Queen Mother Hospital for Animals, Royal Veterinary College, North Mymms, UK
CPR algorithms can be beneficial as we have to act quickly with not much time to think. The author uses the algorithm CABDEF to help with CPR (although on-line posters are also available). The CAB component is what is classed as basic life support, and the most important component of CPR. Once CAB is completed. DEF which is classed as advanced life support should be performed. It is important to identify a team leader, who will give directions so the CPR flows well.
Cardiac compressions are the most important part of CPR and must commence within 10 seconds of identifying cardiac arrest. Cardiac arrest should be identified by auscultation of the heart; however, if the witness is unsure, they should start compressions immediately if the animal is not breathing and nonresponsive. Compressions are performed at 100–120 compressions per minute. There are 2 methods of compressions. For animals greater than 10 kg, thoracic compressions are performed. This involves creating enough intrathoracic pressure to compress the heart. Hands are placed at the highest/widest part of the thorax while the animal is in lateral recumbency. The thorax is compressed approximately 1/3–½ of the width of the thorax (which is usually achieved when compressing as hard as possible). It is important to have a compression to decompression ratio of 1:1 and to decompress (recoil) completely. Recoil is important for venous return and complete filling of the right atrium of the heart. Each cycle of compressions performed by one individual should last approximately 2 minutes; or shorter if compressions are ineffective (efficacy of compressions will be discussed below). It is important to time the change in cycles well to minimise interruptions in CPR. When performing compressions in animals <10 kg (or very flat-chested dogs), external cardiac compressions are performed over the heart which is between the 3rd and 5th ribs (at the point of the elbow). It can be performed by 2 methods:
1. Thumb and fingers are placed on each side of the heart and compressions are performed by one hand.
2. Fingers of each hand are placed on each side of the heart and compressions are performed by using both hands.
The efficacy of CPR can be monitored by 3 methods:
1. Femoral pulse palpation: Each time an effective compression is performed, the femoral pulse should be palpable.
2. Capnography: ETCO2 of 15–20 mm Hg should be achieved with successful compressions; less than this is ineffective compression. ETCO2>35 mm Hg suggests return of spontaneous circulation (ROSC).
3. Doppler: The Doppler probe which is used to measure blood pressure can be applied to the surface of the cornea with any type of conduction gel. Every time an effective compression is performed, a pulse should be audible. However care must be taken not to interpret interference noise as a pulse.
To maintain patency of the airways, endotracheal intubation must be performed. If intubation is difficult, a ridged canine urinary catheter can be used as a stylet. Once the ET tube is placed, the cuff must be inflated and the tube secured with a tie. Once intubated, there may be more airway secretions which accumulate in the ET tube. A sterile suction catheter can be used to aspirate respiratory secretions. If intubation is impossible, emergency tracheostomy can be performed; however, majority of the time intubation should be possible with the guide of the canine urinary catheter.
Ventilation should be performed either using an anaesthetic circuit with 100% oxygen and a rebreathing bag, with the maximum pressure of each breath 20 cm H2O. An Ambu bag attached to an oxygen source can also be used and may be safer as there is a safety release valve so the pressure of each breath cannot exceed 20 cm H2O, no matter how hard you compress the Ambu bag. Ventilation is performed by giving 1 breath every 6–8 seconds.
Drugs can be administered via a peripheral IV catheter, jugular catheter, intraosseous catheter, or transtracheally. If IV access is not possible, transtracheal method should be considered, in which case the dose should be increased to 3–10x the standard dose. When administering drugs transtracheally, a ridged canine urinary catheter should be advanced via the ET tube to the bronchial bifurcation. The drug is administered via the urinary catheter followed by air or sterile water to flush the content of the urinary catheter. Drugs which can be administered transtracheally include adrenaline, atropine, vasopressin, naloxone and lidocaine.
Adrenaline (Epinephrine) (0.1 mg/kg)
Adrenaline is an α and β adrenergic receptor agonist which results in vasoconstriction, coronary dilation, bronchodilation and positive inotropy and chronotropy. It is the vasoconstrictive effect of the peripheral vasculature which is desired, as it will improve circulation of the major body organs (heart, lung, brain). The 1:10000 concentration is used as the 1:1000 volume is too small for dosing. Adrenaline should be administered immediately, then every 3–4 minutes (or every second cardiac cycle).
Vasopressin (0.8 U/kg)
Vasopressin causes vasoconstriction by acting on the v1 receptors on the endothelium. It can be used interchangeably with adrenaline. The benefits of vasopressin are that it can work in an acidaemic environment and has a different mechanism of action to adrenaline. However, it is much more expensive and not proven to be more beneficial than adrenaline.
Atropine (0.05 mg/kg)
Atropine is a parasympatholytic (decreases vagal tone), which will help improve the sympathetic nervous system (i.e., vasoconstriction, inotrope, chronotrope). Atropine should also be administered immediately. However, compared to adrenaline, it is much longer acting and the ideal frequency of administration is unknown. The author administers atropine every 6–8 minutes.
Lidocaine (2 mg/kg)
Lidocaine is a sodium channel blocker which is commonly used to treat ventricular tachycardia (VT) or used as an analgesic. Lidocaine is indicated if there is ventricular fibrillation or pulseless VT (VT during a cardiac arrest) when defibrillation and precordial thump is unsuccessful.
Naloxone (0.04 mg/kg)
Naloxone is an opiate antagonist, and should be administered to any animal in cardiac arrest which has received a pure mu agonist or partial agonist opiate (even if it is not the cause of the arrest).
Butorphanol (0.1 mg /kg)
Butorphanol is a partial opiate agonist/antagonist. If naloxone is unavailable, butorphanol can be used instead to reverse some of the effect of opiates.
Atipamezole (50 mcg/kg)
Atipamezole is an α2 adrenergic antagonist, which can be administered to reverse the effect of medetomidine, dexmedetomidine and xylazine.
Flumazenil (0.01 mg/kg)
Flumazenil is a benzodiazepine antagonist, which can be administered to reverse the effects of any benzodiazepine.
50% Glucose (Dextrose) (1 mL/kg)
When placing an IV catheter blood should be collected to measure blood glucose, electrolytes and blood gas. Glucose administration is indicated in any hypoglycaemic or hyperkalaemic patient. Although 50% glucose is normally diluted, dilution is not necessary during CPR.
Sodium Bicarbonate (1 mEq/kg)
Sodium bicarbonate is indicated in patients with diagnosed or suspected metabolic acidosis. These can include diabetes with ketoacidosis, urethral obstructions, azotaemia, just to name a few. The author also administers sodium bicarbonate 20 minutes into CPR.
10% Calcium Gluconate (1 mL/kg)
Calcium gluconate is indicated in any hypocalcaemic or hyperkalaemic patient. Although standardly it is given over 20 minutes, it can be given more rapidly over 1–2 minutes during CPR.
An ECG should be performed as soon as possible. There are only 4 arrhythmias seen in cardiopulmonary arrest:
1. Asystole: Most common form seen in dogs and cats. CPR should be performed as normal.
2. Ventricular fibrillation (VF): VF is described by continuous irregular ventricular waveforms which represent the fluttering of the heart. Treatment of VF is defibrillation with a defibrillator. If this is not available, precordial thump may convert the rhythm. Precordial thump involves applying large amount of force rapidly over the heart (almost like a punch!). If this is not successful, lidocaine may be administered. The aim of treatment of VF is to convert it into asystole.
3. Pulseless ventricular tachycardia: This is when the ECG looks like VT, however there is no heartbeat. This is treated the same as VF.
4. Pulseless electrical activity (PEA): This includes any wave forms (including normal sinus rhythm), with no heartbeat. This is treated the same as asystole.
Defibrillation is performed with a defibrillator. The defibrillator is set at 2–3 J/kg and plenty of conduction gel is applied to both defibrillator paddles. When a cycle of compressions is completed, the animal is placed in dorsal recumbency, and the paddles are held over both sides of the thorax. The person performing the defibrillation yells "clear!" ensuring no one is touching the table or animal before defibrillation is performed. Commence compressions immediately.
Fluids should be administered in hypovolaemic patients. If the animal died of cardiac reasons, fluids should not be administered; however, after each drug is administered, a small bolus (5–20 ml in total) of fluids should be administered. There is no ideal dose of fluids; however, the author uses 20 mL/kg of Hartmann's or CSL as quickly as possible. Hypertonic saline is also a good choice as it increases the intravascular volume quickly, and it reduces cerebral oedema which commonly occurs during cardiac arrest.
Cause of Arrest
The following is a list commonly used in human medicine called the 6 Hs (hypovolaemia, hypoxaemia, hydrogen ion (acidosis), hypo/hyperkalaemia, hypothermia, hypoglycaemia) and 6 Ts (tachy/bradycardia, toxins, tamponade (cardiac), tension pneumothorax, thrombosis, trauma).
Identification of Return of Spontaneous Circulation (ROSC)
ROSC can be identified by palpable pulses when compressions are not performed, and confirmed by auscultation of the heart. ROSC can be suspected if ETCO2 increases above 35 mm Hg. A normal ECG trace does not mean there is ROSC, as the rhythm may be PEA as discussed above.
ECG, SpO2, ETCO2, blood pressure, temperature, PCV/TS, electrolytes, and blood gas should be monitored. The animal may need to remain intubated and ventilation if hypoventilating.
After CPR, it is important to debrief with the team to improve team moral. It is also important to discuss what went well, and what could be improved to improve CPR in future patients.
References are available upon request.