CPCR: Is it worth it? (Proceedings)

Cardiopulmonary resuscitation (CPR) or the more comprehensive term cardiopulmonary cerebral resuscitation (CPCR) is still one of the most important interventions performed in human or veterinary medicine. This is especially true in veterinary medicine when dealing with emergency and critical critically ill patients. Although not performed frequently, it must be utilized in the appropriate situation and using the most effective techniques in order to maximize the possibility of success. Currently estimates of successful CPCR in veterinary patients are reported to be less than 10% but this may vary from institution to institution. This figure compares favorably with survival rates reported for out of hospital cardiopulmonary arrest in human patients but is lower than reported survival rates for in hospital arrests.

Advance directives

Advance directives are a formal component of human medicine and have gained more importance in veterinary medicine. In generic terms, advance directives refer to information provided to a physician by a patient detailing what care should be provided if the patient in unable to make decisions. In veterinary patients this equates to the primary care clinician having a detailed discussion with the pet owner concerning medical management of a pet who has had a life threatening medical event. In more general terms and for the purpose of this discussion, this addresses whether the patient should be resuscitated if cardiopulmonary arrest occurs. This discussion should be documented in the medical record. There should be a way of identifying which patients have "do not attempt resuscitation" (DNAR) orders and the nursing staff and all clinicians should be made aware of these orders.

Comparative CPR

In the year 2000, the American Heart Association published new guidelines for cardiopulmonary resuscitation and emergency cardiovascular care which made significant changes in previous guidelines. These guidelines represented changes in protocols that should not be immediately extrapolated to veterinary patients. The most significant changes included the incorporation of immediate electrical defibrillation into Basic Life Support (BLS). Previous to this recommendation, BLS included A, B, C, Airway, Breathing and Circulation. Defibrillation was included in Advanced Life Support (ALS). The successful impact of rapid electrical defibrillation on survival in patients with cardiopulmonary arrest led to this change in recommendations. The overall impact of this recommendation has been the increased availability and access to automated external defibrillators (AED). This technology allows minimally trained lay rescuers as well as trained rescuers the ability to defibrillate patients with ventricular fibrillation. The emphasis on early defibrillation is based on reports that as many as 70% of people in the United States who experience out of hospital cardiopulmonary arrest are initially in ventricular fibrillation.

Although ventricular fibrillation does occur in dogs and rarely in cats, it is not necessarily the most common rhythm disturbance recognized and is not recognized in the majority of veterinary patients who experience cardiopulmonary arrest.

It has also been found that what would be considered standard drug therapy in veterinary patients, utilizing epinephrine and atropine has not significantly improved survival in people and therefore these drugs are considered marginal. These drugs would still be considered the cornerstone of therapy for veterinary CPCR.

In the year 2010 there were additional modifications of the guidelines. The most significant of these changes is the recommendation for chest compressions to be started immediately and changing the well known acronym from A, B, C to CAB. This recommendation is again of questionable value in veterinary patients.

Techniques

Basic life support

In veterinary medicine basic life support (BLS) or basic CPR is still considered the A, B, C's (Airway, Breathing, and Circulation). Due to the limited access to defibrillators in veterinary practice and the nature of the underlying diseases contributing to cardiopulmonary arrest in veterinary patients, it is unlikely that this will change. It is also important for the veterinary clinician to recognize that many dysrhythmias occur in the critically ill dog and cat. We cannot extrapolate the information that approximately 70% of people who experience cardiopulmonary arrest out of hospital have ventricular fibrillation as their initial rhythm. The common dysrhythmias associated with cardiopulmonary arrest in dogs and cats include, ventricular asystole, pulseless electrical activity (PEA) (formerly known as electrical mechanical dissociation (EMD) and ventricular fibrillation. Therefore the utility of early defibrillation in veterinary patients is unknown unless the patient has documented ventricular fibrillation.

Current recommendations for veterinary BLS include assessing and providing an airway if required, assessing and providing breathing if required and assessing and providing circulation if required.

Airway

Although there are no real changes in the recommendations for this procedure, the priority is to establish an airway. In most cases this is accomplished by per oral endotracheal intubation. Confirmation of correct tube placement is recommended by visual assessment, digital palpation and/or by ETCO2 monitoring. It has been suggested that airway be prioritized below circulation and breathing. In this scenario rather than A, B, C, the priority would be C, A, B. If following this protocol, external thoracic compressions should be started immediately and prior to providing an airway. This recommendation is based on research performed to evaluate bystander (out of hospital) CPR. In some experimental animal models, most notably pigs, it has been shown that animals can be resuscitated from ventricular fibrillation by only performing chest compressions. It has been hypothesized that when performing external chest compressions, enough passive ventilation occurs for the patient to survive. Another important factor is that during ventricular fibrillation, oxygen delivery is dependent on blood flow, not ventilation.

Breathing

The only innovations with breathing have been the use of simultaneous compression ventilation protocols by some clinicians. When utilizing the thoracic pump theory, it has been demonstrated in animal models that cardiac output can be improved by increasing intrathoracic pressure. Conventional external thoracic compressions utilize the cardiac pump theory in combination with interposed respirations. The cardiac pump involves direct compression of the myocardium to promote cardiac output. The thoracic pump utilizes compression of the thoracic cavity to indirectly improve cardiac output by effecting blood flow through the giant vessels. The incorporation of simultaneous ventilations dramatically increases intrathoracic pressure. There are however, potential complications associated with this technique which include barotrauma and pulmonary contusions.

Circulation

The circulation component of BLS has not changed significantly and has been discussed in part under the breathing section with reference to simultaneous compression and ventilation protocols. There are currently two theories explaining the mechanisms of cardiac out during external thoracic compressions. The cardiac pump theory suggests that direct compression of the myocardium accounts for the majority of cardiac output. In veterinary patients this would apply in patients with body weights of < 15 kg. In patients with body weights of > 15 kg, the thoracic pump would better explain cardiac output. The latter suggests that compressions of the widest portion of the thorax increase intrathoracic pressures which indirectly lead to increased cardiac output. This may be combined with simultaneous ventilation which should further increase intrathoracic pressures. The use of interposed abdominal compressions during CPR has become more common and has been shown to increase venous return by up to 25%.

Advanced cardiac life support

In veterinary medicine advanced life support (ALS) has included all procedures outside of BLS. So the pneumonic of D, E, F still applies.

Drugs

Although there are no longer strong recommendations for epinephrine and atropine for therapy of cardiopulmonary arrest in people, these drugs still play a valuable role in veterinary CPR. The most commonly reported dysrhythmias identified during cardiopulmonary arrest in the dogs and cats are bradyarrhythmias, asystole and pulseless electrical activity (PEA) formerly known as electrical mechanical dissociation. Atropine and epinephrine may be useful for management of these dysrhythmias. The current recommendation for the use of epinephrine is using the low dose protocol (.01-.02 mg/kg). This protocol calls for the use of the standard concentration of epinephrine (1:1000) diluted 10 fold and administered at a volume of 1 ml/10 kg of body weight. This is compared with the high dose epinephrine protocol which utilizes the same volume administered (1ml/10 kg) but using the 1:1000 dilution epinephrine straight from the bottle. There is some renewed interest in the use of vasopressin and recent publications have shown some increase in survival in people who have failed conventional medical therapy. Vasopressin can be administered at a dosage of .8 U/kg. This equates to approximately .5 ml/10 kg of body weight. The advantage of vasopressin may be its performance in the face of hypoxia and acidemia. In addition its mechanism of action is via direct stimulation of receptors in vascular smooth muscle and it does not appear to increase myocardial oxygen consumption. Atropine is still administered at .04 mg/kg or approximately 1 ml/10 kg body weight.

Routes of administration that are recommended include intravenous through a central line, intravenous via a peripheral line with bolus to follow, intraosseous administration and intratracheal administration. The drugs that can be safely administered via the intratracheal route can be remembered by the pneumonic NAVLE and include Naloxone, Atropine, Vasopressin, Lidocaine and Epinephrine. The doses should be twice the intravenous dose and should be diluted in sterile water.

ECG

In veterinary CPCR it is still imperative to know what dysrhythmia is present. Because it is difficult to predict which rhythm is present, intervention and outcome will be dependent on this diagnosis. The most common reported dysthymias include ventricular asystole, PEA and ventricular fibrillation.

Fibrillation control

Defibrillation is prioritized lower and still included in ALS for veterinary patients for two reasons. First and foremost, patients must be in ventricular fibrillation for this intervention to be utilized. Secondly, it still appears that many veterinary hospitals do not have access to this equipment. The advances in the area of defibrillation are the use of biphasic defibrillators which may be more effective than the monophasic defibrillators.

Fluid resuscitation

Fluid resuscitation should be a component of CPR. However, aggressive volume expansion should only be utilized if it is suspected that volume depletion has contributed to the cardiopulmonary arrest. Volume overload can have detrimental effects on these patients. An increase in central venous pressure and right sided pressures may lead to reduced coronary perfusion pressures.

Monitoring

Monitoring of patients during CPR is imperative but difficult. Physical examination along with palpation of peripheral pulses, venous blood gas analysis, venous lactate concentration and end tidal capnography are the most reliable tools. Other suggested techniques include the use of a Doppler blood pressure probe on the cornea to evaluate cerebral artery perfusion but this is unreliable in the author's experience. Return of spontaneous ventilation is often a limiting step as it appears that return to spontaneous circulation occurs more commonly. The frequent cessation of external thoracic compressions in order to examine the ECG or auscultate the thorax may lead to decreased success.

Post-resuscitative Care

Due to poor survival rates, it is often unexpected that post-resuscitative care is required. It is however imperative that the clinician anticipate survival so this care can be provided immediately. If post-resuscitative care cannot be provided then the CPR was wasted. Post-resuscitative care is primarily symptomatic and is dependent on close monitoring and excellent nursing care. Oxygen supplementation should be considered in all of these patients and ventilatory support may also be required. Fluid therapy should be continued as required and inotropic and pressor therapy may be indicated to maintain mean arterial pressures. Neurologic dysfunction may be common following resuscitation and infusion of mannitol at .5-1 g/kg over 30 minutes may help treat cerebral edema and has free radical scavenging properties. It has also been demonstrated that hyperglycemia may have a negative impact on outcome and therefore should be monitored. Due to the lack of evidence for their use, corticosteroids should be avoided. If possible prognosis should not be based on neurologic signs within the first 24-48 hrs due to the possibility of significant change. Nutritional support via feeding tubes or total parenteral nutrition should be provided if patients have not returned to reasonable function within 48-72 hrs.

References

American Heart Association Circulation 2010 122 S676.

Cole SG. J Vet Emerg Crit Care 2004 12(4) 261.

Cole SG. J Vet Emerg Crit Care 2003 13(1) 13.

Haldane S. Compend Cont Educ Pract Vet 2004 26(10) 780.

Haldane S. Compend Cont Educ Pract Vet 2004 26(10) 791.

Hofmeister EH. J Am Vet Med Assoc 2009;235:50.

Plunkett SJ. J Vet Intern Med, 2008, 22:9.

Waldrop, J.EJ Vet Emerg Crit Care 2004;14(1):22.