Cardiopulmonary Resuscitation

Chapter 5


Cardiopulmonary Resuscitation




Veterinary protocols for cardiopulmonary resuscitation (CPR) have been largely adapted from those promoted by the American Heart Association for humans. The Reassessment Campaign on Veterinary Resuscitation (RECOVER) recently completed a systematic review of the literature relevant to veterinary CPR and developed the first evidence-based, consensus CPR guidelines for small animals (Fletcher et al, 2012). The clinical questions examined during the evidence evaluation process were partitioned into five domains: (1) Preparedness, (2) Basic Life Support (BLS), (3) Advanced Life Support (ALS), (4) Monitoring, and (5) Post–Cardiac Arrest Care.



Preparedness


Delays in initiation of CPR for patients with cardiopulmonary arrest (CPA) consistently result in poor outcomes. It is therefore crucial that veterinary practices are well-prepared for early recognition of CPA and immediate initiation of high-quality resuscitation. CPR training that includes both didactic components and opportunities to practice psychomotor skills is recommended for all veterinary personnel who may be called on to assist in a crisis. Refresher training and drills at minimum 6-month intervals have been shown to improve performance, and structured assessment and feedback after training maximizes the effectiveness of that training. A fully stocked crash cart that is routinely audited for content should be available in an easily accessible central location. Cognitive aids such as CPR algorithm charts and emergency drug dosing charts improve adherence to guidelines and individual performance during CPR. After every CPR attempt, a debriefing session during which team performance is discussed and critically evaluated can identify misconceptions and improve future team performance.



Diagnosis of Cardiopulmonary Arrest


CPA is an important differential diagnosis for any acutely unresponsive patient. It is a clinical diagnosis based on the presence of unconsciousness, lack of breathing, and absence of a palpable pulse. Regardless of the clinician’s index of suspicion for CPA in an individual patient, a rapid assessment focused on ruling out CPA should be undertaken immediately when an animal is unresponsive. A standardized approach based on evaluation of airway, breathing, and circulation (ABC) quickly identifies CPA and allows immediate intervention should the diagnosis be made. Because the benefits of starting CPR immediately in a patient with CPA far outweigh the risks of performing CPR on an unresponsive patient not in CPA, the clinician should not delay starting CPR in any patient in which there is a suspicion of CPA.


Thorough airway evaluation is crucial in acutely presenting unresponsive patients. Airway obstruction prevents effective ventilation and delivery of oxygen to the tissues, and failure to identify obstruction likely leads to unsuccessful resuscitation. The airway should be visually inspected by opening the mouth, pulling out the tongue, and examining the oral cavity, pharynx, laryngeal area, and arytenoids. Digital palpation from the oral cavity back to the laryngeal area may reveal foreign objects, masses, or swellings causing airway obstruction. If direct visualization and digital palpation are obstructed by conformation or swollen tissues, a laryngoscope should be used if available. If fluid is preventing visualization, swab the mouth out or use suction to clear the area. If the patient is roused by manipulation of the airway, use caution to avoid being bitten.


When evaluating an unresponsive patient, assessment of breathing and particularly the rapid identification of apnea (respiratory arrest) are of primary concern. If there are no obvious chest excursions on initial visual inspection, breathing can be further assessed by lightly touching the chest and feeling for chest movements, auscultation of the chest for lung sounds, or placing cotton or a slide in front of the nares and looking for movement or fogging, respectively.


Rapid assessment of the circulation via palpation of the femoral or dorsal metatarsal pulses, palpation for an apex heartbeat, or cardiac auscultation should be undertaken; however, if CPA cannot be definitively ruled out, CPR should be initiated immediately rather than performing further diagnostic assessment of the circulation. This is important because (1) several studies in human medicine have shown that pulse palpation is an insensitive test for diagnosis of CPA, which also may be the case in dogs and cats, and (2) a large body of literature indicates that even small delays in initiating CPR in pulseless patients reduce the likelihood of successful resuscitation. Therefore the ABC assessment described above should take no more than 10 to 15 seconds to complete.



Basic Life Support


BLS is the crucial first step in CPR and should be initiated immediately following diagnosis of CPA using the circulation, airway, breathing (CAB) concept. BLS includes delivery of chest compressions, establishment of an airway, and positive pressure ventilation. Circulation should be addressed first because ventilation is ineffective if there is no cardiac output, and evidence suggests a worsening outcome the longer chest compressions are delayed.



Circulation: Chest Compressions


Patients with CPA have no forward blood flow out of the heart and no delivery of oxygen to the tissues. Immediate consequences are the exhaustion of cellular energy stores, cell depolarization, and thus loss of organ function. If persisting for more than a few minutes, this results in increasing severity of ischemic organ injury and sets the stage for escalating reperfusion injury on reinstitution of tissue blood flow. The initial goal of chest compressions is therefore to provide (1) pulmonary blood flow for oxygen upload and carbon dioxide (CO2) elimination and (2) tissue perfusion for oxygen delivery to restore cellular metabolic activity. Experimental evidence suggests that even well-executed external chest compressions produce only approximately 30% of normal cardiac output, so great care must be taken to perform compressions with proper technique. Chest compressions should be started as soon as possible after diagnosis or suspicion of CPA. Delay in the start of high-quality chest compressions reduces the likelihood of return of spontaneous circulation (ROSC).


Chest compressions should be delivered with the dog or cat in lateral recumbency (with a few exceptions noted below). The recommended compression depth is one third to one half the width of the chest. Regardless of species or size, the chest compression rate should be 100 to 120 per minute. Use of aids to ensure correct compression rate, such as a metronome or a song with the correct tempo (e.g., “Staying Alive”) is recommended. Leaning or residual compression of the chest between compressions must be avoided to allow full elastic recoil. Chest compressions should be delivered without interruption in cycles of 2 minutes, and a new compressor should take over after each cycle to attenuate the effect of rescuer fatigue. Any interruption in compressions should be as short as possible because it takes approximately 60 seconds of continuous chest compressions before coronary perfusion pressure (CPP) reaches its maximum. CPP in turn is a critical determinate of myocardial blood flow and the likelihood of ROSC.


The physiology of blood flow generation is fundamentally different during CPR compared with spontaneous circulation. Two distinct theories have been advanced to explain how chest compressions lead to systemic blood flow. The cardiac pump theory is based on the concept that the left and right ventricles are directly compressed, increasing the pressure in the ventricles, opening the pulmonic and aortic valves, and providing blood flow to the lungs and the tissues, respectively. Recoil of the chest between compressions due to the elastic properties of the rib cage creates negative pressure within the chest, thereby improving filling of the ventricles before the next compression. The thoracic pump theory is based on the concept that external chest compressions raise overall intrathoracic pressure, which forces blood from intrathoracic vessels into the systemic circulation, with the heart acting as a passive conduit.


Given the chest wall stiffness in medium and large dogs, blood flow generated by the thoracic pump mechanism likely predominates in these patients. Therefore it is recommended that the chest be compressed over the highest point on the lateral thoracic wall with the patient in lateral recumbency. In contrast, in very keel- or deep-chested dogs (e.g., Doberman pinschers, sight hounds) it is reasonable to do chest compressions directly over the heart as the cardiac pump mechanism likely predominates in dogs with this conformation. In markedly barrel-chested dogs (e.g., English bulldogs), compressions over the sternum with the patient in dorsal recumbency may be more effective in eliciting the thoracic pump mechanism than lateral chest compressions. In these and other large dogs with low chest compliance, considerable compression force is necessary for CPR to be effective. The compressor should maintain locked elbows with one hand on top of the other, and the shoulders should be directly above the hands. This allows compressions to be done using the core muscles rather than the biceps and triceps, reducing fatigue and maintaining optimal compression force. If the patient is on a table and the elbows cannot be locked, a stool should be used or the patient should be placed on the floor to allow correct technique.


Most cats and small dogs tend to have higher thoracic compliance and narrower chests than larger dogs, making the cardiac pump mechanism achievable in these patients; therefore chest compressions should be done directly over the heart. Compressions may be performed using the same two-handed technique as described above for large dogs or may be done using a single-handed technique in which the compressing hand is wrapped around the sternum and compressions are achieved from both sides of the chest by squeezing. Circumferential compressions of the chest using both hands also may be considered.



Airway and Breathing: Ventilation


If an endotracheal (ET) tube and laryngoscope are available, the patient should be intubated as soon as possible. Both dogs and cats can be intubated in lateral recumbency, so chest compressions should continue during ET tube placement. If an ET tube is not readily available, mouth-to-snout ventilation provides improved oxygenation and CO2 removal. The patient’s mouth should be held closed firmly with one hand. The neck is extended to align the snout with the spine, opening the airway as completely as possible. The rescuer makes a seal over the patient’s nares with his/her mouth and blows firmly into the nares to inflate the lungs. The chest should be visually inspected during the procedure and the breath continued until a normal chest excursion is accomplished. An inspiratory time of approximately 1 second should be targeted.


In nonintubated patients ventilated using the mouth to snout technique, ventilation cannot be performed simultaneously with chest compressions because air will flow into the esophagus and stomach rather than into the lungs when the lungs are compressed during chest compressions. Therefore 30 chest compressions should be delivered, immediately followed by two breaths. Alternating compressions and ventilations should be continued for 2-minute cycles, and the rescuers rotated every cycle to prevent fatigue. In contrast, chest compressions and ventilations should be performed simultaneously in intubated patients because the inflated cuff of the ET tube allows alveolar ventilation during a chest compression; interruptions in chest compressions are thereby minimized. Intubated patients should be ventilated at a rate of 10 breaths per minute with an inspiratory time of approximately 1 second. If a respirometer is available, a tidal volume of approximately 10 ml/kg should be targeted. This low-minute ventilation is adequate during CPR since pulmonary blood flow is reduced in comparison with an animal with spontaneous circulation. Care should be taken not to hyperventilate the patient because low arterial CO2 tension leads to cerebral vasoconstriction, decreasing cerebral perfusion and delivery of oxygen to the brain.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Cardiopulmonary Resuscitation

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