CIRCULATORY ADJUNCTS Newer Methods of Cardiopulmonary Resuscitation

00. ם EMERGENCY CARDIOVASCULAR CARE 0733–8651/02 $15.00 CIRCULATORY ADJUNCTS Newer Methods of Cardiopulmonary Resuscitation Charles F. Babbs, MD, PhD Conventional cardiopulmonary resuscita- mately the same way as it was in the 1960s. tion—what has come to be called standard In keeping with the engineering maxim that CPR—has changed little since first introduced ‘‘if you keep on doing what you’re doing, in the early 1960s by Kouwenhoven et al.1 you’ll keep on getting what you’ve got,’’ suc- Standard CPR requires chest compressions at cess rates for standard CPR have changed a rate of 80 to 100 compressions/min in con- little over the past 40 years.5–9 Between 15% junction with mouth-to-mouth rescue breath- and 40% of all victims of nontraumatic car- ing.2 Positive pressure ventilations (PPVs) are diac arrest are resuscitated, and approxi- given at an overall rate in the range of about mately 5% to 10% live to leave the hospital. 8 to 12 ventilations/min, depending on Despite lack of significant change on the whether one is doing one- or two-rescuer surface, a neglected community of researchers CPR. Refinements of standard CPR since its has made substantial progress in the labora- introduction in the 1960s have included in- tory and in the clinic during this same 40- creasing the rate of chest compression from year period. Results have lead to much fun- 60 compressions/min to 80 compressions/ damental understanding of the circulatory min or more, which research shows matters physiology and hemodynamics of cardiac ar- little,3 and recently decreasing the tidal vol- rest and resuscitation. A variety of alternative ume of the PPVs under certain circum- forms of CPR have been proposed and tested stances.2, 4 Elimination of the carotid artery in animals and in humans. All of these alter- pulse check in the year 2000 guidelines has native methods apply more kinetic energy to abolished unnecessary delay in starting chest the victim, either by way of an additional compressions by lay rescuers. Otherwise, ex- rescuer or some type of mechanical device. ternal CPR today is performed in approxi- More energetic forms of CPR include high- impulse CPR, interposed abdominal compression–CPR (IAC-CPR), active compression-de- The author has no financial interest, consulting ar- compression CPR (ACD-CPR), and vest CPR. rangement, or research contract with Ambu, CPRxLLC, or any other resuscitation device company that could be These derivatives of standard CPR have come construed as a conflict of interest. to be known as circulatory adjuncts.Bythe From the Department of Basic Medical Sciences, Purdue University, West Lafayette, Indiana CARDIOLOGY CLINICS VOLUME 20 • NUMBER 1 • FEBRUARY 2002 37 38 BABBS year 2000, a convergence had occurred be- sternum and the spine. As a result, forward tween laboratory knowledge about mecha- blood flow occurs through the aortic and pul- nisms of blood flow and clinical experience monic valves without mitral or tricuspid in- with the various adjuncts. This long-delayed competence. In particular, when the cardiac synthesis of research findings has led to im- pump mechanism is operative, the aortic proved forms of practical resuscitation (able valve is open, and the mitral valve is closed to generate more blood flow and systemic during chest compression.13 The cardiac perfusion pressure during cardiac arrest pump mechanism is also operative during through the efforts of the rescuers) based on open-chest cardiac massage. sound basic science principles. If there is one The thoracic pump mechanism was discov- certain truth in resuscitation science, stem- ered in the 1980s as a result of clinical obser- ming from the pioneering work of Redding10, vation by Criley et al14, 15 of cough CPR and Ralston et al,11 Weil and Tang,9 and Kern et extensive laboratory studies at Johns Hopkins al,12 it is this: Improved coronary and sys- University, led by Rudikoff et al16 and Weis- temic perfusion pressures during cardiac ar- feldt.17 This pump is operative to the extent rest and CPR lead to improved immediate that chest compression causes a global rise in resuscitation success and improved likelihood intrathoracic pressure sufficient to force blood of longer-term, neurologically intact survival. from the pulmonary vasculature, through the The improved hemodynamics, however, do heart, and into the periphery. When the tho- not come without a price, because more phys- racic pump mechanism is operative, the mi- ical work has to be done by human or ma- tral and the aortic valves are open simultane- chine to generate more forward flow of blood. ously during chest compression.18–20 The heart Accordingly, adjuncts may not be appropriate acts as a conduit rather than a pumping for the lone rescuer faced with a collapsed chamber. coworker or loved one. Nevertheless, the time The abdominal pump mechanism was dis- has come for serious consideration of alterna- covered independently by workers in Great tive forms of resuscitation when adequate Britain, Japan, Israel, and the United States in equipment and trained personnel are avail- the latter half of the 20th century.21–26 This able, for example, in the emergency depart- mechanism has two components: (1) arterial ment of a hospital. This article focuses on the and (2) venous. The arterial component of the relevant physiology of cardiac arrest and CPR abdominal pump mechanism is operative to that makes possible the generation of sys- the extent that abdominal compression forces temic perfusion pressure substantially greater blood from the abdominal aorta into the pe- than that provided by standard CPR. Thereaf- riphery against a closed aortic valve. Thus, ter, selected practical aspects of applying cur- the aortic valve is closed during abdominal rently approved circulatory adjuncts in a hos- compression. The venous component of the pital setting are discussed. abdominal pump mechanism is operative to the extent that external abdominal pressure forces blood from the inferior vena cava MODERN PHYSIOLOGY OF BLOOD through the tricuspid valve into the right ven- FLOW DURING CARDIOPULMONARY tricle (or through the tricuspid and pulmonic RESUSCITATION valves into the pulmonary vasculature). In this case, the right heart valves are open dur- At least three different mechanisms can move ing abdominal compression. blood during cardiac arrest and CPR: (1) the The operation of these three pumps— cardiac pump, (2) the thoracic pump, and (3) cardiac, thoracic, and abdominal—is related the abdominal pump. The cardiac pump to the fundamental architecture of the cardio- mechanism was the first to be recognized by vascular system. The physiology of the the original discoverers of closed-chest CPR.1 pumps has been demonstrated in relatively This pump mechanism is operative to the simple mathematical models that represent extent that external chest compression the essential features of the human cardiovas- squeezes the cardiac ventricles between the cular system.24, 27–31 Successful circulatory ad- CIRCULATORY ADJUNCTS 39 juncts generate greater blood flow than stan- and small people and large and small experi- dard CPR either by enhancing the operation mental animals. The author of this article uses of one of these pumps or by simultaneously the simplest realistic model of CPR physiol- invoking the action of multiple pumps. High- ogy to illustrate the actions of the basic pump impulse CPR,32–34 for example, aims to en- mechanisms and to demonstrate selected cir- hance the action of the cardiac pump mecha- culatory adjuncts. The exact values of vascu- nism. Vest CPR35, 36 aims to enhance the action lar compliances and resistances and other of the thoracic pump mechanism through the technical details of the model, which can be action of a pneumatic vest that is inflated and implemented in a spreadsheet, are fully de- deflated rapidly at a rate of 60 to 150 times/ scribed elsewhere.31 min. ACD-CPR37, 38 aims to improve filling of As a point of reference and calibration, Fig- the either the cardiac pump or the thoracic ure 1 illustrates pressures throughout a sim- pump by creating negative pressure in the plified cardiovascular system for a nonar- thorax between chest compressions. IAC- rested circulation of a hypothetical 70-kg CPR39–43 aims to invoke the abdominal pump man. In this instance, cardiac pump generates together with either chest pump. IAC-CPR left ventricular pressures (Ppump) of 122/2 and its derivative, Lifestick (Datascope, Fair- mm Hg at a heart rate of 80 beats/min (bpm). field, NJ) CPR,44 aim to combine thoracic and Systemic arterial blood pressure is 119/82 (the abdominal pumps alternately, in such a way data point representing the exact minimum that the thoracic pump primes the abdominal diastolic pressure at 82 mm Hg is not plotted pump during one half of the cycle and the on the chart), mean arterial pressure is 95 mm abdominal pump primes the thoracic pump Hg, and cardiac output is 5.0 L/min. These during the other half of the cycle. are classical textbook values for the normal Regarding the question of the effectiveness human circulatory system.31 Note the essen- of these various adjuncts, it would be difficult tially normal arterial pulse waveforms and to compare and contrast all such methods in low systemic venous pressures. a single animal or clinical model; however, it Figure 2 illustrates the action of a pure is reasonable to use a mathematical model of cardiac pump CPR in the same circulatory the circulation that includes all three pumps.31 model during cardiac arrest. Steady-state con- Such a model can be used to simulate the ditions are shown after stable pressures have effects of various circulatory adjuncts in ex- been achieved by 20 prior compressions. In actly the same circulatory system, without this simulation, only the right and left ventri- model-to-model variation.

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