Optimum compression to ventilation ratios in CPR under realistic, practical conditions: a physiological and mathematical analysis
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.
References (26)
- et al.
A reappraisal of mouth-to-mouth ventilation during bystander-initiated cardiopulmonary resuscitation. A statement for healthcare professionals from the Ventilation Working Group of the Basic Life Support and Pediatric Life Support Subcommittees, American Heart Association
Resuscitation
(1997) - et al.
Randomized controlled trials of staged teaching for basic life support. 2. Comparison of CPR performance and skill retention using either staged instruction or conventional teaching
Resuscitation
(2001) - et al.
Randomised controlled trials of staged teaching for basic life support. 1. Skill acquisition at bronze stage
Resuscitation
(2000) - et al.
The need for ventilatory support during bystander CPR
Annals of Emergency Medicine
(1995) - et al.
Mechanical ventilation may not be essential for initial cardiopulmonary resuscitation
Chest
(1995) - et al.
Efficacy of chest compression-only BLS CPR in the presence of an occluded airway
Resuscitation
(1998) - et al.
Theoretical advantages of abdominal counterpulsation in CPR as demonstrated in a simple electrical model of the circulation
Annals of Emergency Medicine
(1984) - et al.
Interposed abdominal compression as an adjunct to cardiopulmonary resuscitation
American Heart Journal
(1994) - et al.
The composition of gas given by mouth-to-mouth ventilation during CPR
Chest
(1994) American Heart Association in collaboration with the International Liaison Committee on Resuscitation. Guidelines 2000 for cardiopulmonary resuscitation and emergency cardiovascular care: international consensus on science
Circulation
(2000)
Bystander cardiopulmonary resuscitation. Concerns about mouth-to-mouth contact
Archives of Internal Medicine
31st Bethesda Conference—Emergency Cardiac Care (1999)
Journal of the American College of Cardiology
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Current recommendations for paediatric resuscitation
2018, BJA EducationImproving CPR Performance
2017, ChestCitation Excerpt :In animal models of ventricular fibrillation using compression-only CPR (no ventilation), oxygen saturation remained > 70% for 10 min.50 Mathematical modeling reveals that increasing the chest compression to ventilation ratio to up to 60:2 improves overall oxygen delivery.51 This begs the question of whether ventilation is warranted during CPR.
Modern BLS, dispatch and AED concepts
2013, Best Practice and Research: Clinical AnaesthesiologyCitation Excerpt :Even while a reduction in the number of ventilations per minute resulted in a decrease in pO2 and increase in pCO2, the difference was small while the increase of the C:V ratio to 30:2 resulted in a substantial increase in the number of compressions delivered in one minute [11]. Another model analysis showed that the optimal C:V ratio was close to 30:2 with lower ratio resulted in too little flow and a further increase resulted in an undesirable low pO2 [12]. Even while human data on various C:V ratio's was lacking the C:V ratio was increased from 15:2 to 30:2 in the Guidelines 2005.
Cardiopulmonary resuscitation update
2012, Emergency Medicine Clinics of North America
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Member and Chair, Research Working Group, Emergency Cardiovascular Care Programs, American Heart Association.