cat, it is vital to judge and correct the degree of hypovolaemia present prior to using anaesthetic agents which will obtund the compensatory mechanisms in play in the conscious animal resulting in severe hypotension and essentially hypoxia due to decreased organ perfusion. Differences in clinical assessment, physiological responses and tolerance to acute fluid loading differ dramatically from those in dogs and so our stabilisation needs to be tailored to each individual situation.

Providing intravenous fluid therapy is perhaps the mainstay of stabilisation. Our aim is to restore the circulating blood volume back to as close to normal as possible. Most anaesthetic agents will cause vasodilation therefore preventing the patient's compensation for hypovolaemia.

It is important to understand that fluid therapy is a dynamic therapeutic plan. Patients require reassessment regularly. Many end-points have been proposed to monitor the efficacy of fluid therapy; however, a combination of clinical parameters and laboratory measurements must be assessed and trends monitored regularly to adequately gauge the patient's fluid requirements, their response to those fluids and to tailor the ongoing protocol to suit the patient's needs. Instead of having fixed formulas to calculate fluid deficits alongside maintenance requirements and ongoing losses, the emphasis is more on how the individual patient responds to the therapy administered when deciding how much is enough prior to anaesthesia. We are aiming to anaesthetise a cat that can maintain normotension, and have a normal to slightly elevated heart rate without the presence of severe vasoconstriction. It is not acceptable to place the patient on intravenous fluids and then plan to do the anaesthesia in, say, 3 hours. Every cat will be different in their response to fluids; therefore, continual patient assessment using some simple assessment techniques is required.

Assessment of blood samples is an important guide to fluid therapy; however, haemodynamic status should be assessed regularly and anaesthesia performed once these variables have improved.

Tissue perfusion can be monitored by a number of simple assessments:

 Mental status. Hypoperfusion in cats produces marked mental depression. As volume is restored mental status should improve.

 Mucous membrane colour. Mucous membrane colour and capillary refill time (CRT) are subjective.

 measures of tissue perfusion. Pale mucous membranes may be suggestive of vasoconstriction but also anaemia. Prolonged capillary refill time should improve as perfusion is restored.

 Temperature of the extremities. The difference in toe web temperature and rectal temperature should be less than 4°C in cats and will be increased in the presence of shock and vasoconstriction. However, in the late stages of shock, cats often become globally hypothermic. Rectal temperature should be evaluated and sometimes a 'cut-off' line of temperature can be noted on the limbs and as perfusion improves with fluid therapy this line demarcating vasoconstriction will move distally down the limbs. Hypothermia occurs during late shock in cats and it is important to correct volume deficits before trying to warm the patient up externally.

 Heart rate. Cats are well known for not reading textbooks and certainly this is true in shocked cats. Initially during compensatory shock, heart rate increases and vasoconstriction occurs in an attempt to maintain cardiac output, blood pressure and therefore tissue perfusion. However, as these compensatory mechanisms fail, the patient's heart rate will decrease back to normal values (inappropriate bradycardia) and there is a resultant hypotension.

 Arterial blood pressure. Mean arterial blood pressure (MAP) is related to cardiac output and systemic vascular resistance and is the driving pressure for systemic flow. It can be used as a guide to tissue perfusion. As a general rule flow to some or all tissues, including the kidney, is compromised when MAP falls below 60 mmHg. However, during hypovolaemia, compensatory mechanisms maintain MAP within the normal range initially and therefore it can be an inaccurate guide for effective volume replacement. Blood pressure should be monitored and assessed in conjunction with other parameters such as heart rate and indicators of tissue perfusion. Subjective assessment of blood pressure is difficult with palpation. The difference between systolic and diastolic pressures can be appreciated but the pressure required to occlude flow is the only indicator of MAP. Lack of peripheral pulses indicates vasoconstriction. In cats, hypotension is a late indicator of shock and will often occur alongside a fall in heart rate once the compensatory mechanisms dealing with hypovolaemia fail. Systolic blood pressure can easily be measured in conscious cats with the Doppler. In a population of cats admitted to an intensive care facility, if hypotension was recorded on admission, they were less likely to survive to discharge especially if the systolic pressure was less than 80 mmHg. This means we should avoid anaesthetising cats until we have restored enough blood volume for them to be able to maintain normotension. This is particularly evident as our inhalational agents will all result in vasodilation, resulting in hypotension and all the normal compensatory mechanisms will be obtunded under anaesthesia. Systolic blood pressure should be maintained above 90 mmHg or mean arterial blood pressure above 60–70 mmHg before anaesthesia is commenced. Balanced anaesthesia will help to minimise the requirement for inhalational agents such as isoflurane and therefore minimise the vasodilation and hypotension that will result.

 Urine output. During the early stages of fluid resuscitation, placement of a urinary catheter can be extremely useful or bladder filling should be assessed.

A number of laboratory values can be used in conjunction with the above markers of intravascular volume to help guide therapy prior to anaesthesia:

 Lactate. Normal lactate levels are below 2.5 mmol/l in cats. As a result of anaerobic respiration, lactate levels will rise with values over 10 mmol/l suggestive of severe hypovolaemia (if other causes have been ruled out). With effective fluid therapy, lactate levels should fall, optimally returning to the normal range.

 Packed cell volume (PCV). PCV will usually be elevated in dehydrated patients or normal in dehydrated, anaemic animals. Effective fluid therapy should be aimed at decreasing the PCV to the normal range.

 Total protein. Total protein will fall in association with crystalloid fluid therapy.

 Renal parameters. Serial documentation of blood urea nitrogen and creatinine can be used to monitor the efficiency of therapy and to look at ongoing prerenal dysfunction.

 Electrolytes. In dehydrated patients, sodium is often elevated and potassium decreased. Electrolytes should be serially monitored as a guide to fluid therapy and supplementation of electrolytes such as potassium. Hypochloraemia may be seen in cats that are vomiting purely gastric fluid and this can lead to an alkalosis developing. In these cats, fluid therapy using 0.9% sodium chloride should be used to help correct the acid-base abnormalities.

 Blood gas analysis. Venous blood gas analysis should be performed (preferably in conjunction with arterial analysis) to provide information on the pH and the partial pressure of oxygen. Metabolic acidosis results from tissue ischaemia and pH should increase and the base deficit decrease (less negative) as fluid therapy is optimised.

Intravenous fluid therapy using crystalloids should be the primary goal of stabilising the vomiting cat prior to anaesthesia, whilst ensuring normal electrolyte status. Correcting hypovolaemia will improve the cardiovascular response of the patient to anaesthesia and therefore reduce the risks associated with decreased renal perfusion or gastrointestinal tract perfusion.

Some simple goals to aim for include:

 Normotension

 Normal (to slightly elevated) heart rate

 Warm extremities

 A palpable pulse in the extremities

The majority of problems resulting from anaesthesia are a result of inadequate stabilisation so taking a little bit of extra time to monitor responses to ongoing fluid therapy will lead to a safer anaesthetic and recovery for the patient.