Elsevier

Resuscitation

Volume 54, Issue 2, 1 August 2002, Pages 147-157
Resuscitation

Optimum compression to ventilation ratios in CPR under realistic, practical conditions: a physiological and mathematical analysis

https://doi.org/10.1016/S0300-9572(02)00054-0Get rights and content

Abstract

Objective: To develop and evaluate a practical formula for the optimum ratio of compressions to ventilations in cardiopulmonary resuscitation (CPR). The optimum value of a variable is that for which a desired result is maximized. Here the desired result is assumed to be either oxygen delivery to peripheral tissues or a combination of oxygen delivery and waste product removal. Method: Equations describing oxygen delivery and blood flow during CPR as functions of the number of compressions and the number of ventilations delivered over time were developed from principles of classical physiology. These equations were solved explicitly in terms of the compression/ventilation ratio and evaluated for a wide range of conditions using Monte Carlo simulations. Results: As the compression to ventilation ratio was increased from 0 to 50 or more, both oxygen delivery and the combination of oxygen delivery with blood flow increased to maximum values and then gradually declined. For variables typical of standard CPR as taught and specified in international guidelines, maximum values occurred at compression/ventilation ratios near 30:2. For variables typical of actual lay rescuer performance in the field, maximal values occurred at compression/ventilation ratios near 60:2. Conclusion: Current guidelines overestimate the need for ventilation during standard CPR by two to four-fold. Blood flow and oxygen delivery to the periphery can be improved by eliminating interruptions of chest compression for these unnecessary ventilations.

Sumàrio

Objectivo: Desenvolver e avaliar uma fórmula prática para a relação óptima de compressão para ventilação na ressuscitação cardiopulmonar (RCP). O valor óptimo de um parâmetro é aquele para o qual o resultado desejado é maximizado. Neste caso o resultado desejado é assumido ser o aporte de oxigénio aos tecidos periféricos ou uma combinação de aporte de oxigénio com a remoção de produtos indesejáveis. Método: Foram desenvolvidas, a partir de princı́pios de fisiologia clássica, equações que descrevem o aporte de oxigénio e o débito sanguı́neo durante RCP em função do número de compressões e ventilações ao longo do tempo. Estas equações foram resolvidas explicitamente em termos da razão compressões/ventilações e avaliadas para uma grande variedade de condições utilizando simulações de Monte Carlo. Resultados: À medida que a relação de compressões para ventilações aumentava de 0 até 50 ou mais, quer o aporte de oxigénio quer a combinação deste com o débito sanguı́neo aumentaram até valores máximos e depois declinaram gradualmente. Para os parâmetros tı́picos da RCP standard tal como é ensinada e especificada nas recomendações internacionais, os valores máximos ocorreram com razões compressão/ventilação próximas de 30:2. Para parâmetros tı́picos da actuação real no terreno de reanimadores não profissionais, os valores máximos ocorreram com razões compressão/ventilação próximas de 60:2. Conclusão: As recomendações actuais sobrestimam duas a quatro vezes a necessidade de ventilações durante a RCP. O débito sanguı́neo e o aporte de oxigénio à periferia podem ser melhorados eliminando as interrupções das compressões torácicas para realizar estas ventilações desnecessárias.

Resumen

Objetivo: Desarrollar y evaluar una formula practica para la relación ventilación compresión óptima en la resucitación cardiopulmonar (RCP). El valor óptimo de un parámetro es aquel para el cual el resultado deseado es maximizado. Aquı́ se asume que el resultado deseado es la entrega de oxı́geno a tejidos periféricos o una combinación entrega de oxı́geno y remoción de productos de desecho. Método: Basándose en los principios de fisiologı́a clásica, se desarrollaron ecuaciones que describen la entrega de oxı́geno y el flujo sanguı́neo durante la RCP como funciones de el número de compresiones y el número de ventilaciones entregadas en el tiempo. Estas ecuaciones fueron resueltas esplı́citamente en términos de relación ventilación/compresión y fueron evaluadas para un amplio rango de condiciones usando las simu,aciones de Monte Carlo. Resultados: A medida que se aumentaba la relación compresión/ventilación de 0 a 50 ó mas, tanto la entrega de oxı́geno como la entrega de oxı́geno combinada con flujo sanguı́neo aumentaron a valores máximos y luego disminuyeron gradualmente. Para los parámetros tı́picos de de la RCP estándar como es enseñada y especificada en las guı́as internacionales, los valores máximos ocurrieron con relaciones compresión / ventilación cercanos a 30:2. Para parámetros tı́picos de desempeño de reanimadores legos en la ewscena, los valores máximos ocurrieron en relaciones compresión/ventilación cercanas a 60:2. Conclusión: Las actuales guı́as sobreestiman la necesidad de ventilación durante el RCP estándar por 2 a 4 veces. El flujo sanguı́neo y la entrega de oxı́geno a la periferia puede ser mejorado eliminando las interrupciones de la s compresiones torácicas para estas ventilaciones innecesarias.

Introduction

Current adult cardiopulmonary resuscitation (CPR) by one or two rescuers is based on the traditional ABC's—airway, breathing, circulation—with a 15:2 compression/ventilation ratio [1]. That is, the rescuer compresses the chest 15 times, pauses to give two mouth-to-mouth ventilations, and then continues with chest compressions2. The 15:2 ratio is essentially the same as the normal ratio of heart rate to breathing in a quietly resting adult with a heart rate of 75 beats per min and a respiratory rate of ten breaths per min, namely 7.5:1 or 15:2. Recently, the issue of the most desirable compression/ventilation ratio has been reopened because of the reluctance of many rescuers, both lay and professional, to perform mouth-to-mouth rescue breathing, owing to the fear of contracting serious communicable diseases such as AIDS [2], [3], [4]. Moreover, the relatively long pauses in chest compression required for ventilation lead to disturbingly long interruptions in chest compressions and associated blood flow. In turn, the average systemic perfusion pressure over a complete compression/ventilation cycle may be much lower than is generally appreciated.

Consider, for example, a set of 15 compressions at a compression rate of 100/min [1], which requires 9 s to deliver. If a rescuer takes 5 s to administer two slow, deep rescue breaths of 700–1000 ml each, as specified in current Guidelines [1], then chest compressions are only being delivered 9/14ths of the time. The 5 s pause for ventilation following every 15 chest compressions has been shown in experimental models to reduce coronary perfusion pressure by 50% [5]. This loss of perfusion pressure must be rebuilt during each subsequent set of compressions, and typically requires about five to ten compressions before the previous level is achieved [5]. In some cases the 5 s pause for ventilation may reduce overall mean systemic perfusion below the value of approximately 25 mmHg required for effective resuscitation [6], [7], [8].

Furthermore, actual observations of lay rescuers suggest that the pause in chest compression required to deliver two ventilations is rarely as brief as 5 s. Recent videotape analysis of lay rescuers in action shows that the interruption of chest compression for rescue breathing consistently requires about 16 s to perform [9], [10]. The act of delivering two slow, deep rescue breaths is not just the blowing into the mouth of the victim, but the physical task of stopping compressions, leaving the chest, moving to the head, performing a head tilt/chin lift maneuver to open the airway, taking in a breath, bending over, getting a good mouth to mouth seal, blowing in the breath, rising up, taking in a second breath, bending over again, recreating a good seal, blowing in the second breath, watching the chest rise, leaving the head and returning to the chest, finding the proper hand position, and finally beginning to compress the chest again! This complex set of tasks is much more difficult kinesthetically for the once trained, but unpracticed, rescuer than is the rhythmic repetition of chest compression.

Hence in a practical, real world setting, with a compression rate of 100/min (the new value specified in the year 2000 international guidelines [1]), chest compressions would be interrupted for ventilations a majority of the time (9 s for 15 compressions, 16 s for two ventilations). In this case chest compressions would be delivered during only 36% of the total resuscitation time.

Accordingly, some authors have begun to explore the use of other compression to ventilation ratios such as 50:5 [9], [10], [11], [12], during which chest compressions are sustained for a greater proportion of the time. The ultimate extension of this concept of increasing the number of chest compressions between ventilation ventilations is ‘continuous chest compression CPR’ without any ventilations. Such a strategy has been extensively studied in a swine model of resuscitation and has shown identical outcome results to standard 15:2 compression to ventilation CPR [11], [12], [13], [14], [15], [16], [17]. Recently, Hallstrom et al. [18] have reported a clinical study of simplified, dispatcher assisted CPR, in which no ventilations are given. In this study, the results of CPR without ventilations were no worse than those of standard CPR. Such research begs the question as to how much, if any, ventilation is needed in the early treatment of cardiac arrest [16], [19]—or more generally—what is the optimum compression to ventilation ratio?

The optimum value of a variable is that for which a desired result is maximized. Hence the optimum compression to ventilation ratio depends on the particular principle or criterion one chooses to define ‘desired result’. One such principle is that the main purpose of the circulation is to deliver oxygen to peripheral tissues. An extension of this principle is that the purpose of the circulation is not only to deliver oxygen but also to remove metabolic waste products. That is, there may be some independent benefit of circulation even if little or no oxygen is delivered, for example, to clear lactic acid made during prior ischemia and anaerobic metabolism. In this case the function of effectiveness of the circulation can be viewed as the product of some function of blood flow multiplied by some function of oxygen delivery.

The present paper takes a mathematical and physiological approach to finding the optimum compression/ventilation ratio in CPR, where the optimum is defined either in terms of oxygen delivery alone or as a combination of oxygen delivery and perfusion. The results show that 15:2 is optimal for less than 1% of patients resuscitated with an ideal ventilation technique and virtually none of the patients resuscitated with average ventilation technique characteristic of lay rescuers.

Section snippets

Approach

To define an optimum value of a variable, x, one has to plot a desired result as a function of x, and then find the value of x at which the desired result is maximized. Suppose, for example, that the desired result is oxygen delivery. According to the Fick principle, oxygen delivery is equal to cardiac output (forward blood flow) multiplied by the arteriovenous difference in oxygen content (A-V O2 difference). In this case it is necessary to express blood flow and A-V O2 difference during CPR

Discussion

Having solved the problem several different ways, it would appear that the optimal number of compressions to be followed by two ventilations is between 30 and 70, or for simplicity, we can say somewhere in the neighborhood of 50, rather than 15. This 50:2 optimum applies to current standard, one- or two-rescuer CPR delivered by typical lay rescuers. In the future the optimal compression/ventilation ratio may be somewhat less than 50:2 when more effective methods using both chest and abdominal

Conclusion

It is only now in the era of serious consideration of CPR simplification [16], [19] and reluctance to perform mouth-to-mouth ventilation [2] that we have begun to reconsider how much ventilation is really needed. The currently recommended 15:2 compression to ventilation ratio is based upon an overly optimistic estimate of the amount of pulmonary capillary perfusion that can be generated during standard CPR. At least half of these ventilations are unnecessary. Valuable time for perfusion is

A call for action

One would hope that adjustment of compression to ventilation ratio for basic life support could be accomplished rather quickly worldwide. Innovation by adding something new requires proof of safety and efficacy of the new method. There is a long hard road to consensus guidelines, upon which many decision makers must agree. However, innovation by subtracting something—in this case needless ventilations—may be an easier task. By simply eliminating interruptions of chest compression we can

Acknowledgements

The helpful comments and suggestions of Douglas Chamberlain, University of Wales College of Medicine, and of Anthony J. Handley, Colchester, Essex, England are gratefully acknowledged.

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