Cardiac arrest & heart failure
Cardiac arrest is the complete absence of a heartbeat. This is a very serious situation and needs correcting within a few minutes. Brain cells are particularly at risk from a lack of oxygen (hypoxia) and irreversible brain damage is likely unless the circulation to the brain is restored within 3 minutes. Resuscitation may not be appropriate for tigers with very severe injuries – such as a fractured spine.
Heart failure means no effective output from the heart. Therefore, the term includes not only cardiac arrest, but also uncoordinated beating of different parts of the heart such as occurs with ventricular fibrillation. Most anaesthetic drugs have a depressant effect on cardiovascular function and anaesthetic overdoses are the commonest cause of heart failure during anaesthesia. However, some animals that need to be anaesthetised may be very old or sick, or suffering from metabolic problems that cannot be assessed by visual examination. Such animals have a high risk of heart failure during anaesthesia. Other causes include respiratory failure or hypoxia, hypothermia, pH and electrolyte imbalances in the blood, and extreme pre-anaesthetic excitement or stress.
The signs of heart failure include an absence of a previously palpable pulse, no heart sounds audible with a stethoscope, cyanosis (blue coloration) or pallor of mucous membranes, a prolonged capillary refill time, and cold extremities. These signs are rapidly followed by wide dilatation of pupils, and cessation of breathing or agonal gasping.
Immediately these signs are detected the anaesthetist should look at their watch and set a stopwatch running. The administration of any anaesthetic drugs including gaseous agents should be discontinued, and:-
- Check that the passage of air into the lungs is unobstructed. See respiratory failure.
- Establish and maintain breathing. See respiratory failure. This may have to be by positive-pressure ventilation delivered via a resuscitation bag (or “Ambu” bag) attached to the endotracheal tube). If oxygen is available it should be provided immediately, and doxapram administered intravenously to re-establish breathing if it has ceased.
- With the tiger laid on it’s side apply external cardiac massage, i.e. intermittent pressure on chest wall directly over the heart once per second. Each compression should be held for a brief period to maximise elimination of the blood from heart and chest.
- Place head down to assist blood flow to the brain.
- One ventilation of the lungs should be delivered for every 4-5 chest compressions.
- External cardiac massage can be helped by placing a solid surface or sandbag under the area of the heart, and applying continuous pressure to the abdomen.
- Give intravenous or intra-cardiac adrenaline at 0.01 mg/kg. This is equivalent to 0.1ml “Adrenaline injection 1:1000” per 10 kgs of animal diluted in physiological saline, lactated Ringers solution or even sterile water. For intracardiac injections long needles are required (5 – 7.5cms) and are inserted between the 4th and 6th ribs to access the heart. Continue heart compressions after administration.
- Repeat adrenaline every 3 – 4 minutes if necessary.
- Administer intravenous fluids rapidly (eg: 20mls per kg of Lactated Ringers solution or Hartmann’s solution).
- If the heart re-starts, give glycopyrrolate by slow intravenous injection at 0.01 mg/kg. This is equivalent to 0.5ml “Robinul” per 10 kgs of animal.
- If glycopyrrolate is not available give atropine by slow intravenous injection at 0.02 – 0.06 mg/kg. This is equivalent to approx. 0.3 – 1ml of a 0.6mg/ml solution per 10 kgs or between 0.2 and 0.6 ml of a 10 mg/ml solution per 100kgs.
Restoration of effective cardiovascular function after a cardiac arrest is difficult, and prevention is infinitely better than attempting a cure. A common situation during anaesthesia where too much anaesthetic has been given is the weakening of pulse strength and slowing of the heart. Timely administration of subcutaneous or very slow intravenous atropine or glycopyrrolate at this point can restore both and prevent heart failure developing.