Letter by Rottenberg Regarding Article “Improved Neurological Outcome With Continuous Chest Compressions Compared With 30:2 Compressions-to-Ventilations Cardiopulmonary Resuscitation in a Realistic Swine Model of Out-of-Hospital Cardiac Arrest”
Ewy and colleagues1 published an animal study of witnessed out-of-hospital sudden ventricular fibrillation that compared 24-hour neurologically normal survival in swine receiving continuous-chest-compression (CCC CPR) versus swine receiving CPR with a compression/ventilation ratio of 30:2 (30:2 CPR). They concluded that the rate of neurologically normal survival 24 hours after resuscitation in animals receiving CCC CPR was significantly better than that in animals receiving 30:2 CPR. However, that study does not realistically simulate human bystander CPR in the majority of cardiac arrest victims. Moreover, the results of the human study2 cited by Ewy et al likely only suggests that CCC CPR is more effective for out-of-home arrests.
In humans, the tongue, soft palate, and/or epiglottis can act as a 1-way valve that prevents gasping and leads to forced exhalation during sternal compression without reexpansion of the lung (or prevents passive ventilation from recoil of the chest). Therefore, only animal studies in which the airway is occluded at end-expiration realistically simulate what likely occurs during bystander CPR in the majority of arrest victims. Dorph and colleagues3 used an inspiratory impedance threshold valve to achieve airway occlusion at end-expiration during CPR in a swine model of sudden ventricular fibrillation that compared CCC CPR with 30:2 CPR. They concluded that during cardiac arrest with an obstructed airway, pulmonary ventilation should be strongly recommended.
Whitehurst and colleagues4 showed that adequate passive ventilation from recoil of the chest is highly dependent on the quality/technique of CPR. The blood gas profiles from the animals receiving CCC CPR indicated that their compression technique did not move sufficient air to allow physiologically significant pulmonary gas exchange. Moreover, CCC CPR with an open airway failed to achieve a blood gas profile measurably superior to that in animals receiving CCC CPR with a completely blocked endotracheal tube, which was occluded by clamping the endotracheal tube. The study did not specify whether it was done at end-expiration. Therefore, unlike as evidenced by the study by Ewy et al,1 not all patients who are fortunate enough to maintain an adequately open airway during chest compression/decompressions receive the substantial level of ventilation suggested in their study.
It is possible and perhaps likely that the manual CPR provided in the study by Ewy and colleagues1 was performed by a male rescuer and is therefore not a valid simulation of bystander CPR performed by the typical rescuer in the home, who is most often an older woman. Herlitz and colleagues5 in their study of over 24 000 out-of-hospital arrests found that the chance of survival is much lower for in-home arrests versus out-of-home arrests and concluded that having an arrest at home was a strong independent predictor of adverse outcome. The human study cited by Ewy and colleagues, which was used to support their argument that CCC CPR is more effective than conventional CPR and to suggest that the same applies to 30:2 CPR, did not perform separate analyses for in-home versus out-of-home arrests; therefore, it is reasonable to have the opinion that the results of this study are flawed. It is also reasonable to believe it likely that the results of this study showing that CCC CPR is superior to conventional CPR apply primarily to victims outside the home.
Ewy GA, Zuercher M, Hilwig RW, Sanders AB, Berg RA, Otto CW, Hayes MM, Kern KB. Improved neurological outcome with continuous chest compressions compared with 30:2 compressions-to-ventilations cardiopulmonary resuscitation in a realistic swine model of out-of-hospital cardiac arrest. Circulation. 2007; 116: 2525–2530.
Herlitz J, Eek M, Holmberg M, Engdahl J, Holmberg S. Characteristics and outcome among patients having out of hospital cardiac arrest at home compared with elsewhere. Heart. 2002; 88: 579–582.