Chest compressions: biomechanics and injury.
Tsai et al (2) described the imaging features of cardiac arrest in clinically unstable patients during contrast-enhanced CT imaging. Their study revealed important hemodynamic information about cardiac arrest. When blood flow slows, arterial and venous pressures drop dramatically to undetectable levels and normal pressure loss between different vascular systems occurs. As a result, the contrast agent can pool in dependent parts of the right side of the body, especially in the venous system and the right lobe of the liver. If medical professionals are aware of these specific imaging features, prompt cardiac resuscitation can be initiated in an attempt to avoid permanent brain damage and death. (2)
A comprehensive overview of CPR can help prepare technologists for emergency cardiac situations and better serve patients. (3,4) A recent study indicated that an opportunity exists to improve CPR quality and, it is hoped, patient survival by focusing on the correct depth and rate of chest compressions with minimal "hands-off" periods. (4) It is well documented in the literature that properly performed chest compressions manipulate the chest wall and can result in injury to the sternum and ribs. (5-10) On careful examination, chest radiographs reveal compression-induced fractures in some patients who survived a cardiac event with subsequent CPR. (11) On the other hand, negative findings after chest radiography of nonsurvivors do not necessarily exclude broken ribs or a broken sternum. (12) This article discusses the biomechanics of chest compressions and the risks for rib cage fractures in adult CPR. Additionally, the methodology for studying injuries to the chest wall will be examined, taking into account the revised guidelines for chest compressions recommended by the American Heart Association (AHA). (7,8)
A review of the literature on patient care, physiology, biomechanics, emergency medical services and diagnostic imaging was conducted to examine rib cage injuries resulting from chest compressions during adult CPR. The review included print journals and online databases. Keyword use in the print and online database searches required each term used singularly with the combining-word form AND, as follows: adverse reaction or biomechanics or "cardiac output" or "chest wall" or "contrast media" or force or fracture or hemodynamic or injury or mechanics or radiograph or rib or "rib cage" or sternal or sternum or x-ray AND "cardiopulmonary resuscitation" or AND "chest compressions" or AND CPR. The words listed in quotations denote the actual inclusion of the quotations to narrow the search field. The PubMed/Medline database was also searched according to the same keyword usage. These articles were reviewed and included based on the following criteria: CPR given for adults; studies on chest compressions and hemodynamics, with reference to human participants or with strong crossover from animal studies; concentration on rib and sternal injuries; and an overall intent and consistency for applications involving radiology. This produced 7 articles for both manipulation of the chest wall and compression-induced injuries, and 24 articles that supported these areas of interest.
Cardiac arrest can be accompanied by the following cardiac rhythms: ventricular fibrillation, ventricular tachycardia, asystole or pulseless electrical activity. (13) During CPR for early intervention in cardiac arrest, chest compressions are necessary as the external mechanism to promote internal blood flow to the body tissues. The hemodynamic imitated is cardiac output. Cardiac output is the product of stroke volume and heart rate and supports diffusion of oxygen into the interstitial fluids surrounding the capillary beds. (13,14)
Radionuclide imaging studies during CPR have indicated that chest compressions produce only 27% to 32% of adult average resting cardiac output. (15,16) With chest compressions, virtually all the blood is directed toward the organs above the diaphragm. Cerebral blood flow is approximately 50% to 90% of normal; myocardial flow is between 20% and 50%, and abdominal viscera and the lower extremities receive only 5% of normal blood flow. Blood flow to vital organs drops with time despite a constant effort. Administering adrenaline increases flow to the brain and myocardium, but flow to other organs decreases regardless. (17)
Investigations into the controlling factors for cardiac output derived from chest compressions produced 2 theories involving different biomechanics: the cardiac pump theory and the thoracic pump theory. (18,19) Historically, blood flow during chest compressions was presumed to result from direct compression of the cardium between the sternum and the vertebral column; this is known as the cardiac pump theory. (18) In the 1970s an alternate explanation developed. This is most often attributed to a report by Criley, Blaufuss and Kissel on self-administered cough-induced cardiac compressions. They described 8 patients undergoing coronary angiography who were successfully resuscitated from ventricular fibrillation. Three of these patients remained conscious and alert for 24 to 39 seconds after ventricular fibrillation by coughing every 1 to 3 seconds. Cough-CPR, which is accomplished by abrupt, forceful coughing, was therefore reported to maintain consciousness by rhythmic compression of the heart. (20) The knowledge gained through the study of cough-CPR helped researchers formulate the thoracic pump theory of CPR. According to this theory, increased intrathoracic pressure or the change in intrathoracic pressure during chest compressions forces blood from the thoracic vessels into the systemic circulation, with the heart acting as a conduit and not as a pump. (18,19)
Eventually, as a result of a study by Maier et al (19) on the physiology of chest compressions, it became apparent that both theories contribute to cardiac output, depending on the physiological situation. Their analysis concluded with the recommendation that maximal stroke volume and coronary blood flow are achieved by brief compressions of moderate force and that increasing the rate of compressions significantly improves cardiac output. The term "high-impulse CPR" was used to define the technique. (19) Maier et al noted an unexpected but significant advantage of this technique regarding blood flow dynamics to the coronary vessels--a 65% to 75% improvement compared with the control group. This was credited to the brief compression durations that permitted refilling of the coronary vessels, although it also was observed that a negative coronary flow velocity resulted during initial onset in the cycles of chest compressions when left ventricular pressure exceeded aortic pressure. (19)
As it turned out, high-impulse CPR is the centerpiece for exhibiting the relative contribution of cardiac pump theory by involving the localized area between the sternum and the cardiac surface where regional pleural pressure is similar to intracardiac pressures. Because the heart lies immediately posterior to the sternal body, (21) it is suspected that this regional high-pressure zone represents an area of direct transfer of force from the sternum to the heart. (19) For effective adult chest compressions, AHA guidelines recommend to "push hard and push fast" at a rate of about 100 compressions per minute, with a compression depth of 1 1/2 to 2 inches (approximately 4 to 5 cm). In addition, the palm of the hand maintains contact with the sternum, allowing the chest to recoil completely after each compression with approximately equal compression and relaxation times. (8) With regard to handedness, CPR is performed with fewer errors when the rescuer's dominant hand is in contact with the sternum. (22)
Rib and Sternal Injuries
The analysis by Maier et al (19) not only helped describe the mechanics of chest compressions, but also allowed researchers to draw conclusions regarding levels of risk for rib cage injury due to the force transfer from manipulation of the chest wall. In a study consisting of a heterogeneous patient group, investigators demonstrated that a force of 245 to 800 N on the sternum was required to obtain a compression depth of 3.8 cm. Furthermore, it was extrapolated that a sternal force of 700 N (154 pounds) was required to obtain the minimum depth (1 1/2 inches or approximately 4 cm) for patients who have stiff chest walls, and this would likely result in compression-induced injury. (6) In a study on force distribution across the palm of the hand during simulated chest compressions, the hypothenar eminence was demonstrated to exert greater force than the thenar eminence to produce compressions with the minimum depth. The authors determined that the potential for sternal fractures is reduced when the right hand contacts the sternum with the rescuer kneeling on the right side of the patient, and vice versa if the rescuer is left handed. (23) This places the hypothenar eminence more caudal to the sternal angle, which is a better position for efficacious compressions over the sternal body and heart, especially if the role of hand dominance is considered. (22,23)
Despite best practices for hand placement, compression-induced rib cage fractures occur and are well documented in the literature. (9-12) These risks also are recognized by laypeople who attend basic life support classes. (24) The degree of risk for injury is related both to intrinsic conditions, such as the frailty or stiffness of the chest wall, and to extrinsic conditions. (6) However, the benefit of CPR during cardiac arrest is also recognized, (13,24) particularly in terms of its potential to enhance quality of life. (25) Regarding such risks and benefits, Oschatz et al (11) compared outcomes for adult survivors who received basic life support performed by bystanders vs advanced cardiac life support performed in the emergency department. Adverse effects from chest compressions performed by the basic life support group and the advanced cardiac life support group, respectively, were as follows: pneumothorax, 3% vs 2%; soft-tissue emphysema, 2% vs 1%; and serial rib fractures, 8% vs 8%. The authors explained the relatively small number of rib fractures by the fact that the study participants were cardiac arrest survivors and thus chest radiography, rather than autopsy, was the method for assessing complications. (11)
Most of the findings in the literature on compression-induced fractures associated with CPR are based on examinations of patients who died. (9,10,12) Lederer et al (12) reported on whether findings of chest radiography correlated with postmortem findings in adult patients who underwent CPR after out-of-hospital cardiac arrest. The results indicated that fractures associated with CPR are underreported in conventional radiographic investigations, with fractures diagnosed by radiological exam in only 9 of 19 patients compared with fractures diagnosed by autopsy in 18 patients. The total number of fractures (both rib and sternal) found on anteroposterior (AP) chest radiographs was 18, compared with 92 found at autopsy. Sternal fractures were detected by radiographic investigation only when special lateral sternum radiographs were taken. According to the discussion, increased object-to-film distance associated with positioning for AP chest radiographs might have complicated radiologic investigation of the anterior chest wall, creating considerable difficulty in detecting fracture dislodgement and nondisplaced fractures of the anterior ribs and sternum. This difficulty was acknowledged as the plausible explanation for why sternal fractures were detected only when the additional lateral sternum radiograph was made. (12)
In a recent autopsy report by Black, Busuttil and Robertson, a sternal fracture incidence of 14% and a rib fracture incidence of 29% were observed. (10) However, in a broad review conducted by Hoke and Chamberlain (9) on skeletal injuries secondary to CPR, the incidence of sternal fractures was shown to range from 1% to 43%, and rib fractures ranged from 13% to 97%. The authors attributed these wide variances to essential differences in the patients, the settings, the protocols, data collection and data presentation. (9) Hoke and Chamberlain reviewed 15 studies that related to conventional CPR in adults. Thirteen of those 15 studies were based on autopsy only. One study was based on autopsy and chest radiographs, and another study was based on chest radiographs only. The chest radiographs-only study represents the only recent study on survivors. The Hoke and Chamberlain review (9) concluded that there are no sound methodological studies on thoracic fractures resulting from chest compression and the studies that are available do not permit interstudy comparison. They offered recommendations for study designs that implement statistically sufficient sample sizes, precisely defined homogenous study groups, thorough autopsies and control of all potential interacting factors, especially age. (9)
Working collaboratively, the International Liaison Committee on Resuscitation (ILCOR) and the AHA have wide-reaching influence on resuscitative health care. These organizations revisit CPR guidelines every 5 years and issue updates as necessary. They are committed to continually reviewing new science to provide medical professionals with state-of-the-art resuscitation treatments. (26) To accomplish this, the ILCOR and the AHA established 6 task forces: basic life support, advanced cardiac life support, acute coronary syndromes, pediatric life support, neonatal life support and an interdisciplinary task force to consider overlapping topics such as educational issues. Furthermore, they implemented steps in their evidence evaluation process that cover the integration of evidence as well as a hierarchy for levels of evidence. In accordance with the latest symposium, the AHA recently revised its guidelines regarding the compression-ventilation ratio for lone rescuers providing chest compressions. The new recommendation calls for a 30:2 compression-ventilation ratio, (7,8) which represents a 100% increase in the number of compressions (30:2 vs 15:2). This reflects the growing data on positive outcomes related to minimizing the number of interruptions during chest compressions. (27-30)
This revision to the guidelines offers an ideal opportunity to study possible changes in the frequency of CPR-related compression injuries to the rib cage in adult survivors using radiographic evaluation. The author suggests adopting the following 5 criteria for study protocols consistent with recommendations by Hoke and Chamberlain (9):
1. Implement a prospective study on survivors of cardiac arrest who received CPR.
2. Use a statistically sufficient sample size to determine relevance.
3. Enroll participants according to homogenous study group criteria.
4. Investigate radiographically, using both chest radiographs and lateral sternum radiographs.
5. Control for all potential interacting factors, especially age.
To the extent possible, the chest radiographs-only study by Oschatz et al should serve as a basis of comparison. As for the initial stages in protocol development, it might be of value to consider 3 basic design elements: gross inspection of the patient before and after the chest compression period, palpation of the chest wall by the physician-investigator responsible for detecting gross evidence of body displacement or fractures, and radiographic data interpretation by a radiologist. (5)
After baseline data are collected and general agreement is reached among the researchers, future investigators should consider alternate projections of the sternum or other radiographic techniques that meet the challenges of imaging the mediastinum in the modern trauma room. (21,31) The author hypothesizes that these recommendations will further help address the difficulties experienced by Lederer et al, who incorporated the lateral sternum radiographs into their study design.
Patient outcomes following cardiac arrest are enhanced by early intervention through CPR. By using radiography to study compression-induced injuries, useful information can be provided to the health care community to evaluate revised treatment guidelines. Radiology will continue to serve as a means to investigate the factors that contribute to CPR, such as biomechanics and the quality of chest compressions given by responders. This article reiterates the suggestion that study protocols should be adopted to promote interstudy comparisons. Radiologic technologists are well suited to play active roles in such studies.
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Answers to Directed Reading Quizzes DR10004018, DR10004019, DRI0005001, DRI0005002 and DRI0005003
Directed Reading Quiz DRI0004018, "Blunt Abdominal Trauma" and DRI0004019, "Health Literacy" were published in the Vol. 76, No. 2 issue of Radiologic Technology. The expiration date to submit these quizzes to the ASRT was January 31, 2007. Answers to the quiz questions are:
Directed Reading Quiz DRI0005001, "Panoramic Radiography;" DRI0005002, "Medical Imaging and CNS Infections;" and DRI0005003, "Radiologic Appearance of Uncommon Breast Diseases" were published in the Vol. 76, No. 3 issue of Radiologic Technology. The expiration date to submit these quizzes to the ASRT was February 28, 2007. Answers to the quiz questions are:
DRI0004018 1. C 2. C 3. D 4. A 5. B 6. C 7. A 8. C 9. A 10. B 11. C 12. C 13. D 14. A 15. B 16. C 17. A 18. D 19. D 20. B 21. A 22. B 23. A 24. B 25. D 26. C 27. C 28. B 29. -- 30. C 31. B 32. B 33. B 34. C 35. A 36. D 37. C DRI0004019 1. B 2. B 3. C 4. C 5. A 6. A 7. C 8. B 9. A 10. A 11. C 12. A 13. D 14. B 15. D 16. A 17. B 18. C 19. D 20. A 21. B 22. C DRI0005001 1. B 2. A 3. D 4. C 5. A 6. C 7. A 8. A 9. D 10. B 11. C 12. A 13. C 14. A 15. -- 16. B 17. D 18. C 19. D 20. A 21. C 22. -- 23. A 24. B 25. C DRI0005002 1. C 2. B 3. D 4. A 5. B 6. B 7. D 8. C 9. D 10. B 11. B 12. D 13. C 14. D 15. C 16. D 17. C 18. C 19. D 20. B 21. C 22. C 23. A 24. C 25. B DRI0005003 1. A 2. C 3. C 4. D 5. B 6. A 7. D 8. B 9. D 10. A 11. B 12. A 13. C 14. D 15. C 16. A 17. D 18. B 19. C 20. A 21. D 22. A 23. B 24. D 25. B 26. C 27. A 28. C 29. B
Kevin L. Wininger, B.S., R.T.(R), RKT, earned a bachelor of science degree in exercise science in 1995, graduating cum laude from the University of Toledo in Ohio. He then moved to Florida to work in the physical medicine and rehabilitation service of the newly-built West Palm Beach Veterans Administration Medical Center. He served our veterans with pride and while working at the medical center developed an interest in radiology. In 2006 he graduated with an associate of science degree in radiologic technology from Keiser University in Lakeland, Fla. He was awarded the university's presidential honors and voted valedictorian. Mr. Wininger recently passed the ARRT certification exam and relocated to Columbus, Ohio, where he is pursuing employment as a radiologic technologist and exploring a master's degree from The Ohio State University.
Reprint requests may be sent to the American Society of Radiologic Technologists, Communications Department, 15000 Central Ave. SE, Albuquerque, NM 87123-3909.
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|Author:||Wininger, Kevin L.|
|Article Type:||Clinical report|
|Date:||Mar 1, 2007|
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