The Mnemonic ABCDEF Is Useful in Remembering the Elements of CPR

The Mnemonic ABCDEF is useful in remembering the elements of CPR. Each member of the team can then take responsibility for the treatment priorties of life support.

Airway (A) Establish a patent airway. Intubate the patient with a well-fitting, cuffed endotracheal tube. Check that an existing tube is in the correct position and has not become blocked. In the event of upper airway obstruction, a long cannula can be passed to the tracheal bifuraction and oxygen insufflated until an emergency tracheostomy can be performed.

Breathing (B) Perform intermittent positive pressure ventilation (IPPV) with 100% oxygen and an appropriate anaesthetic breathing system. Alternatively an Ambu bag can be used which will allow manual lung inflation with either air or 100% oxygen. Initially give the patient two breaths rapidly of 1-1.5 sec duration, and then ventilate once every 3-5 secs, interspersed with external chest compression. Inflation of the lungs should provide supranormal chest wall expansion. It is allso important to allow the lungs to deflate fully after each ventilation to facilitate venous return.

Circulation (C) Support of the circulation is essential to restore blood flow to vital organs and preserve cerebral function. It can be accomplished through external or internal cardiac compression. The breed of dog (size and shape) and chest wall compliance determine the effectiveness of the chosen method.

 External Cardiac Compression Blood flow during external cardiac compression is achieved by one of two mechanisms: the ‘cardiac pump’ and ‘thoracic pump’.

Cardiac Pump: This is used in animals weighing less than 20kg, or very narrow- chested dogs, e.g Greyhound. The aim is to squueze the ventricles by compression of the ribcage over the cardiac area. This is performed with the patient positioned on a hard surface in right lateral recumbency with a slight head down inclination. One or both hands are placed between the fourth to fifth intercostal space, at the costochondral junction, and the chest wall is compressed with the arms extended, force being applied by bending at the waist. The chest should be compressed by approximately one third. A sandbag can be placed under the opposing chest wall for support. In cats, and patients weighing less than 5kg, the thumb and forefingers can be used to compress the chest. The rate of commpressions is approximately 90-120 per minute. The patient should be ventilated once every fifth or sixth compression.

Thoracic Pump: This technique is suited to larger dogs, or for those with ‘barrel’ chests. The chest is compressed at its widest part to increase intra-thoracic pressure. This is transmitted to the intrathoracic vasculature, which leads to improved perfusion. Additional ways of improving venous return with this technique are; Synchronous lung inflation with chest wall compression. Abdominal binding of the hindlimbs and abdomen. This will direct blood flow from the abdomen towards the head. Intermittent abdominal compression. The abdomen is slowly manually compressed as another means of improving blood flow. Placing the patient in dorsal recumbency and compressing the sternum. The effectiveness of CPR must be monitored within 3-4 mins of onset. If the resuscitation efforts are not sufficient, the resuscitation technique must be changed.

The following signs indicate generation of forward blood flow: - Palpation of a pulse during compression; - Constriction of pupil - Improvement in mucous membrane colour - ECG changes - Internal cardiac compression

Internal cardiac compression has been shown to be more effective than external chest compression in the artifical circulation of blood and perfusion of tissues during CPR.

Internal cardiac compression is indicated in the following conditions; - failed external cardiac compression - pneumothorax or haemothorax - percardial tamponade - flail chest - diaphragmatic hernia - intra-operative thoracic or abdominal surgery.

The decision to perform a thoracotomy should be made after 5-10 mins if the initial techniques employed are ineffective.

A rapid clip of the third to sixth intercoatal space on the left side will remove the majority of long hair, and time should not be wasted in performing an aseptic surgical preparation of the site. The VN should ensure that a surgical pack containing a loaded scalpel is prepared, and rib spreaders are available. Internal compression allows visual and palpable assessment of ventricular filling, evaluation of the heart rhythm, accurate intraventricular injections, cross-clamping of the descending aorta, and assessment of lung inflation.

Disadvantages of internal compression include stopping cardiac compressions while performing a thoracotomy, potential damage to thoracic organs, unfamiliarity with the technique and postoperative sepsis.

Drugs (D) Drug therapy during CPA is dependent on the situation and the type of cardiac rhythm; therefore ECG monitoring is generally required.

The principal drugs used in the treatment of CPA are adrenaline, atropine, and lignocaine. Additional drug therapy is based on the response to the initial stages of CPR. There is considerable controversy regarding the value and place of many of the drugs suggested for use in CPR. Routes of drug administration during CPR

- Central Venous Route This is the route of choice. Drugs are deposited near the heart, and provide high concentrations in a short period of time. Placement of a jugular catheter is required such that its tip lies within the cranial vena cava. This technique is useful in dogs and cats only when a central venous catheter has been preplaced.

- Intraosseus Route This route is useful when vascular access is limited. There is rapid drug uptake from this site and large volumes can be administered. An intraosseus or spinal needle is inserted into the femur, humerus, wing of ileum or tibial crest.

- Intratracheal Route Drugs may be administered into the trachea (IT route) by passing a urinary catheter through the endotracheal tube to the bifurcation of the trachea. Double the dose of the chosen drug is required and it should be diluted with saline to provide adequate volume to both distribute the drug and improve absorption. A limited number of drugs can be administered by this route. Drug uptake from this site may be slow and final blood concentrations reduced, but it is better than peripheral venous administration of drugs if there is poor forward flow of blood during CPR.

- Peripheral Venous route This route is not ideal, but is often more convenient than others. Drugs take an excessive length of time to reach the heart, but following the drug with an intravenous bolus of sterile saline soluton enhances response to peripheral venous injection.

- Intracardiac Route This route of administration should be avoided unless open-chest CPR is being performed. Potential complications associated with this route of administration include coronary laceration, cardiac tamponade, myocardial trauma and refractory arrythmias.

Fluid administration during CPR should be conservative unless hypovolaemia has been identified as an underlying cause of cardiac arrest because aggressive fluid therapy can rapidly lead to volume overload.

The VN should ensure that drugs which are kept in the crash box remain within expiry dates. The practice of maintaining preloaded syringes with emergency drugs is not justified in veterinary practice. Unless the crash box is used on a routine basis, the drugs life expentancy will be diminished; therefore the waste and costs involved do not make it practical. Additionally, the large range in animal body weights results in doses being variable. Electrical defibrillation (E) The purpose of defibrillation is to convert the chaotic electrical activity of the fibrillating heart to sinus rhythm. The discharge of an electrical current through the myocardium aims to allow the pacemaker (sinoatrial node) to resume its normal rhythm. Defibrillation should be performed as soon as ventricular fibrillation is diagnosed, as success is inversely related to the elapsed time since fibrillation.

 External defibrillation: Conductive gel is applied to the defibrillator paddles, which are then placed firmly over the heart on each side of the chest. No personnel should be in contact with the patient at the time of discharge, therefore the operator should inform staff to ‘clear’. Energy level = 1-5J/kg.

 Internal defibrillation: The internal paddles should be covered in saline soaked swabs to ensure good contact. The paddles cradle the heart opposite each other, and the procedure is carried out as above. Energy level = 0.1-0.5 J/kg.

When a defibrillator is not available a precordial thump may be effective. This involves delivering a sharp blow with a clenched fist over the precordium. Chemical defibrillation agents are of unknown value, but can be tried when a defibrillator is not available. Bretylium tosylate has been used at a dose of 5-10 mg/kg.

Follow-Up (F) After successful CPR, continued support is required.

 Continue IPPV until the patient is breathing spontaneously. Provide a method of supplementing oxygen in the recovery period.  Correct Acidosis: Establish normal renal function or administer sodium bicarbonate if blood gas analysis is available.  Cardiac Support: Inotropes may be required to maintain cardiac output and improve renal blood flow.  Fluid Therapy: Fluids should be administered to maintain blood pressure and urine output.  Minimise cerebral oedema: Position patient with head inclined upwards, hyperventilate to reduce PaCO2 and administer corticosteroids and/or mannitol if cerebral oedema is suspected.  Assess neurological function  Monitor and maintain urinary output at 1-2ml/kg/hr.  Maintain body temperature.

Intensive monitoring is very important in the post-resuscitative period. Special attention should be paid to monitoring the respiratory, cardiovascular, central nervous and urinary systems. It is essential to monitor as many physiological variables as possible for each system to give an overview of the patient’s status and prognosis.