Veterinary Specialists

What does it mean to "stabilise" a critically ill small animal patient before anaesthetic induction?

A L Leisewitz, F Kettner, K Joubert , A Zambelli, N Keller, E Scheepers, M Bohm.

Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Onderstepoort.

  1. What is an ‘unstable’ patient?

A patient that presents in shock or one that is unable to adequately compensate physiologically during an intervention such as anaesthesia. Shock can be defined and suspected under the following circumstances:

A dog or cat that is weak or collapsed. A depressed level of consciousness or sudden decrease in mentation are indicators of decompensation. These changes should be viewed in light of the nature of the trauma the patient has experienced (for example head trauma), or the possibility of a toxicity. Patients that have a sudden change in consciousness should be re-evaluated;
Shock typically causes a low rectal temperature (although temperature may be high in cases of sepsis or blood stream infections) and cold peripheral extremities;
Shock is associated with a tachycardia (although cats may have normal pulse and be in shock) or bradycardic; Tachycardia may be the result of pain in trauma patients and if the heart rate fails to decrease after an appropriate dose of an analgesic, hypovolaemia should be suspected. Patients that are tachycardic or normocardic where there is a suspicion of hypovolaemia or shock should be challenged with a fluid bolus equal to about 10% of their blood volume (approximately 10 ml/kg in dogs) given over 5-15 minutes. Changes in heart rate and blood pressure (the quality of the pulse) should be assessed. Before treating a patient that is bradycardic with atropine the following contraindications should be excluded: hypoxia, hypothermia and electrolyte imbalances (increased blood K+).
Capillary refill time is typically prolonged in established shock but in hyperdynamic shock and early septic shock this may not be the case (mucous membranes are also usually pale but may be injected in hyperdynamic or septic shock);
An attempt to feel the quality of the femoral or other peripheral arterial pulse should be made. If a peripheral pulse cannot be felt, arterial blood pressure is below 60 mmHg and this requires urgent attention.
Respiratory rate and pattern can be good indicator of patient stability. A fast respiratory rate with shallow breathing and an irregular pattern are indicators of an unstable patient. Rapid deep breathing cannot be sustained over a prolonged period and should warn about possible respiratory muscle failure.
Urine production is the ‘poor-man’s’ blood pressure machine. Measuring urine production is cheap and easy, requiring minimal invasiveness and time. It provides an objective means of evaluating blood pressure over time, rather than an instantaneous assessment at the time of the examination. Production in excess of 0.5ml/kg/hour usually indicates normal blood pressure
Blood glucose is often abnormal in unstable patients. It may both be high (as in some cases of septic shock and head trauma) and low (e.g. canine babesiosis)

Patients in compensated shock are more problematic as they may suddenly decompensate or may become unstable following the administration of anaesthesia.

A patient that has lost a large quantity of blood and is anaemic may be unable to maintain oxygen delivery under anaesthesia. Patients with long bone fractures can loose significant quantities of blood into the haematoma surrounding the fracture. Haematocrit is a poor indicator of acute blood loss. In these cases cardiovascular signs maybe a better indicator of the need for blood pressure support.

Patients showing severe dyspnoea should be regarded as unstable even if not in shock.

  1. What does it mean to ‘stabilise’ a patient (what are the therapeutic end-points?)?

Stabilisation is the treatment that is given in the interim period from admission until the primary problem can be, or is, addressed. The benefit of stabilisation should always be greater than the cost of delaying primary treatment.
To restore arterial blood pressure and tissue oxygenation to as close to normal as possible. Blood pressure and tissue oxygenation cannot be directly assessed in most general practice environments in South Africa so we are forced to use markers of these indices. These have been listed above in question 1.
These markers should be showing a consistent tendency towards normalisation. It is important to appreciate that trends in the observations are very important and the establishment of trends requires frequent monitoring during the post event period. In critically ill animals it is probably unrealistic to expect a complete normalisation of these measurements and observations. This is a very important point to remember. There is no way that leaving a shocked patient in a cage overnight on a drip only to be re-examined the next morning for the first time can be seen as ethical practice.

  1. What are the general therapeutic interventions used in stabilising a patient before any anaesthetic can be administered?

Intravenous fluid and colloid infusion (at shock doses) to expand circulating volume and thus increase blood pressure.

Caution in fluid administration must be exercised in patients with brain trauma, anuric renal failure, hypoproteinemia, lung oedema and lung contusions as fluid overload may be more rapidly lethal in these cases. Fluid overloading is more rapidly a problem in cats and more caution must be exercised when fluid loading this species.

The correction of any abnormal life-threatening biochemical disorders (such as severe hypoprotienaemia, hyperkalaemia, hypoglycaemia, hyper or hypoclacaemia or ketoacidosis), the correction of anaemia due to haemolysis (to a haematocrit of at least 20%) or whole blood loss (until blood pressure is above 70 mmHg – the approximate level at which femoral pulses can be palpated).

Ensure adequate urine production.
The correction of any life threatening respiratory abnormality such as serious thoracic effusion or flail chest. This should be accompanied by the administration of nasal oxygen.
The needle drainage of acute pericardial effusion that threatens cardiac output to the point of causing cardiogenic shock.
The needle trocharisation of life threatening gastric gas distension in gastric dilatation volvulus syndrome (GDV).
The correction (or at least an attempt to) of life threatening cardiac arrhythmias.
The arrest of any significant bleeding that can be stopped without surgical intervention.
The correction of acute bleeding tendencies due to clotting factor or platelet deficiencies to the point that spontaneous haemorrhage has subsided.
The supplementation of oxygen initially has few contraindications and can be safely used in most patients. Ventilation should be assessed for adequacy in terms of respiratory rate and tidal volume. Patients with slow respiratory rates (< 8 – 10) and low tidal volume should be ventilated to maintain adequate oxygenation with 100% oxygen until a diagnosis is made. Emergencies where ventilation is a consideration are usually caused by diseases of the neuromuscular junction, brain stem trauma or pleural space occupying lesions.
If blood pressure is still not adequately supported once adequate volume resuscitation has occurred, administer positive inotropes (dobutamine constant rate infusion) or vasoconstrictors (dopamine constant rate infusion).
Peritoneal lavage to removed purulent septic material should exploratory celiotomy not be possible at that stage (by mean of local anaesthesia and a keyhole incision into abdomen through which a Foley’s catheter can be passed to lavage the abdomen).
Pain should be addressed. Patients will often remain unstable if left in severe pain.
Severely hypothermic patients must have this need appropriately addressed if they are to be stabilised.
  1. How long should stabilisation take?

As fast as possible. Volume resuscitation in hypovolaemic shock should not take longer than 30-60 minutes.
This said the time required to stabilise a patient can be highly variable and will depend on the underlying condition. Some patients never reach sufficient stability to allow for interventions under anaesthetic. It must however be stressed that continuous and close monitoring of unstable patients is absolutely essential if life saving interventions that require a general anaesthetic are necessary. Only close and frequent monitoring will allow a well-timed decision as to when the best time to administer an anaesthetic might be.
It is important to remember that stabilisation should be achieved in the shortest safe period and once a patient is showing trends towards stabilisation urgent procedures under anaesthetic should be performed. In other words medical interventions may only provide a short window of opportunity during which more permanently stabilising procedures that require general anaesthesia (GA) can be applied. Delaying invasive interventions unnecessarily will cost a life in many situations.
Generally speaking stabilization should not take longer than a few hours. In most cases it should be achieved within the first hour or two after presentation.

  1. Under what emergency situations can one not afford to wait before administering an anaesthetic?

If a patient needs such an urgent intervention to save its life (like an obstructed trachea in a dog that you cannot relieve without an anaesthetic such as a puff adder bite that needs an emergency tracheostomy or a trauma patient that will die without an immediate insertion of an ET tube or a bulldog with heat stroke that has such bad laryngeal oedema or collapse that it is cyanotic and deteriorating in front of your eyes).
If a patient is in life-threatening status epilepticus (such as with strychnine poisoning or following cranial trauma).
A dog or cat with a flail chest or pleural effusion that will not tolerate a conscious needle drainage or drain insertion under local anaesthetic for stabilisation.
A patient with rupture of an organ that is haemorrhaging causing haemothorax or haeomoperitoneum (such as a ruptured liver or spleen). These patients will require emergency transfusion whilst surgical intervention to stop the bleeding occurs.
A patient in a state of collapse following a neurotoxic snakebite that requires mechanical ventilation.
Patients with a ruptured gastro-intestinal tract, strangulation or volvulus lesion should undergo surgical correction as soon as possible. The longer contamination of the peritoneal cavity is present the poorer the outcome is.

  1. When should primary intervention to correct the underlying problem be initiated?

Intervention to correct the underlying primary problem should be performed when the risk of delaying further treatment is greater than the benefit of continuing stabilisation. Complete normalisation may not yet have been achieved.
When further stabilisation is no longer possible due to ongoing deterioration of the patient’s condition.
When the patient is stable enough (i.e. not yet normalised) to undergo intervention to correct the primary problem.
As an example, a patient with septic peritonitis may benefit from surgical treatment before stabilisation to "normal" levels, as it is quite likely that this cannot be achieved before the abdomen has been lavaged.

  1. Which emergency situations usually need stabilisation before anaesthetics can be administered?

Cats with obstructive uropathy that are hyperkalaemic. These cats should have their bladders needle drained and the hyperkalaemia should be resolved before catheterisation under general anaesthetic.
Patients with clinically significant pulmonary contusion (usually following road traffic accidents).
Dogs with GDV should be trocharised and urgently fluid loaded at the same time before an anaesthetic is given. Some of these patients can be stomach tubed by mouth without anaesthetic but most will require at least a short anaesthetic for this. If these dogs are in shock it is my/our opinion that the stomach should be deflated and the vascular compartment fluid and colloid loaded before surgical anaesthetic is administered.
All patients in shock as a result of trauma that is not resulting in immediate life threatening danger (such as listed above). Fight wounds and dogs involved in motor vehicle accidents are common examples. Adequate resuscitation is normally required before radiographic examinations, wound care or complete assessments can be made. It is important to remember however that a patient that has been injured may only destabilise after admission and as such trauma patients should be closely monitored.
All dogs with a blood loss anaemia (and MAP below 70 mmHg) or haemolytic anaemia (with haematocrits below 20%)
Pyometra/septic peritonitis cases may need stabilisation although most cases are sufficiently stabile to intervene immediately. The general rule of thumb in any septic patient is to surgically drain or remove the source of sepsis without any or the minimum of delay. Any stabilisation without doing this can be expected to be short lived.
Cases that are clinically dehydrated should be rehydrated in the shortest possible time before urgent anaesthetics are administered.
Cases in low output cardiac failure that is not stable on treatment.

8. Anaesthesia

The appropriate use of anaesthetic agents is dependent on the clinical situation of the patient and a thorough knowledge of the clinical pharmacology of drugs used. There is little evidence to support the use of specific drugs and their influence on outcome is negligible. The most important determinate of outcome is the treatment and maintenance of stable cardiovascular and respiratory function (ensuring an adequate oxygen delivery to all cells in the body).

Essentially patients can be classified into three categories:

          1. those who are stable may receive a standard induction or slightly reduced induction dose;
          2. those who are unstable should have the anaesthetic slowly titrated to effect;
          3. those who are moribund or near death where minimal anaesthesia should be used.

General principles to be remembered when anaesthetising compromised patients include the optimisation of tissue perfusion and oxygen delivery to all organ systems while achieving analgesia, muscle relaxation and unconsciousness.

A. Pre-anaesthetic Management

In certain cases prompt surgical treatment is life saving (uncontrolled haemorrhage, perforated intestines, ruptured pyometra or uterus, etc) but generally the accompanying shock is more dangerous than the initial trauma. In these cases the treatment of shock is the priority before surgical correction is attempted.

Oxygen supplementation

This is almost always indicated and can be achieved with a facemask, nasal catheter, endotracheal tube or oxygen chamber. The use of an oxygen chamber is not ideal, as it does not allow access to the patient. Oxygen supplementation is especially important in cases with respiratory and cardiac compromise. In anaemic patients correcting haematocrit is more important than oxygen therapy. Remember not to stress animals attempting to administer oxygen. Free flow oxygen passed the face may be the best way of administration in cases that are overly anxious.

Analgesics and sedatives and premedications

Useful premedications include morphine and diazepam and these are usually indicated. Acetylpromazine and alpha-2 agonists are generally contraindicated. These are required for the management of pain, fear and apprehension. Analgesia should be applied as soon as possible and be maintained post trauma or post-surgery.

Anticholinergics

These are not routinely used in trauma patients as they increase myocardial oxygen demand and reduce the threshold for arrhythmias. Atropine or glycopyrrolate may be used when vagal influence affect cardiac function or secretions need to be controlled. Trauma patients are often not starved and this increases the incidence of aspiration. Methods to reduce aspiration include position of the head lower than the abdomen, immediate intubation with a cuffed endotracheal tube and suctioning if reflux occurs.

B. Induction Agents

It is always best to remember that the drug with which you have the most personal experience is most likely to be the safest drug in your hands. Always remember that anaesthetic drugs administered to a patient in shock in the same way as they would be used in a healthy patient will aggravate shock. This is because these drugs have potent respiratory and cardiovascular depressive effects that will exacerbate the state of shock.

Intravenous agents:

Barbiturates are known to decrease myocardial contractility, depress baroreceptor reflexes, vasodilatation, decreases venous return and depress respiration. They are poor analgesics and have no muscle relaxation. Barbiturates are arrhythmogenic when given rapidly intravenously. Propofol has less of myocardial depressant effect and fewer arrhythmias than thiopentone. The degree of myocardial depression is a function of dose and rate of injection. Barbiturates are highly protein bound and the acid-base balance; albumin content and concurrent drug administration influence their pharmacokinetics. Critically ill patients are often acidotic and hypoproteinemic and thus require lower anaesthetic doses.

Propofol causes similar haemodynamic effects as thiopentone and cannot be recommended over thiopentone unless the patient is cardiovascularly stable.

Both these agents should be given very slowly intravenously to effect to achieve a plane of anaesthesia just sufficient for intubation. Rapid administration of these agents in states of myocardial hypoxia can be fatal and it is therefore essential to supplement oxygen and ensure adequate blood pressure before induction. Propofol has a greater tendency than thiopentone to cause induction apnoea, especially if given too fast. This may prove fatal in hypoxaemic animals.

Ketamine is one of the few drugs with indirect cardiovascular stimulatory properties. It raises blood pressure secondary to a sympathetic increase in heart rate and cardiac output. It is contraindicated in any patient with myocardial disease or in a patient in maximal sympathetic stimulation (which is often the case in severe trauma or haemorrhagic shock). Ketamine is a poor muscle relaxant and spontaneous movement is possible. Ketamine increases intracranial pressure and is contra-indicated in cranial trauma. Benzodiazepines may be used with ketamine to enhance muscle relaxation.

Opioid induction

This mode of induction is the preferred method for induction in compromised patients but needs to be used with caution and a complete description of the use of this modality is beyond the scope of this short review. Opioids are most commonly combined with benzodiazepines and the opioid drugs most commonly used include fentanyl, morphine and midazolam. Mixed agonist-antagonists include buprenorphine and are not preferred due to their ceiling effects and an inability to titrate the analgesia.

Because opioids (especially fentanyl) result in respiratory depression pre-oxygenation is advisable. Bradycardia may occur and is treatable with anticholinergics and in refractory cases adrenaline may be used. Morphine is associated with a dose dependent histamine release and may result in hypotension. Adequate fluid loading can minimise this negative effect and should not be a reason for withholding morphine administration. Opioid combinations (eg fentanyl and morphine) have been used successfully. Opioid induction (with for example diazepam and fentanyl) is slower than conventional methods and if rapid intubation is required it cannot be recommended. Opioids are administered to the point where tracheal intubation is possible.

Inhalation agents

These are as hypotensive as barbiturates and are only safer because the period of time from drug application to surgical anaesthesia is longer thus allowing for a longer period of time for the body to establish homeostasis for their depressant effects on blood pressure. Halothane and isoflurane both cause a dose dependent cardiovascular depression. Isoflurane is the least depressant at equipotent minimum alveolar concentration values. Isoflurane is also less arrhythmogenic and causes a quicker induction than halothane. If a traumatised patient is alert it is likely to struggle with gas induction. This struggle may result in increased circulating catecholamines which may result in arrhythmias (especially with halothane) as halothane sensitises the myocard to adrenaline. Inhalation agents have no muscle relaxing or analgesic properties.

Some dogs in severe shock require no anaesthesia and only some analgesia in order for surgery to be performed. A less depressant form of analgesia using a local anaesthetic block should be considered. Epidural anaesthesia can be useful tool for abdominal or hind limb procedures. Patients should be fluid loaded as epidural anaesthesia can result in vasodilatation. The lowest possible dose of anaesthetic should be used and all intravenous agents should be titrated to effect.

Maintenance of anaesthesia.

Some patients may require mechanical ventilation. The fact that a patient is breathing does not necessarily mean it is adequately ventilating. Remember also that an anaemic, pale patient may not show cyanosis. Close monitoring of the measures of blood pressure, perfusion and tissue oxygenation must be maintained constantly throughout the period of anaesthesia. In addition to close physical assessment of these measures of the patient’s condition, additional monitoring that is very useful includes ECG, pulse oximetery, blood pressure (invasive arterial and or central venous pressure), and capnography. Haemodynamic stability is maintained with fluids, ionotropic agents (dobutamine constant rate infusion), vasoconstrictors (dopamine or adrenalin constant rate infusion) and colloids. Direct arterial blood pressure measurement and central venous pressure can be used to monitor haemodynamic stability. Urinary output is easily monitored with a catheter and is an important indicator of renal perfusion and fluid balance.

A combination of inhalation agents, opioids, benzodiazepines or ketamine can be given for maintenance.

Hypothermia kills and patients should be warmed or the body temperature maintained using hot warm bottles, warm water blankets, heating lamps or warm air heating devices during all anaesthetic procedures and post operative care should pay special attention to this aspect.

Useful references include (but are not limited to) the following texts:

Manual of Canine and Feline Emergency and Critical Care

Ed: L King, R Hammond.

Published by BSAVA, United Kingdom

1999

ISBN 0 905214 40 4

Emergency Medicine in Small Animal Practice

The Compendium Collection

Published by Veterinary Learning Systems, Trenton, New Jersey.

1997

ISBN 1 884254 24 1

Small Animal Emergency and Critical Care. A Manual for the Veterinary Technologist.

A M Battaglia

Published by WB Saunders Co.

2000

ISBN 0 7216 7773 8

Emergency Procedures for the Small Animal Veterinarian.

SJ Plunkett

WB Saunders, Philadelphia

1993

ISBN 0 7216 6781 3

Self Assessment Colour Review of Small Animal Emergency and Critical Care Medicine.

R Kirbey

Mason Publishing Ltd, London, UK.

1998

ISBN 1 874545 65 6

Fluid Therapy in Small Animal Practice. 2nd Edition.

Editor: SP DiBartola

WB Saunders Co. Philadelphia

2000

ISBN 0 7216 7739 8.

Some useful drugs and doses are tabulated below.

NAME

DOSAGE

ROUTE

STRENGTH

PREMEDICATION

ATROPINE SULPHATE

0.02 mg/kg

i/v

0.5 mg/ml

(ATORPINE)

0.04 mg/kg

s/c

MIDAZOLAM

0.3 mg/kg

i/m or i/v

5 mg/ml

DIAZEPAM

0.2 mg/kg

i/v

5 mg/ml

INDUCTION

THIOPENTONE SODIUM

10 mg/kg (with premed)

i/v

25 mg/ml

15 mg/kg (without premed)

PROPOFOL

4 - 6 mg/kg (with premed)

i/v

10 mg/ml

6-8 mg/kg (without premed)

KETAMINE

5 - 10 mg/kg

i/v

100 mg/ml

10 - 20 mg/kg

i/m

FENTANYL

40 microgram/kg

i/m or i/v

0.2 mg/ml

OTHER

ADRENALINE

1 ml/10 kg

i/v

1/1000

ATROPINE

1 ml/10 kg

i/v

0.5 mg/ml

LIGNOCAINE

1 ml/10 kg

i/v

2%

ANALGESICS

MORPHINE SULPHATE

0.2-0.4 mg/kg

s/c or i/v

10 mg/ml

(MORPHINE)

FENTANYL

10 – 20 microgram/kg

i/m or i/v

0.2 mg/ml

BUPRENORPHINE

0.02 mg/kg

i/m

0.3 mg/ml

EPIDURALS

MORPHINE SULPHATE

0.1 mg/kg

EPIDURAL

10 mg/ml

(MORPHINE)

ROPIVACAINE

1 mg/kg

EPIDURAL

7.5 mg/ml

(Published Newsletter 38, November 2004)