Understanding Cardiopulmonary Resuscitation (CPR)

Sudden cardiac arrest is one of the leading causes of death in both clinical and non-clinical settings.ⁱ A number of different situations can lead to cardiac arrest, including heart attack, recreational use of drugs, heart failure, obesity, or diabetes.ⁱⁱ It may not always be clear what precipitated an event of sudden cardiac arrest. In such situations, and many other situations involving cardiac arrest, cardiopulmonary resuscitation (CPR) is a quick, non-invasive, and potentially life-saving intervention. Understanding how CPR works and receiving training can empower civilians to intervene in an emergency.

Though CPR is conventionally considered to be a mix of both chest compressions and mouth-to-mouth, the American Heart Association (AHA) now recommends that civilian interventionists in out-of-hospital settings use a Hands-Only approach to reduce disease transmission risk. For healthcare providers and those with CPR certification, conventional CPR involves: “chest compressions and … breathing at a ratio of 30:2 compressions-to-breaths. In adult victims of cardiac arrest, it is reasonable for rescuers to perform chest compressions at a rate of 100 to 120/min and to a depth of at least 2 inches (5 cm) for an average adult, while avoiding excessive chest compression depths (greater than 2.4 inches [6 cm]).” 

Recently, mechanical CPR (mCPR) devices have become available in hospital and some EMS settings. The primary goal of these devices is to mitigate caregiver mental/physical fatigue and increase the accuracy of CPR technique, although early studies have not noted a detectable improvement in patient outcome when comparing mCPR to manual CPR.ⁱⁱⁱ 

The goal of cardiopulmonary resuscitation is to reestablish blood flow in the absence of normal contractions caused by electrical signals in the heart. The compression phase propels blood to the brain, the coronary arteries, and the rest of the body through one-way valves.⁵ This mechanically generated circulation allows for continued transportation of oxygen and nutrients. A key point to understanding CPR is that body tissues, especially the brain, depend on oxygen and other nutrients, so proper CPR when indicated can postpone organ failure and brain death until more advanced interventions can be performed.  

When CPR was first described in 1960, the scientific understanding was that the heart was being manually compressed through the sternum and against the spine, creating a pressure system which emptied the ventricles and produced circulation.^iv This hypothesis became known as the cardiac compression theory. Around two decades later, the thoracic pump model was proposed, wherein heightened intra-thoracic pressure expels blood from the heart and chest to generate forward flow.^v Neither theory has been conclusively proven to be the true mechanism of action. The difficulty in determining a “correct” mechanism likely stems from the fact that several parameters, including positive pressure ventilation and compression depth, seem to influence overall contribution of either mechanism.^vi In reality, the current understanding is that both mechanisms at least partially contribute to CPR-mediated forward blood flow.^vii 

The rise in intrathoracic pressure from CPR is correlated with a rise in intracranial pressure, thought to be mediated through the venous/epidural plexus and spinal fluid.⁵ The impact of intrathoracic pressure on intracranial pressure and, by extension, cerebral perfusion, has been a topic of increasing focus in recent years.^viii A 2021 study published in Critical Care has suggested that maintaining a “head up position” following active chest compression-decompression improves coronary perfusion pressure and cerebral profusion pressure, all while lowering intracranial pressure.^ix The authors’ meta-analysis suggests that simple modifications to body positioning throughout the CPR process can improve interventional efficacy and reduce the risk of long-term brain damage. Such modifications can be easily implemented by non-medical professionals. 

The American Heart Association trains more than 16 million individuals globally in CPR technique using a network of over 400,000 instructors.^x Continuing to train laypeople in CPR and redefining CPR best practices will be critical in improving the odds of cardiac arrest victim survival. 

References

[1] Cave, D. M., Gazmuri, R. J., Otto, C. W., Nadkarni, V. M., Cheng, A., Brooks, S. C., Daya, M., Sutton, R. M., Branson, R., & Hazinski, M. F. (2010). Part 7: CPR techniques and devices: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation, 122(18 Suppl 3), S720–S728. doi:10.1161/CIRCULATIONAHA.110.970970

[2] Cleveland Clinic: Sudden Cardiac Death (Sudden Cardiac Arrest). Retrieved from https://my.clevelandclinic.org/health/diseases/17522-sudden-cardiac-death-sudden-cardiac-arrest

[3] Poole, K., Couper, K., Smyth, M. A., Yeung, J., & Perkins, G. D. (2018). Mechanical CPR: Who? When? How?. Critical care (London, England), 22(1), 140. doi:10.1186/s13054-018-2059-0

[4] Kouwenhoven, WB, Jude, JR, and Knickerbocker, GG. (1960). Closed-chest cardiac massage. JAMA, 173, 1064–1067. doi:10.1001/jama.1960.03020280004002

[5] Rudikoff, M. T., Maughan, W. L., Effron, M., Freund, P., & Weisfeldt, M. L. (1980). Mechanisms of blood flow during cardiopulmonary resuscitation. Circulation, 61(2), 345–352. doi:10.1161/01.cir.61.2.345

[6] Georgiou, M., Papathanassoglou, E., & Xanthos, T. (2014). Systematic review of the mechanisms driving effective blood flow during adult CPR. Resuscitation, 85(11), 1586–1593. doi:10.1016/j.resuscitation.2014.08.032

[7] Ewy G. A. (2018). The mechanism of blood flow during chest compressions for cardiac arrest is probably influenced by the patient’s chest configuration. Acute medicine & surgery, 5(3), 236–240. doi:10.1002/ams2.336

[8] Kiehna, E. N., Huffmyer, J. L., Thiele, R. H., Scalzo, D. C., & Nemergut, E. C. (2013). Use of the intrathoracic pressure regulator to lower intracranial pressure in patients with altered intracranial elastance: a pilot study. Journal of neurosurgery, 119(3), 756–759. doi:10.3171/2013.4.JNS122489

[9] Huang, C. C., Chen, K. C., Lin, Z. Y., Chou, Y. H., Chen, W. L., Lee, T. H., Lin, K. T., Hsieh, P. Y., Chen, C. H., Chou, C. C., & Lin, Y. R. (2021). The effect of the head-up position on cardiopulmonary resuscitation: a systematic review and meta-analysis. Critical care (London, England), 25(1), 376. doi:10.1186/s13054-021-03797-x

[10] CPR statistics. AHA CPR First Aid Blog. Retrieved from https://cprblog.heart.org/cpr-statistics/