Introduction
Canine cardiopulmonary resuscitation (CPR) represents an emergency procedure that, if delivered correctly, can significantly increase the chances of survival in dogs suffering from cardiopulmonary arrest (CPA).[1] In 2012, the Reassessment Campaign on Veterinary Resuscitation (RECOVER) initiative published the first evidence-based, consensus guideline for CPR in small animals.[2] This initiative provides standardized guidelines to decrease the variability in the approach to canine CPR. While this initiative offers five domains of CPR for dogs, the discussion here is primarily limited to Basic Life Support (BLS) measures. BLS includes recognizing CPA, initiating chest compressions, airway management, and providing ventilation. BLS can be provided by both lay rescuers and medical professionals.
Anatomy and Physiology
Two main theories exist to describe how external chest compressions produce blood flow during canine CPR. The cardiac pump theory states that the ventricles are directly compressed during chest compressions, causing blood to be advanced through the circulatory system. The exact mechanism depends on the position of the canine. Cardiac pump theory postulates that the ventricles are compressed between the sternum and spine in the dorsal recumbent position and between the ribs in the lateral recumbent position. The thoracic pump theory suggests that pressure changes in the chest mediate flow rather than blood flowing from direct compression of the ventricles. As the chest is compressed, overall intrathoracic pressure increases, exerting that force upon the ventricles and the aorta and collapsing the vena cava, causing blood to flow out of the thorax.[3][4][5]
Allowing full recoil of the chest when performing CPR creates negative pressure in the thoracic cavity. This produces a gradient favoring blood returning from the peripheral to central circulation and into the cardiopulmonary circulation, allowing oxygen and carbon dioxide exchange. The evidence on the predominant mechanism in canine CPR is limited, but both proposed mechanisms likely contribute to blood flow in most cases.
Experimental evidence demonstrated higher left ventricular pressures and aortic blood flow when compressions were administered to canines placed in the lateral recumbent position compared to those in the dorsal recumbent position. Additionally, clinical data shows higher rates of ROSC when canines were placed in the lateral recumbent position.[5] Although there is little published evidence, profound anatomic differences between dog breeds suggest that there may be some benefit to different canine positioning and rescuer hand placement when administering chest compressions.[6]
Direct compression of the heart is difficult in most medium and large breeds, given the size of their thorax. In this case, the thoracic pump mechanism likely predominates. These dogs should be placed in either left or right lateral recumbency, and the rescuer's hands should be placed over the widest portion of the thorax to administer compressions to maximize intrathoracic pressure.[6]
Indications
Canine CPR should be initiated in cases of non-traumatic cardiopulmonary arrest. If resources and the situation allow, rescuers may consider CPR in canines with cardiac arrest secondary to traumatic causes but should understand that successful resuscitation is unlikely. Indications for CPR include a canine that is unresponsive and apneic or exhibiting agonal breathing.[1][7] Due to the difficulty of palpating a canine's pulse and obtaining an electrocardiogram (ECG) to confirm CPA, rescuers should not delay initiating CPR in an unresponsive and apneic canine to obtain these measures.[6][8]
Preparation
As for community emergency medical service (EMS) personnel, scene safety should be a top priority for rescuers attempting to intervene for a canine patient.[9] While tactical EMS personnel are prepared to operate in less secure environments than their civilian counterparts, rescuers should prepare to administer canine CPR by ensuring the scene is safe enough to do so.[10] Preparation of a canine patient, especially a law enforcement or military working dog, should include the utilization of a muzzle. Applying a muzzle before any evaluation or treatment attempts will help ensure a safe environment for rescuers and the canine.[11]
Technique or Treatment
Quick action is crucial as multiple studies in both human and animal patients have shown that early recognition of CPA and initiation of CPR is associated with increased rates of return of spontaneous circulation (ROSC), which is the primary goal of CPR.[12][13] Initial assessment of a canine that is apneic and unresponsive should begin with a rapid airway, breathing, and circulation (ABC) assessment, a mainstay of human BLS measures.[6]
Once CPA has been recognized, chest compressions should begin immediately. If doubt persists about whether the canine is experiencing CPA, CPR should be started without further delay, and the patient should be reassessed after an initial 2-minute cycle.
Most dog breeds, including breeds traditionally utilized as police and military working dogs, should be placed in the lateral recumbency position (lying on their side).[14] The rescuer's hands should be placed over the broadest part of the dog's chest.
Chest compressions should be delivered at 100-120 per minute. Experimental data in dogs shows increased ROSC and 24-hour survival rates when compressions were administered at rates of 100-120 per minute.[15] There is also some evidence to suggest that higher rates of up to 150 compressions per minute may be preferable. However, the rate should not fall below 100 per minute.
Compressions should achieve a depth of approximately half the width of the canine's thorax. Veterinary literature has demonstrated a linear relationship between mean arterial pressure and depth of chest compressions. This is also reflected in human clinical trials.[16][17][18] While the depth of the compressions is important, allowing full recoil of the chest between each compression is also vital. There are reductions in coronary and cerebral perfusion when rescuers do not allow the chest to recoil after compressions fully.[6][8]
Unless an intervention is made to ventilate the patient, canines requiring CPR will become hypoxic and hypercapnic. This will reduce the chances of achieving ROSC.[19][20] Securing the canine airway early in CPA is emphasized in the RECOVER guidelines. However, canine endotracheal intubation techniques will not be reviewed here. If the necessary supplies and trained rescuers are available, intubation should be attempted early. Rescuers may also consider a bag-valve mask, but achieving a tight fit with a mask designed for human patients may prove difficult. Studies have shown that ventilating canine patients during CPR with a poorly fitting mask does not provide effective rescue breaths. If intubation or other non-invasive ventilation methods are unavailable, rescuers may utilize "mouth-to-snout" rescue breathing.[21][22] While holding the dog's mouth closed with their hands, the rescuer should place their mouth over the canine's nares, creating a seal, and blow into the nares until a normal chest rise is achieved.[6][8] It is important to note that an injured canine may act unpredictably, and mouth-to-snout ventilation places the rescuer administering breaths in a particularly vulnerable position. At a minimum, a muzzle should be placed on the canine. Additionally, it is recommended that if the canine's handler can, they assist in resuscitation efforts.[23]
Canines that have been intubated should be ventilated at a rate of 10 breaths per minute or one breath delivered every 6 seconds. Breaths should be delivered in this case without any pause in compressions. If utilizing non-invasive ventilation or mouth-to-snout ventilations, 30 chest compressions should be delivered, followed by a brief pause in compressions to allow for 2 rescue breaths to be given. Emphasis should be placed on as few interruptions in compressions as possible to increase the chance of achieving ROSC.[24][25]
CPR should be continued in cycles of 2 minutes. Switching compressors after each 2-minute cycle has decreased the overall number of interruptions in compressions.[26] Rescuer fatigue has been shown to contribute to a decrease in the depth of compressions and an increase in leaning on the patient. Rescuer leaning does not allow for full chest recoil and impairs cardiac output.[27][28] This underscores the importance of switching compressors every 2 minutes.
Automatic External Defibrillator (AED) devices have been a part of human BLS for decades and are widely available.[29] The widespread adoption of AED education for the layperson and emphasis on early application in human out-of-hospital CPA is associated with a significant improvement in survival.[30] AEDs designed for human use are effective at recognizing shockable rhythms and delivering shocks to canine patients when used with ECG gel or clipping the fur. In one study evaluating AED effectiveness in canines, 90% of canines required both shaving of their fur and the application of ECG gel.[31] Trimming fur and applying gel makes AED application in the pre-hospital setting for canines more challenging than for a human patient. An AED is still a validated tool that can increase the chances of achieving ROSC if applied appropriately.
Once CPR has been initiated, the canine patient should be transported to a veterinary hospital as quickly and safely as possible. CPR should be continued until the canine becomes responsive and begins breathing spontaneously, or a veterinarian takes over care of the patient.
If ROSC is achieved, rescuers should seek veterinary care immediately and continue assessing their patients' ABCs. These canines are at extreme risk of CPA recurrence. The proportion of canine and feline patients that survive to discharge after ROSC ranges from 2% to 10%.[14][32] This highlights the increased risk for CPA recurrence as well as the importance of post-cardiac arrest (PCA) care.
While a veterinarian best delivers PCA care, rescuers should understand some fundamental concepts. PCA syndrome is believed to be the cause of such high mortality in PCA patients. It is described as a combination of multiorgan failure, anoxic brain injury, cardiogenic shock, and the sequelae of preexisting diseases.[33] Optimization of hemodynamics, respiratory function, body temperature, and neuroprotective therapies are the fundamentals of PCA care in the critical care setting.
Clinical Significance
Early cardiopulmonary resuscitation significantly increases survival in CPA.[34] The vast majority of canine CPR occurs in the veterinary hospital setting. Human studies have demonstrated that early initiation of CPR in the out-of-hospital setting increases the chances of achieving ROSC and the patient's full recovery.[35] Recognition of canine CPA and knowledge of the steps in canine CPR will equip rescuers to give these patients the best chance of achieving ROSC and, ultimately, survival.[6]
Enhancing Healthcare Team Outcomes
Many healthcare workers trained to treat human patients may be called on to act as first responders to a canine patient. Canine CPR shares many similarities with human CPR, and if done correctly, can save lives. The evidence available in human patient outcomes in out-of-hospital CPA shows that teams led by non-physicians in administering CPR are non-inferior to teams led by physicians.[36][37] While no studies have been conducted to assess these dynamics in veterinary medicine, the available data suggests that the most important aspect of team dynamics impacting patient outcomes lies not in who is leading the team but in how effectively the team communicates with one another. This includes establishing clear roles, utilizing focused, clear communication to task individuals specifically, and using closed-loop communication.[38]
