LETTERS
Critique of hyperglycemia and surgical site infection To the Editor: ichards et al.1 have done an excellent study on stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries and found that stress hyperglycemia was associated with surgical site infection in this prospective observational cohort of stable nondiabetic patients with orthopedic injuries. We have tried several times to prove this hypothesis but gave up halfway because of the difficulty in getting enough qualified cases. However, we find a shortcoming in the study of Richards et al. through our limited experience, which is that there was a significant difference between nonhyperglycemic and hyperglycemic patients in terms of age (p = 0.03) and American Society of Anesthesiologists (ASA) class (p = 0.006). As we know, patients with more severe comorbid medical disease, as defined by ASA Class 3 or 4, were more likely to be hyperglycemic, and they also have low resistance to disease, which lead to high risk for postoperative infection. At the same time, the younger patients usually have good hyperglycemia reaction to stress and also have low risk for postoperative infection. Thus, there is no significant difference between nonhyperglycemic and hyperglycemic patients in terms of age, and ASA class is the prerequisite for study on stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries. We hope that there will be such a study in large-scale clinical centers that can meet the prerequisites and give us the exact results about the relationship between stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries.
R
*The authors declare no conflicts of interest. Yueju Liu, MD Han Li, MD Third Hospital of Hebei Medical University and Key Orthopaedic Biomechanics Laboratory of Hebei Province Shijiazhuang, China
REFERENCE 1. Richards JE, Hutchinson J, Mukherjee K, et al. Stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries. J Trauma Acute Care Surg. 2014; 76(4):1070Y1075.
TO THE
EDITOR
Pediatric casualties in the war zone To the Editor: dwards et al.1 report particularly interesting and uncommon data about war pediatric blast injuries. They highlight the specifications of surgical procedures performed for this particular population admitted to a US Department of Defense Medical Treatment Facility (MTF) at the Role 3 (combat hospital) echelon of care, named ‘‘Role 3,’’ during a period of 8 years of war in Iraq and Afghanistan. They emphasize the requirements in pediatric operative resources and expertise but also point out the restrictions on humanitarian admissions in this context. We would like to subjoin two comments. First, data collected by French military physicians in Kabul multinational Role 3, under French command, from 2010 to 2014, showed that pediatric patients (defined as e15 years) could account for one third of emergency or surgical care. In 1 year, up to 60 children were admitted to the intensive care unit, in 88% of cases for penetrating trauma (shrapnels, gunshot wounds) with at least two associated major injuries. A mean Injury Severity Score (ISS) of 25.2 (range, 12Y43) and a mean Trauma and Injury Severity Score (TRISS) of 9.96% (range, 0.9Y46.9%) illustrate the severity of the injuries in these patients, with an in-hospital mortality of 5%. Thus, approximately 100 pediatric anesthetic and surgical procedures (damage control surgery and repeated procedures) were performed monthly. These data enhance those collected by Edwards et al. during a period of 8 years (1,213 patients e 15 years) and the relevance of the questions concerning pediatric operative resources and expertise in Role 3 MTF. They highlight the question of trauma rehabilitation and long-term outcome of these patients in war zone. Besides, the authors point out the limitations of their study due to the lack of data before the admission of the wounded children in Role 3. We can highlight this topic differently thanks to in-press data, collected in 2011 by French military physicians in Region Command East (Kapisa), for each French Role 1 MTF (prehospital echelon of care). They describe their complete prehospital trauma care activity, including 90 wounded children (mean age, 9.4 years; range, 8 months to 15 years), that is, 25% (349) of the overall wounded patients. Among injuries, 59% were directly related to explosions (improvised explosive device, mortar, or other exploding munitions) or to gunshot wounds. Physicians and paramedics
E
performed tactical care and medical procedures: tourniquets and hemostatic dressings, venous or intraosseous access, orotracheal intubation, and sedation. The severity, assessed by a mean Pediatric Trauma Score2 of 7.2 (range, j4 to +11), was illustrated by injury locations (limbs, 50%; head and neck, 18%; abdomen, thorax, and perineum, 20%). Eighty-two pediatric patients needed further urgent surgical procedures: 3 died before evacuation, and the 50 more severely injured were transferred to an ISAF Role 3 (61%), through ISAF (International Security and Assistance Force) tactical medical evacuation. The others, 29 casualties transported to local hospitals and 8 discharged to home, were not followed up. These limited prehospital data highlight the extent of the issue of severe war pediatric injuries. The awesome series of Edwards et al. is probably a terrible, but representative, part of the reality for children in those countries. *The authors declare no conflicts of interest.
Pierre-Fran0ois Wey, MD Fabrice Petitjeans, MD Pascal Precloux, MD Anesthesia and Intensive Care Department Desgenettes French Military Teaching Hospital Lyon, France
REFERENCES 1. Edwards MJ, Lustik M, Carlson T, et al. Surgical interventions for pediatric blast injury: an analysis from Afghanistan and Iraq 2002 to 2010. J Trauma Acute Care Surg. 76:854Y858. 2. Tepas JJ, Mollitt DL, Talbert JL, Bryant M. The pediatric trauma score as a predictor of injury severity in the injured child. J Pediatr Surg. 1987;22:14Y18.
Nonsuperiority does not imply equivalence To the Editor: read with interest the article by Ekeh et al. titled ‘‘Successful placement of intracranial pressure monitors by trauma surgeons,’’ since we have recently published an article on the safe placement of intracranial pressure monitors by midlevel practitioners.1 However, I wish to point out two major flaws in the design and interpretation of the study by Ekeh et al. In their study, they do not specify the study design or adequately explain their statistical analysis. On the basis of the
I
J Trauma Acute Care Surg Volume 77, Number 3
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
521
J Trauma Acute Care Surg Volume 77, Number 3
Letters to the Editor
information they have provided, the only possible inference is that the design is a superiority comparison between the complication rates of neurosurgeons versus trauma surgeons using the two-tailed W2 test withYates correction. This yielded a p value of 0.3762, and, presumably, because this was greater than their > level (which was also not specified), they concluded that there was no difference in the complication rates. This p value is the probability of the difference in the rates being the measured size or larger given that the null hypothesis is true (i.e., there truly is no difference between the rates). If this probability is small enough, then one can reject the null hypothesis, which proves the alternative hypothesis that one measured rate is different from the other. However, the common error here is believing that not finding a difference means equality. A statistical test should be thought of as a diagnostic test. It has a sensitivity, an > level, and a specificity. The ‘‘disease’’ is whether there is a difference between the two treatment groups in the outcome being measured. The sensitivity is the probability of the test being positive when the disease is present (the > level). The specificity is the probability of the test being negative when the disease is absent, which varies based on the magnitude of the difference found. Here, we are concerned with the probability of the test being negative when the disease is present, which is 1-A or power. Power is the probability of rejecting the null hypothesis when the null hypothesis is, in fact, false or, conversely, as in this study, erroneously concluding that there is no difference because no difference was found. After some searching and some help, I found a calculator for retrospective power calculations for the W2 test.2 The power calculation assumes an uncorrected W2 test. The p value in this study for the uncorrected W2 test is 0.2134. The calculated power is 0.179. That means that there is a 17.9% probability that there is a difference between the complication rates of neurosurgeons versus trauma surgeons. If it is assumed that this study used the conventional significance level of 0.05, then the actual value for the question their study asked was more than 3.5 times higher. Therefore, the conclusions drawn in the article are not supported by the statistical analysis. The authors summarized the literature in Table 3. None of those studies, except ours, used a study design and statistical analysis appropriate to the question asked, nor did one further study that we found while researching our article.3 Despite this, it appears that the authors have a large enough sample, and an appropriate analysis will support their conclusion. *The author declares no conflict of interest. 522
William Matthew Bowling, MD Hurley Comprehensive Surgical Group Flint, Michigan
REFERENCES 1. Young PJ, Bowling WM. Mid-level practitioners can safely place intracranial pressure monitors. J Trauma. 2012;73:431Y434. 2. PS: Power and sample size calculation. Available at: http://biostat.mc.vanderbilt.edu/wiki/Main/ PowerSampleSize. Accessed March 7, 2014. 3. Ehtisham A, Taylor S, Bayless L, et al. Placement of external ventricular drains and intracranial pressure monitors by neurointensivists. Neurocrit Care. 2009;10:241Y247.
Re: Nonsuperiority does not imply equivalence In Reply: e thank Dr. Bowling for his comments on our article.1 Dr. Bowling states that we ‘‘do not specify the study design or adequately explain the statistical analysis.’’ Below, we provide our rejoinder. Regarding the study design, we state in the article that a chart review was conducted in which trauma surgeons and neurosurgeons were compared on complication rates following placement of intracranial pressure monitors. Thus, our investigation was a cross-sectional study. Further, Dr. Bowling notes that we have conducted a superiority study, presumably, he means, rather than a noninferiority study. For randomized controlled trials that compare drugs or medical devices, choosing between a noninferiority study and a superiority study is an important planning consideration2 but has no relevance to a cross-sectional study. Consequently, referring to our study as a ‘‘superiority comparison’’ is gratuitous. Regarding the statistical analysis, we state that the W2 test was used for the 2 2 comparison between type of surgeon (trauma surgeon vs. neurosurgeon) and complication (yes or no). The trauma surgeons had nine complications for their 298 cases, while the neurosurgeons had one complication for their 112 casesV3.0% versus 0.9%. The W2 test with Yates correction yielded p = 0.38. When he states that we do ‘‘not I adequately explain the statistical analysis,’’ Dr. Bowling may be referring to the Yates correction, which he raises in his letter but we do not mention in our article. Unlike in the past, when the Yates calculation was done manually or with a calculator, today’s statistical software calculates the Yates correction automatically, when needed. (The Yates procedure subtracts
W
0.5 from the difference between each observed value and its expected value for each cell in a 2 2 table. This formula is used when at least one cell has an expected count of less than 5.) Researchers today seldom report this detail in their published articles. Dr. Bowling is concerned that we did not specify > in our Methods section. We made inferences about statistical significance at the traditional 0.05 level. We are not sure why Dr. Bowling states ‘‘the common error (in statistical inference) I is believing that not finding a difference means equality.’’ We indicate that we found no statistically significant difference between trauma surgeons and neurosurgeons in complication rates but never use the term equal or equality. Dr. Bowling gives his view that ‘‘a statistical test should be thought of as a diagnostic test’’ when he associates the clinical diagnostic testing model (sensitivity and specificity) with the hypothesis testing model. We do not subscribe to this analogy. Clinical diagnostic testing, medicine’s approach to examining agreement, differs from the broader research approach of hypothesis testing, which examines relationships. Clinicians want to know the proportion of agreements (sensitivity and specificity) between a new (perhaps less invasive, less costly) measure and a reference standard. To illustrate: how sensitive and specific is a given (dimerized plasmin fragment D) Ddimer level for predicting (i.e., agreeing with) a diagnosis of pulmonary embolism (PE) as determined with angiography? When a researcher misguidedly uses a statistical test (e.g., W2 test) to determine whether a positive D-dimer level is related to a PE diagnosis, he or she should not be surprised when the p value is less than 0.05 (typically much lower) since a statistical test only tells us the probability of a relationship being due to chance. Physicians know beforehand that D-dimer levels and PEs are related. Accordingly, using a statistical test is trivial in the context of clinical diagnostic testing. In contrast, as noted above, the hypothesis testing model is used to examine relationships. Thus, in our study, we assessed whether trauma surgeons had a greater complication rate than neurosurgeons. In sum, the two paradigmsVclinical diagnostic testing and hypothesis testingVare not interchangeable. Although his calculations miss by a wide margin and he relies on ‘‘some searching and some help (to find) a calculator,’’ Dr. Bowling next addresses the issue of ‘‘retrospective power,’’ more properly called post hoc power (i.e., the probability of detecting the observed difference in complication rates (3.0% vs. 0.9%) given the study’s actual sample sizes (n = 298 cases for trauma surgeons and n = 112 cases for neurosurgeons) and the calculated p value ( p = 0.38). * 2014 Lippincott Williams & Wilkins
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Critique of hyperglycemia and surgical site infection To the Editor: ichards et al.1 have done an excellent study on stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries and found that stress hyperglycemia was associated with surgical site infection in this prospective observational cohort of stable nondiabetic patients with orthopedic injuries. We have tried several times to prove this hypothesis but gave up halfway because of the difficulty in getting enough qualified cases. However, we find a shortcoming in the study of Richards et al. through our limited experience, which is that there was a significant difference between nonhyperglycemic and hyperglycemic patients in terms of age (p = 0.03) and American Society of Anesthesiologists (ASA) class (p = 0.006). As we know, patients with more severe comorbid medical disease, as defined by ASA Class 3 or 4, were more likely to be hyperglycemic, and they also have low resistance to disease, which lead to high risk for postoperative infection. At the same time, the younger patients usually have good hyperglycemia reaction to stress and also have low risk for postoperative infection. Thus, there is no significant difference between nonhyperglycemic and hyperglycemic patients in terms of age, and ASA class is the prerequisite for study on stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries. We hope that there will be such a study in large-scale clinical centers that can meet the prerequisites and give us the exact results about the relationship between stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries.
R
*The authors declare no conflicts of interest. Yueju Liu, MD Han Li, MD Third Hospital of Hebei Medical University and Key Orthopaedic Biomechanics Laboratory of Hebei Province Shijiazhuang, China
REFERENCE 1. Richards JE, Hutchinson J, Mukherjee K, et al. Stress hyperglycemia and surgical site infection in stable nondiabetic adults with orthopedic injuries. J Trauma Acute Care Surg. 2014; 76(4):1070Y1075.
TO THE
EDITOR
Pediatric casualties in the war zone To the Editor: dwards et al.1 report particularly interesting and uncommon data about war pediatric blast injuries. They highlight the specifications of surgical procedures performed for this particular population admitted to a US Department of Defense Medical Treatment Facility (MTF) at the Role 3 (combat hospital) echelon of care, named ‘‘Role 3,’’ during a period of 8 years of war in Iraq and Afghanistan. They emphasize the requirements in pediatric operative resources and expertise but also point out the restrictions on humanitarian admissions in this context. We would like to subjoin two comments. First, data collected by French military physicians in Kabul multinational Role 3, under French command, from 2010 to 2014, showed that pediatric patients (defined as e15 years) could account for one third of emergency or surgical care. In 1 year, up to 60 children were admitted to the intensive care unit, in 88% of cases for penetrating trauma (shrapnels, gunshot wounds) with at least two associated major injuries. A mean Injury Severity Score (ISS) of 25.2 (range, 12Y43) and a mean Trauma and Injury Severity Score (TRISS) of 9.96% (range, 0.9Y46.9%) illustrate the severity of the injuries in these patients, with an in-hospital mortality of 5%. Thus, approximately 100 pediatric anesthetic and surgical procedures (damage control surgery and repeated procedures) were performed monthly. These data enhance those collected by Edwards et al. during a period of 8 years (1,213 patients e 15 years) and the relevance of the questions concerning pediatric operative resources and expertise in Role 3 MTF. They highlight the question of trauma rehabilitation and long-term outcome of these patients in war zone. Besides, the authors point out the limitations of their study due to the lack of data before the admission of the wounded children in Role 3. We can highlight this topic differently thanks to in-press data, collected in 2011 by French military physicians in Region Command East (Kapisa), for each French Role 1 MTF (prehospital echelon of care). They describe their complete prehospital trauma care activity, including 90 wounded children (mean age, 9.4 years; range, 8 months to 15 years), that is, 25% (349) of the overall wounded patients. Among injuries, 59% were directly related to explosions (improvised explosive device, mortar, or other exploding munitions) or to gunshot wounds. Physicians and paramedics
E
performed tactical care and medical procedures: tourniquets and hemostatic dressings, venous or intraosseous access, orotracheal intubation, and sedation. The severity, assessed by a mean Pediatric Trauma Score2 of 7.2 (range, j4 to +11), was illustrated by injury locations (limbs, 50%; head and neck, 18%; abdomen, thorax, and perineum, 20%). Eighty-two pediatric patients needed further urgent surgical procedures: 3 died before evacuation, and the 50 more severely injured were transferred to an ISAF Role 3 (61%), through ISAF (International Security and Assistance Force) tactical medical evacuation. The others, 29 casualties transported to local hospitals and 8 discharged to home, were not followed up. These limited prehospital data highlight the extent of the issue of severe war pediatric injuries. The awesome series of Edwards et al. is probably a terrible, but representative, part of the reality for children in those countries. *The authors declare no conflicts of interest.
Pierre-Fran0ois Wey, MD Fabrice Petitjeans, MD Pascal Precloux, MD Anesthesia and Intensive Care Department Desgenettes French Military Teaching Hospital Lyon, France
REFERENCES 1. Edwards MJ, Lustik M, Carlson T, et al. Surgical interventions for pediatric blast injury: an analysis from Afghanistan and Iraq 2002 to 2010. J Trauma Acute Care Surg. 76:854Y858. 2. Tepas JJ, Mollitt DL, Talbert JL, Bryant M. The pediatric trauma score as a predictor of injury severity in the injured child. J Pediatr Surg. 1987;22:14Y18.
Nonsuperiority does not imply equivalence To the Editor: read with interest the article by Ekeh et al. titled ‘‘Successful placement of intracranial pressure monitors by trauma surgeons,’’ since we have recently published an article on the safe placement of intracranial pressure monitors by midlevel practitioners.1 However, I wish to point out two major flaws in the design and interpretation of the study by Ekeh et al. In their study, they do not specify the study design or adequately explain their statistical analysis. On the basis of the
I
J Trauma Acute Care Surg Volume 77, Number 3
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
521
J Trauma Acute Care Surg Volume 77, Number 3
Letters to the Editor
information they have provided, the only possible inference is that the design is a superiority comparison between the complication rates of neurosurgeons versus trauma surgeons using the two-tailed W2 test withYates correction. This yielded a p value of 0.3762, and, presumably, because this was greater than their > level (which was also not specified), they concluded that there was no difference in the complication rates. This p value is the probability of the difference in the rates being the measured size or larger given that the null hypothesis is true (i.e., there truly is no difference between the rates). If this probability is small enough, then one can reject the null hypothesis, which proves the alternative hypothesis that one measured rate is different from the other. However, the common error here is believing that not finding a difference means equality. A statistical test should be thought of as a diagnostic test. It has a sensitivity, an > level, and a specificity. The ‘‘disease’’ is whether there is a difference between the two treatment groups in the outcome being measured. The sensitivity is the probability of the test being positive when the disease is present (the > level). The specificity is the probability of the test being negative when the disease is absent, which varies based on the magnitude of the difference found. Here, we are concerned with the probability of the test being negative when the disease is present, which is 1-A or power. Power is the probability of rejecting the null hypothesis when the null hypothesis is, in fact, false or, conversely, as in this study, erroneously concluding that there is no difference because no difference was found. After some searching and some help, I found a calculator for retrospective power calculations for the W2 test.2 The power calculation assumes an uncorrected W2 test. The p value in this study for the uncorrected W2 test is 0.2134. The calculated power is 0.179. That means that there is a 17.9% probability that there is a difference between the complication rates of neurosurgeons versus trauma surgeons. If it is assumed that this study used the conventional significance level of 0.05, then the actual value for the question their study asked was more than 3.5 times higher. Therefore, the conclusions drawn in the article are not supported by the statistical analysis. The authors summarized the literature in Table 3. None of those studies, except ours, used a study design and statistical analysis appropriate to the question asked, nor did one further study that we found while researching our article.3 Despite this, it appears that the authors have a large enough sample, and an appropriate analysis will support their conclusion. *The author declares no conflict of interest. 522
William Matthew Bowling, MD Hurley Comprehensive Surgical Group Flint, Michigan
REFERENCES 1. Young PJ, Bowling WM. Mid-level practitioners can safely place intracranial pressure monitors. J Trauma. 2012;73:431Y434. 2. PS: Power and sample size calculation. Available at: http://biostat.mc.vanderbilt.edu/wiki/Main/ PowerSampleSize. Accessed March 7, 2014. 3. Ehtisham A, Taylor S, Bayless L, et al. Placement of external ventricular drains and intracranial pressure monitors by neurointensivists. Neurocrit Care. 2009;10:241Y247.
Re: Nonsuperiority does not imply equivalence In Reply: e thank Dr. Bowling for his comments on our article.1 Dr. Bowling states that we ‘‘do not specify the study design or adequately explain the statistical analysis.’’ Below, we provide our rejoinder. Regarding the study design, we state in the article that a chart review was conducted in which trauma surgeons and neurosurgeons were compared on complication rates following placement of intracranial pressure monitors. Thus, our investigation was a cross-sectional study. Further, Dr. Bowling notes that we have conducted a superiority study, presumably, he means, rather than a noninferiority study. For randomized controlled trials that compare drugs or medical devices, choosing between a noninferiority study and a superiority study is an important planning consideration2 but has no relevance to a cross-sectional study. Consequently, referring to our study as a ‘‘superiority comparison’’ is gratuitous. Regarding the statistical analysis, we state that the W2 test was used for the 2 2 comparison between type of surgeon (trauma surgeon vs. neurosurgeon) and complication (yes or no). The trauma surgeons had nine complications for their 298 cases, while the neurosurgeons had one complication for their 112 casesV3.0% versus 0.9%. The W2 test with Yates correction yielded p = 0.38. When he states that we do ‘‘not I adequately explain the statistical analysis,’’ Dr. Bowling may be referring to the Yates correction, which he raises in his letter but we do not mention in our article. Unlike in the past, when the Yates calculation was done manually or with a calculator, today’s statistical software calculates the Yates correction automatically, when needed. (The Yates procedure subtracts
W
0.5 from the difference between each observed value and its expected value for each cell in a 2 2 table. This formula is used when at least one cell has an expected count of less than 5.) Researchers today seldom report this detail in their published articles. Dr. Bowling is concerned that we did not specify > in our Methods section. We made inferences about statistical significance at the traditional 0.05 level. We are not sure why Dr. Bowling states ‘‘the common error (in statistical inference) I is believing that not finding a difference means equality.’’ We indicate that we found no statistically significant difference between trauma surgeons and neurosurgeons in complication rates but never use the term equal or equality. Dr. Bowling gives his view that ‘‘a statistical test should be thought of as a diagnostic test’’ when he associates the clinical diagnostic testing model (sensitivity and specificity) with the hypothesis testing model. We do not subscribe to this analogy. Clinical diagnostic testing, medicine’s approach to examining agreement, differs from the broader research approach of hypothesis testing, which examines relationships. Clinicians want to know the proportion of agreements (sensitivity and specificity) between a new (perhaps less invasive, less costly) measure and a reference standard. To illustrate: how sensitive and specific is a given (dimerized plasmin fragment D) Ddimer level for predicting (i.e., agreeing with) a diagnosis of pulmonary embolism (PE) as determined with angiography? When a researcher misguidedly uses a statistical test (e.g., W2 test) to determine whether a positive D-dimer level is related to a PE diagnosis, he or she should not be surprised when the p value is less than 0.05 (typically much lower) since a statistical test only tells us the probability of a relationship being due to chance. Physicians know beforehand that D-dimer levels and PEs are related. Accordingly, using a statistical test is trivial in the context of clinical diagnostic testing. In contrast, as noted above, the hypothesis testing model is used to examine relationships. Thus, in our study, we assessed whether trauma surgeons had a greater complication rate than neurosurgeons. In sum, the two paradigmsVclinical diagnostic testing and hypothesis testingVare not interchangeable. Although his calculations miss by a wide margin and he relies on ‘‘some searching and some help (to find) a calculator,’’ Dr. Bowling next addresses the issue of ‘‘retrospective power,’’ more properly called post hoc power (i.e., the probability of detecting the observed difference in complication rates (3.0% vs. 0.9%) given the study’s actual sample sizes (n = 298 cases for trauma surgeons and n = 112 cases for neurosurgeons) and the calculated p value ( p = 0.38). * 2014 Lippincott Williams & Wilkins
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
