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Cerebral Oxygenation and Acceleration in Pediatric and Neonatal Interfacility Transport
Air Medical Journal xxx (2016) 1e5
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Original Research
Cerebral Oxygenation and Acceleration in Pediatric and Neonatal Interfacility Transport Michael E. Valente, MD 1, Judy A. Sherif, RN 1, Colleen G. Azen, MS 2, Phung K. Pham, MS 1, Calvin G. Lowe, MD 1, 3, * 1 2 3
Division of Emergency and Transport Medicine, Children's Hospital Los Angeles, CA, USA Southern California Clinical and Translational Science Institute, Children's Hospital Los Angeles and University of Southern California, Los Angeles, CA, USA Department of Pediatrics, Keck School of Medicine, University of Southern California, Los Angeles, CA
a b s t r a c t Objective: The purpose of this study is to measure peak acceleration forces during interfacility transport; examine whether drops in cerebral oxygenation occurred; and test the associations between cerebral oxygenation, acceleration, and patient positioning. Methods: A cerebral oximeter (INVOS-5100C; Somanetics, Minneapolis, MN) monitored regional saturation of oxygen (rSO2 [cerebral oxygenation]) in pediatric and neonatal patients (N ¼ 24) transported between facilities by ground ambulance, helicopter, or fixed wing aircraft. An accelerometer (GP1; SENSR, Georgetown, TX) bolted to the isolette or gurney recorded z-axis (aligned with the spine) accelerations. Results: The z-axis peak accelerations (absolute values of g) by transport type were as follows: ground ambulance takeoff mean ¼ 0.16 and landing mean ¼ 0.08, helicopter takeoff mean ¼ 0.16 and landing mean ¼ 0.05, fixed wing aircraft takeoff mean ¼ 0.14 and landing mean ¼ 0.20. During takeoff, 2 of 7 patients in the headetoefront of vehicle position experienced rSO2 drop. During landing, 4 of 13 patients in the headetoeback of vehicle position experienced rSO2 drop. There were no significant associations of rSO2 drop during takeoff and landing with patient positioning or with z-axis peak acceleration. Conclusion: Acceleration forces of pediatric and neonatal interfacility transport are small and comparable in magnitude. The relationship between rSO2 drop and patient positioning was not significant in this pilot study. Copyright © 2016 by Air Medical Journal Associates
The current model of centralized pediatric and neonatal critical care relies on the interfacility transport of critically ill neonates and pediatric patients via ground ambulance, helicopter, and fixed wing aircraft. However, these transports are not without risks. In a prospective study of 141 interhospital critical pediatric transports, 12% of patients were documented as having physiologic deterioration during transport.1 In a larger prospective cohort study of 1,085 transported critically ill pediatric patients, unanticipated adverse events occurred in 5% of transports as well as an outcome of death in 10% of the cohort.2 In order for the centralized model of caring for
Michael E. Valente, MD is now an attending physician with Emergency Medicine Specialists of Orange County, Pediatric Emergency Medicine, Children's Hospital Orange County, Orange, CA, USA. * Address for correspondence: Calvin G. Lowe, MD, 4650 Sunset Blvd, MS#113, Los Angeles, CA 90027. E-mail address: [email protected] (C.G. Lowe). 1067-991X/$36.00 Copyright © 2016 by Air Medical Journal Associates http://dx.doi.org/10.1016/j.amj.2016.01.006
critically ill children to be successful, the risks of transporting these patients need to be understood and minimized. Little research has been conducted on the potential physiologic effects of the movement generated by medical transport. Research using acceleration data from adult volunteers during simulated ground and helicopter transports found that supine patients transported by helicopter were subjected to greater lateral and vertical forces and lesser head-to-toe forces than patients transported by ground ambulance.3 The clinical significance of these acceleration forces on patients has not been investigated. There is growing concern in the transport community about vehicle acceleration on clinical outcomes. For example, when a patient's head is placed toward the rear of a fixed wing aircraft, there is greater intracranial blood pooling and subsequent increased intracranial pressure because of acceleration in the caudal direction during aircraft takeoff.4 Furthermore, if a patient's head is positioned to the front of the aircraft, the ventral acceleration during takeoff is expected to lead to decreased cerebral oxygenation pressure
2
M.E. Valente et al. / Air Medical Journal xxx (2016) 1e5
because of blood pooling in the lower extremities as well as decreases in venous return and mean arterial pressure.5 This risk of decreased cerebral oxygenation has led to the practice of avoiding this positioning for patients who may have poor cardiac output. In current practice, patients with a high risk of morbidity from increased intracranial pressure (eg, patients with head trauma) are positioned such that their heads are aligned toward the front of a fixed wing aircraft during takeoff.4 However, empirical validation of this practice has yet to be obtained. Although acceleration effects on cerebral oxygenation have not been studied in patients during transport, near-infrared range spectroscopy (NIRS), which measures regional saturation of oxygen (rSO2) of the brain, has been used to investigate acceleration effects on cerebral oxygenation in healthy adult aircraft pilots.6 To our knowledge, 1 study has used NIRS to similarly investigate cerebral oxygenation in the interfacility transport of pediatric patients by helicopter.7 The NIRS cerebral oximeter is a noninvasive technology that uses the relative absorption of near-infrared light by oxyhemoglobin and deoxyhemoglobin to continuously monitor the oxygen saturation of a small region of cerebral tissue. Compared with pulse oximeters, which measure the oxygen saturation of pulsatile blood (ie, systemic arterial hemoglobin saturation), NIRS measures rSO2 in nonpulsatile blood (ie, local tissue saturation). NIRS is a venous weighted measure composed of an approximate 5 to 1 ratio of local venous to arterial blood, but it should be noted that this proportion may vary from patient to patient.8 Because of this variability along with technological limitations, standard cerebral rSO2 values applicable to all patients have not been established.9 In the absence of standard values, 1 study has defined abnormal cerebral rSO2 as a 20% or higher reduction from baseline or as a baseline absolute value below 40%.10 Despite its limitations, the nature of rSO2 as a venous weighted measure makes it useful as a continuous measure of the balance of oxygen delivery and consumption (ie, metabolism). This has led to its clinical use as a continuous measure of cerebral oxygenation in numerous settings in which patients are at risk of decreases in cerebral oxygenation, including monitoring during pediatric cardiac and neurosurgical procedures.9 In centrifuge and highperformance aircraft studies, decreasing cerebral rSO2 has been associated with increasing acceleration forces in the head-to-toe direction.6,11,12 In the context of patient transport, there may also be an inverse association between rSO2 and acceleration. Therefore, the objectives of this pilot study were to measure peak acceleration forces during interfacility transport; examine whether drops in cerebral rSO2 occurred; and test the associations between rSO2 drop, acceleration, and patient positioning, specifically the headetoeback of vehicle (HTB) and headetoefront of vehicle (HTF) positions. Methods Study Design and Sample This was a prospective pilot study of pediatric and neonatal patients (convenience sampling) who were transported to an urban tertiary care children's hospital by a dedicated pediatric/neonatal transport team. This specialized team is assembled from a transport staff of dedicated transport physicians, nurses, and respiratory therapists. The transport staff serves an urban, metropolitan area that spans 4,850 square miles (12,562 km2) and transports approximately 2,000 patients annually. Modes of transport include ground ambulance (80%), helicopter (20%), and fixed wing aircraft (5%). Patient enrollment into the study was based on the size of the transport vehicle and the availability of study personnel. Study personnel consisted of off-duty transport staff members. Because of space limitations of the transport vehicles, only 1 study personnel member at a time could accompany the on-duty transport team.
-Z
+Z Figure 1. Acceleration in the z-axis.
The institutional review board approved this study with waiver of consent based on the minimal risks of the study and the possible delay of transport if written consent was mandatory. An information sheet was given to the parents in lieu of the written consent process. After the transport, Health Insurance Portability and Accountability Act authorization was obtained. The data collection period began in February 2011 and ended in January 2012. Equipment and Procedures Study participants received standard of care transport services, including mode of transport, monitoring, and medical care, which were determined by the transport team. In addition to the standard monitoring, each study participant received (at the beginning and end of transport) continuous monitoring of cerebral rSO2 by NIRS technology, cerebral oximeter, and in vivo optical spectroscopy (INVOS-5100C; Somanetics, Minneapolis, MN). Two NIRS probes were used, and both had the following dimensions: 17.25 cm2 for neonates and infants and 26.8 cm2 for pediatric patients. Both probes had an emitter/diode spacing of 20 to 40 mm and 2 light paths, each with a light penetrating depth of 20 mm. The monitoring of cerebral rSO2 began at the initial preparation and stabilization before patient transport, and data were used to determine pretransport baseline cerebral saturation. The cerebral rSO2 data were electronically stored by the INVOS cerebral oximeter during the course of transport. Study personnel logged baseline rSO2 values onto a written data collection form, and their real-time observations during takeoff and landing of “drop in rSO2” (20% drop or greater) and “no drop in rSO2” from baseline were documented categorically as yes or no. The clinical diagnoses of the study patients and whether they required medical intervention during their transport were also documented. Transport vehicle accelerations were electronically recorded using a programmable accelerometer, specifically the GP1 (SENSR, Georgetown, TX). This accelerometer uses a 3-axis microelectromechanical system with a measurement range of ± 10 g, a resolution of 0.001 g, and an output rate of 100 samples per second. The accelerometer was mounted on the study participant's gurney or isolette for the duration of the transport. Specifically, the mounting position was consistently oriented to accurately record z-axis accelerations (aligned with the axis of the spine from head to toe) in the supine position (Fig. 1); z-axis peak accelerations were recorded during takeoff and landing. Takeoff was defined as the initial moment of forward movement in ground transport and as the span from taxiing to the attainment of cruise altitude for air transport. Landing was defined as the moment during final braking in ground transport and as the descent completion to final braking for air transport.
M.E. Valente et al. / Air Medical Journal xxx (2016) 1e5
3
Table 1 Summary Statistics of Z-axis Peak Accelerations (Absolute Value g) by Transport Type Transport Type
Phase
Mean (SD)
95% CI of Mean
Median
Range
Ground ambulance (n ¼ 13)
Takeoff Landing Takeoff Landing Takeoff Landing
0.16 0.08 0.16 0.05 0.14 0.20
0.10-0.21 0.05-0.11 0.05-0.26 0.04-0.06 0.01-0.26 0-0.43
0.16 0.08 0.18 0.05 0.17 0.11
0.05-0.35 0.10-0.16 0.02-0.26 0.03-0.06 0.03-0.24 0.03-0.49
Helicopter (n ¼ 6) Fixed wing aircraft (n ¼ 5)
(0.09) (0.05) (0.10) (0.01) (0.10) (0.18)
CI ¼ confidence interval; SD ¼ standard deviation.
Table 2 Cerebral Oxygenation and Patient Positioning During Transport Patient Positioning (n ¼ 22)
Takeoffa
Landingb
Drop in rSO2
No Drop in rSO2
Drop in rSO2
No Drop in rSO2
Head to front (n ¼ 8) Head to back (n ¼ 14)
2 0
5 13
0 4
8 9
rSO2 ¼ regional saturation of oxygen. a During takeoff, data missing for 2 patients (1 head to front of vehicle and 1 head to back of vehicle) because of technical difficulties with the near-infrared range spectroscopy cerebral oximeter. b During landing, data missing for 1 patient (head to back of vehicle) because of technical difficulties with near-infrared range spectroscopy cerebral oximeter.
Statistical Analyses Given the pilot nature of this study, no sample size calculation was performed, and we opted to enroll as many patients as possible during a 12-month period using convenience sampling. Summary statistics for continuous data included mean, standard deviation (SD), median, range, and 95% confidence interval (CI) of the mean. Overlapping CIs were indicative of no significant difference between means. Percentages or counts were reported for categoric data. Fisher exact tests were used to assess the association between rSO2 drop and patient positioning during takeoff and landing. Finally, point biserial correlations were used to test the associations between rSO2 drop and z-axis peak acceleration during takeoff and landing.
Results A total of 25 transported patients were enrolled in the study. Diagnoses included 12 cardiac, 7 pulmonary, 3 neurologic, 2 gastroenterologic, and 1 chromosomal disorder. One patient was excluded from analyses because of missing cerebral oxygenation and acceleration data resulting from technical difficulties during that patient's transport. Therefore, the study sample (N ¼ 24) included 10 males and 14 females who ranged in age from 0 to 11 years (mean ¼11 months, SD ¼ 2.59 years, median ¼ 15 days). No clinical deteriorations were observed among study patients during their transport, and, therefore, no clinical interventions occurred. The z-axis peak accelerations (absolute values of g) by transport type are presented in Table 1. The mean z-axis peak accelerations across the 3 types of transport vehicles were small and comparable in magnitude as evidenced by the overlapping CIs. The mean pretransport baseline rSO2 of the study sample (N ¼ 24) was 65.38% (SD ¼ 17.50; 95% CI, 57.99-72.76; median ¼ 70; range, 19-87). It should be noted that 2 patients had abnormally low cerebral oxygenation10 before being transported by ground ambulance (pretransport baseline rSO2 values of 19% and 39%). For the remaining sample (n ¼ 22), cerebral oxygenation was recorded during takeoff and during landing, and positioning (HTB or HTF) was also recorded (Table 2). Decreased cerebral oxygenation was not significantly associated with patient positioning during takeoff (P ¼ .11) or during landing (P ¼ .13). There were no significant associations between z-axis peak acceleration and rSO2 drop during takeoff (rpb ¼ 0.01, P ¼ .97) or landing (rpb ¼ 0.33, P ¼ .15).
Because all but 2 patients in the study sample were less than 2 years old, a reanalysis was performed that omitted the 2 older patients (7 and 11 years old, both transported by ground ambulance). The reanalysis did not significantly improve associations of rSO2 drop with patient positioning or with z-axis peak acceleration. Therefore, the variability of age was not considered a concern in this study. Finally, over the course of interfacility transport (excluding takeoff and landing), the percentages of patients (n ¼ 22) who were observed as experiencing a 20% drop or greater from pretransport baseline rSO2 were 55% ground ambulance (6/11), 50% helicopter (3/6), and 40% fixed wing aircraft (2/5). Discussion Commonly discussed areas of risk regarding medical transport include the effects of hypoxia at altitude, gas expansion at altitude, noise, vibration, temperature instability, decreased humidity, inherent decrease in level of care, and the potential for complications related to the movement of patients such as the dislodging or failure of medical equipment as well as the physiologic effects of the movement per se.4,13,14 There exists research on the exposure of neonates to sound and vibration during ground and air transport without an investigation into how neonates are clinically affected by these factors.15 Although there is little evidence to support the practice, it has been advocated that certain patient head positions should be used in patient transport to minimize the negative effects of head-to-toe acceleration forces on blood pooling and cerebral oxygenation. As previously mentioned, this pilot study was conducted in order to measure peak acceleration forces during interfacility transport; examine whether drops in cerebral rSO2 occurred; and test the associations between rSO2 drops, acceleration, and patient positioning during takeoff and landing. Our study compliments others in this field in measuring the cerebral oxygenation of pediatric and neonatal patients during interfacility transport. For example, Stroud et al7 used NIRS technology to examine the effect of altitude on cerebral oxygenation of pediatric patients who were transported by helicopter and found that cerebral oxygenation may change with acute changes in altitude. Moreover, Stroud et al concluded that during interfacility helicopter transports, cerebral oxygenation monitoring could be conducted using NIRS technology.
4
M.E. Valente et al. / Air Medical Journal xxx (2016) 1e5
Of interest in our study was the z-axis acceleration (aligned with the spine) across different transport vehicles because this force is postulated to lead to fluid redistribution into the lower extremities and a reduction in cerebral oxygenation resulting from decreased venous return and cardiac output. In this study, the z-axis peak accelerations (absolute values of g) during takeoff and landing of ground ambulance, helicopter, and fixed wing aircraft were small and comparable with each other. Furthermore, the z-axis peak accelerations were not significantly associated with decreased cerebral oxygenation during takeoff or landing. The results of our study thereby suggest that z-axis peak accelerations of small magnitude may not affect cerebral oxygenation. However, it should be noted that accelerations in our study were smaller in comparison with those reported by Silbergleit et al3 in which the acceleration forces for ground ambulance and helicopter ranged from 0.32 to 0.83 g. Nevertheless, both our study and the study by Silbergleit et al observed acceleration forces that were generally smaller than those reported by Kobayashi et al6 (4-7 g) in the context of adult aircraft pilots. From the adult aircraft pilot literature, there is an association between z-axis acceleration and decreased cerebral oxygenation.6,11,12 This association is potentially caused by different forces subjected based on the location within the aircraft (eg, pilot in the cockpit vs. patient in fuselage) and the resultant incline as well as the speed of the aircraft (eg, commercial vs. fighter jet). Of additional interest in our study was the expected relationship between cerebral oxygenation and patient positioning.5 We found that decreased cerebral oxygenation was not significantly associated with patient positioning during takeoff or during landing. However, it should be noted that patients who experienced a drop in rSO2 during takeoff or landing were positioned such that the force of acceleration was in the head-to-toe direction (ie, 2 patients HTF during takeoff and 4 patients HTB during landing all experienced decreased cerebral oxygenation). Because the supine position is parallel to the long axis of a fixed wing aircraft, gravitational forces from acceleration and deceleration theoretically may have pathological effects on patients. During these critical times, a patient may be at risk of a transient redistribution of blood and shift of body organs.16 Because takeoff acceleration is the strongest force, the presumption is that the faster the acceleration rate, the more severe the effects. Orthostatic intolerance as a possible severe effect may cause a temporary reduction in cardiac output and concomitant reduction in intracranial pressure during takeoff with the head in the forward position.17 Patients with marginal cardiac reserve or those with significant intracranial pathology would be at greatest risk. Further investigation with a larger sample is warranted in order to clarify the association between cerebral oxygenation and patient positioning during transport. Finally, we were concerned about the issue of patient deterioration during transport. Decreases in rSO2 have been found to be early markers of clinical deterioration.9 In our study using NIRS technology, we detected a drop in rSO2 of at least 20% in 11 of 22 patients (55% ground ambulance, 50% helicopter, and 40% fixed wing aircraft). Yet, no clinical deteriorations were observed among our study patients during their transport, and, therefore, no clinical interventions occurred. Nevertheless, we appreciated the usefulness of NIRS technology. The detection of clinical deterioration through physical examinations during transport can be challenging under the conditions of noise, limited space, and movement of the transport vehicle. Therefore, using NIRS technology to continuously monitor rSO2 may facilitate the early detection of changes in clinical status.18 Our pilot study should be interpreted in light of several limitations. To begin with, our sample size was small, which precludes
us from making definitive interpretations from the results. Next, we acknowledge that ground transport entails multiple start and stops because of the nature of traffic. Cerebral oxygenation was not logged continuously en route to obtain rSO2 values during all starts and stops, only during the initial moment of forward movement and final braking in ground transport. Related to this limitation, the cerebral oximeter used in this study was not specifically manufactured for the medical transport setting. Its battery life was limited, which constrained the data collection. Cerebral oxygenation could be continuously monitored, but the caveat was that it could be monitored only for short durations. Future research examining rSO2 on a continuous basis to span the entire duration of transport should be conducted once equipment efficiency advancements have been made. Another limitation of this study is that time measurements spent on peak accelerations and decelerations were not logged because they were short in duration, lasting only seconds. Future research on z-axis peak accelerations of transport vehicles might include formal time measurements. Additionally, clinical details about transport patients and not simply their diagnoses should be obtained to guide interpretations about their positioning during transport (eg, HTF is avoided for patients with poor cardiac output or elevated intracranial pressure). Furthermore, sudden drops in altitude or changes in direction (eg, ground ambulance swerving to avoid obstacles) are inevitable during transport. These details, including lateral and vertical forces in helicopter transport, were not examined in this study, and they may have confounded the results. Finally, we acknowledge that although previous research10 has defined a significant decrease in rSO2 as a 20% or higher reduction from baseline, the standard values of abnormal cerebral oxygenation have yet to be established for pediatric and neonatal patients. Conclusions During the transport of pediatric and neonatal patients between facilities, z-axis peak acceleration forces generated by ground ambulance, helicopter, and fixed wing aircraft are small and comparable in magnitude. Further investigation with a larger sample is warranted to clarify the association between cerebral oxygenation and patient positioning during transport. References 1. Edge WE, Kanter RK, Weigle CG, Walsh RF. Reduction of morbidity in interhospital transport by specialized pediatric staff. Crit Care Med. 1994;22: 1186e1191. 2. Orr RA, Felmet KA, Han Y, et al. Pediatric specialized transport teams are associated with improved outcomes. Pediatrics. 2009;124:40e48. 3. Silbergleit R, Dedrick DK, Pape J, Burney RE. Forces acting during air and ground transport on patients stabilized by standard immobilization techniques. Ann Emerg Med. 1991;20:875e877. 4. Woodward GA, Insoft RM, Kleinman ME, eds. American Academy of Pediatrics Section on Transport Medicine. Guidelines for Air and Ground Transport of Neonatal and Pediatric Patients. 3rd ed. Elk Grove, IL: American Academy of Pediatrics; 2007. 5. Hurd WW, Jernigan JG. Aeromedical Evacuation: Management of Acute and Stabilized Patients. New York, NY: Springer; 2003. 6. Kobayashi A, Kikukawa A, Onozawa A. Effect of muscle tensing on cerebral oxygen status during sustained high þGz. Aviat Space Environ Med. 2002;73: 597e600. 7. Stroud MH, Gupta P, Prodhan P. Effect of altitude on cerebral oxygenation during pediatric interfacility transport. Pediatr Emerg Care. 2012;28: 329e332. 8. Watzman HM, Kurth CD, Montenegro LM, Rome J, Steven JM, Nicolson SC. Arterial and venous contributions to near-infrared cerebral oximetry. Anesthesiology. 2000;93:947e953. 9. Murkin JM, Arango M. Near-infrared spectroscopy as an index of brain and tissue oxygenation. Br J Anaesth. 2009;103(Suppl 1):i3ei13. 10. Austin EH 3rd, Edmonds HI Jr, Auden SM, et al. Benefit of neurophysiologic monitoring for pediatric cardiac surgery. J Thorac Cardiovasc Surg. 1997;114: 707e715, 717; discussion 715-706.
M.E. Valente et al. / Air Medical Journal xxx (2016) 1e5 11. McKinley RA, Tripp LD Jr, Bolia SD, Roark MR. Computer modeling of acceleration effects on cerebral oxygen saturation. Aviat Space Environ Med. 2005;76:733e738. 12. Ryoo HC, Sun HH, Shender BS, Hrebien L. Consciousness monitoring using near-infrared spectroscopy (NIRS) during high þGz exposures. Med Eng Phys. 2004;26:745e753. 13. Schierholz E. Flight physiology: science of air travel with neonatal transport considerations. Adv Neonatal Care. 2010;10:196e199. 14. Woodward GA, Insoft RM, Pearson-Shaver AL, et al. The state of pediatric interfacility transport: consensus of the second National Pediatric and Neonatal Interfacility Transport Medicine Leadership Conference. Pediatr Emerg Care. 2002;18:38e43.
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15. Campbell AN, Lightstone AD, Smith JM, Kirpalani H, Perlman M. Mechanical vibration and sound levels experienced in neonatal transport. Am J Dis Child. 1984;138:967e970. 16. Martin T, Rodenberg HD. Aeromedical Transportation: A Clinical Guide. 2nd ed. Burlington, VT: Ashgate; 2006. 17. The Naval Aerospace Medical Institute. United States Naval Flight Surgeon's Manual. 3rd ed. Washington, DC: The Bureau of Medicine and Surgery, Department of the Navy; 1991. 18. Denault A, Deschamps A, Murkin JM. A proposed algorithm for the intraoperative use of cerebral near-infrared spectroscopy. Semin Cardiothorac Vasc Anesth. 2007;11:274e281.
Contents lists available at ScienceDirect
Air Medical Journal journal homepage: http://www.airmedicaljournal.com/
Original Research
Cerebral Oxygenation and Acceleration in Pediatric and Neonatal Interfacility Transport Michael E. Valente, MD 1, Judy A. Sherif, RN 1, Colleen G. Azen, MS 2, Phung K. Pham, MS 1, Calvin G. Lowe, MD 1, 3, * 1 2 3
Division of Emergency and Transport Medicine, Children's Hospital Los Angeles, CA, USA Southern California Clinical and Translational Science Institute, Children's Hospital Los Angeles and University of Southern California, Los Angeles, CA, USA Department of Pediatrics, Keck School of Medicine, University of Southern California, Los Angeles, CA
a b s t r a c t Objective: The purpose of this study is to measure peak acceleration forces during interfacility transport; examine whether drops in cerebral oxygenation occurred; and test the associations between cerebral oxygenation, acceleration, and patient positioning. Methods: A cerebral oximeter (INVOS-5100C; Somanetics, Minneapolis, MN) monitored regional saturation of oxygen (rSO2 [cerebral oxygenation]) in pediatric and neonatal patients (N ¼ 24) transported between facilities by ground ambulance, helicopter, or fixed wing aircraft. An accelerometer (GP1; SENSR, Georgetown, TX) bolted to the isolette or gurney recorded z-axis (aligned with the spine) accelerations. Results: The z-axis peak accelerations (absolute values of g) by transport type were as follows: ground ambulance takeoff mean ¼ 0.16 and landing mean ¼ 0.08, helicopter takeoff mean ¼ 0.16 and landing mean ¼ 0.05, fixed wing aircraft takeoff mean ¼ 0.14 and landing mean ¼ 0.20. During takeoff, 2 of 7 patients in the headetoefront of vehicle position experienced rSO2 drop. During landing, 4 of 13 patients in the headetoeback of vehicle position experienced rSO2 drop. There were no significant associations of rSO2 drop during takeoff and landing with patient positioning or with z-axis peak acceleration. Conclusion: Acceleration forces of pediatric and neonatal interfacility transport are small and comparable in magnitude. The relationship between rSO2 drop and patient positioning was not significant in this pilot study. Copyright © 2016 by Air Medical Journal Associates
The current model of centralized pediatric and neonatal critical care relies on the interfacility transport of critically ill neonates and pediatric patients via ground ambulance, helicopter, and fixed wing aircraft. However, these transports are not without risks. In a prospective study of 141 interhospital critical pediatric transports, 12% of patients were documented as having physiologic deterioration during transport.1 In a larger prospective cohort study of 1,085 transported critically ill pediatric patients, unanticipated adverse events occurred in 5% of transports as well as an outcome of death in 10% of the cohort.2 In order for the centralized model of caring for
Michael E. Valente, MD is now an attending physician with Emergency Medicine Specialists of Orange County, Pediatric Emergency Medicine, Children's Hospital Orange County, Orange, CA, USA. * Address for correspondence: Calvin G. Lowe, MD, 4650 Sunset Blvd, MS#113, Los Angeles, CA 90027. E-mail address: [email protected] (C.G. Lowe). 1067-991X/$36.00 Copyright © 2016 by Air Medical Journal Associates http://dx.doi.org/10.1016/j.amj.2016.01.006
critically ill children to be successful, the risks of transporting these patients need to be understood and minimized. Little research has been conducted on the potential physiologic effects of the movement generated by medical transport. Research using acceleration data from adult volunteers during simulated ground and helicopter transports found that supine patients transported by helicopter were subjected to greater lateral and vertical forces and lesser head-to-toe forces than patients transported by ground ambulance.3 The clinical significance of these acceleration forces on patients has not been investigated. There is growing concern in the transport community about vehicle acceleration on clinical outcomes. For example, when a patient's head is placed toward the rear of a fixed wing aircraft, there is greater intracranial blood pooling and subsequent increased intracranial pressure because of acceleration in the caudal direction during aircraft takeoff.4 Furthermore, if a patient's head is positioned to the front of the aircraft, the ventral acceleration during takeoff is expected to lead to decreased cerebral oxygenation pressure
2
M.E. Valente et al. / Air Medical Journal xxx (2016) 1e5
because of blood pooling in the lower extremities as well as decreases in venous return and mean arterial pressure.5 This risk of decreased cerebral oxygenation has led to the practice of avoiding this positioning for patients who may have poor cardiac output. In current practice, patients with a high risk of morbidity from increased intracranial pressure (eg, patients with head trauma) are positioned such that their heads are aligned toward the front of a fixed wing aircraft during takeoff.4 However, empirical validation of this practice has yet to be obtained. Although acceleration effects on cerebral oxygenation have not been studied in patients during transport, near-infrared range spectroscopy (NIRS), which measures regional saturation of oxygen (rSO2) of the brain, has been used to investigate acceleration effects on cerebral oxygenation in healthy adult aircraft pilots.6 To our knowledge, 1 study has used NIRS to similarly investigate cerebral oxygenation in the interfacility transport of pediatric patients by helicopter.7 The NIRS cerebral oximeter is a noninvasive technology that uses the relative absorption of near-infrared light by oxyhemoglobin and deoxyhemoglobin to continuously monitor the oxygen saturation of a small region of cerebral tissue. Compared with pulse oximeters, which measure the oxygen saturation of pulsatile blood (ie, systemic arterial hemoglobin saturation), NIRS measures rSO2 in nonpulsatile blood (ie, local tissue saturation). NIRS is a venous weighted measure composed of an approximate 5 to 1 ratio of local venous to arterial blood, but it should be noted that this proportion may vary from patient to patient.8 Because of this variability along with technological limitations, standard cerebral rSO2 values applicable to all patients have not been established.9 In the absence of standard values, 1 study has defined abnormal cerebral rSO2 as a 20% or higher reduction from baseline or as a baseline absolute value below 40%.10 Despite its limitations, the nature of rSO2 as a venous weighted measure makes it useful as a continuous measure of the balance of oxygen delivery and consumption (ie, metabolism). This has led to its clinical use as a continuous measure of cerebral oxygenation in numerous settings in which patients are at risk of decreases in cerebral oxygenation, including monitoring during pediatric cardiac and neurosurgical procedures.9 In centrifuge and highperformance aircraft studies, decreasing cerebral rSO2 has been associated with increasing acceleration forces in the head-to-toe direction.6,11,12 In the context of patient transport, there may also be an inverse association between rSO2 and acceleration. Therefore, the objectives of this pilot study were to measure peak acceleration forces during interfacility transport; examine whether drops in cerebral rSO2 occurred; and test the associations between rSO2 drop, acceleration, and patient positioning, specifically the headetoeback of vehicle (HTB) and headetoefront of vehicle (HTF) positions. Methods Study Design and Sample This was a prospective pilot study of pediatric and neonatal patients (convenience sampling) who were transported to an urban tertiary care children's hospital by a dedicated pediatric/neonatal transport team. This specialized team is assembled from a transport staff of dedicated transport physicians, nurses, and respiratory therapists. The transport staff serves an urban, metropolitan area that spans 4,850 square miles (12,562 km2) and transports approximately 2,000 patients annually. Modes of transport include ground ambulance (80%), helicopter (20%), and fixed wing aircraft (5%). Patient enrollment into the study was based on the size of the transport vehicle and the availability of study personnel. Study personnel consisted of off-duty transport staff members. Because of space limitations of the transport vehicles, only 1 study personnel member at a time could accompany the on-duty transport team.
-Z
+Z Figure 1. Acceleration in the z-axis.
The institutional review board approved this study with waiver of consent based on the minimal risks of the study and the possible delay of transport if written consent was mandatory. An information sheet was given to the parents in lieu of the written consent process. After the transport, Health Insurance Portability and Accountability Act authorization was obtained. The data collection period began in February 2011 and ended in January 2012. Equipment and Procedures Study participants received standard of care transport services, including mode of transport, monitoring, and medical care, which were determined by the transport team. In addition to the standard monitoring, each study participant received (at the beginning and end of transport) continuous monitoring of cerebral rSO2 by NIRS technology, cerebral oximeter, and in vivo optical spectroscopy (INVOS-5100C; Somanetics, Minneapolis, MN). Two NIRS probes were used, and both had the following dimensions: 17.25 cm2 for neonates and infants and 26.8 cm2 for pediatric patients. Both probes had an emitter/diode spacing of 20 to 40 mm and 2 light paths, each with a light penetrating depth of 20 mm. The monitoring of cerebral rSO2 began at the initial preparation and stabilization before patient transport, and data were used to determine pretransport baseline cerebral saturation. The cerebral rSO2 data were electronically stored by the INVOS cerebral oximeter during the course of transport. Study personnel logged baseline rSO2 values onto a written data collection form, and their real-time observations during takeoff and landing of “drop in rSO2” (20% drop or greater) and “no drop in rSO2” from baseline were documented categorically as yes or no. The clinical diagnoses of the study patients and whether they required medical intervention during their transport were also documented. Transport vehicle accelerations were electronically recorded using a programmable accelerometer, specifically the GP1 (SENSR, Georgetown, TX). This accelerometer uses a 3-axis microelectromechanical system with a measurement range of ± 10 g, a resolution of 0.001 g, and an output rate of 100 samples per second. The accelerometer was mounted on the study participant's gurney or isolette for the duration of the transport. Specifically, the mounting position was consistently oriented to accurately record z-axis accelerations (aligned with the axis of the spine from head to toe) in the supine position (Fig. 1); z-axis peak accelerations were recorded during takeoff and landing. Takeoff was defined as the initial moment of forward movement in ground transport and as the span from taxiing to the attainment of cruise altitude for air transport. Landing was defined as the moment during final braking in ground transport and as the descent completion to final braking for air transport.
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Table 1 Summary Statistics of Z-axis Peak Accelerations (Absolute Value g) by Transport Type Transport Type
Phase
Mean (SD)
95% CI of Mean
Median
Range
Ground ambulance (n ¼ 13)
Takeoff Landing Takeoff Landing Takeoff Landing
0.16 0.08 0.16 0.05 0.14 0.20
0.10-0.21 0.05-0.11 0.05-0.26 0.04-0.06 0.01-0.26 0-0.43
0.16 0.08 0.18 0.05 0.17 0.11
0.05-0.35 0.10-0.16 0.02-0.26 0.03-0.06 0.03-0.24 0.03-0.49
Helicopter (n ¼ 6) Fixed wing aircraft (n ¼ 5)
(0.09) (0.05) (0.10) (0.01) (0.10) (0.18)
CI ¼ confidence interval; SD ¼ standard deviation.
Table 2 Cerebral Oxygenation and Patient Positioning During Transport Patient Positioning (n ¼ 22)
Takeoffa
Landingb
Drop in rSO2
No Drop in rSO2
Drop in rSO2
No Drop in rSO2
Head to front (n ¼ 8) Head to back (n ¼ 14)
2 0
5 13
0 4
8 9
rSO2 ¼ regional saturation of oxygen. a During takeoff, data missing for 2 patients (1 head to front of vehicle and 1 head to back of vehicle) because of technical difficulties with the near-infrared range spectroscopy cerebral oximeter. b During landing, data missing for 1 patient (head to back of vehicle) because of technical difficulties with near-infrared range spectroscopy cerebral oximeter.
Statistical Analyses Given the pilot nature of this study, no sample size calculation was performed, and we opted to enroll as many patients as possible during a 12-month period using convenience sampling. Summary statistics for continuous data included mean, standard deviation (SD), median, range, and 95% confidence interval (CI) of the mean. Overlapping CIs were indicative of no significant difference between means. Percentages or counts were reported for categoric data. Fisher exact tests were used to assess the association between rSO2 drop and patient positioning during takeoff and landing. Finally, point biserial correlations were used to test the associations between rSO2 drop and z-axis peak acceleration during takeoff and landing.
Results A total of 25 transported patients were enrolled in the study. Diagnoses included 12 cardiac, 7 pulmonary, 3 neurologic, 2 gastroenterologic, and 1 chromosomal disorder. One patient was excluded from analyses because of missing cerebral oxygenation and acceleration data resulting from technical difficulties during that patient's transport. Therefore, the study sample (N ¼ 24) included 10 males and 14 females who ranged in age from 0 to 11 years (mean ¼11 months, SD ¼ 2.59 years, median ¼ 15 days). No clinical deteriorations were observed among study patients during their transport, and, therefore, no clinical interventions occurred. The z-axis peak accelerations (absolute values of g) by transport type are presented in Table 1. The mean z-axis peak accelerations across the 3 types of transport vehicles were small and comparable in magnitude as evidenced by the overlapping CIs. The mean pretransport baseline rSO2 of the study sample (N ¼ 24) was 65.38% (SD ¼ 17.50; 95% CI, 57.99-72.76; median ¼ 70; range, 19-87). It should be noted that 2 patients had abnormally low cerebral oxygenation10 before being transported by ground ambulance (pretransport baseline rSO2 values of 19% and 39%). For the remaining sample (n ¼ 22), cerebral oxygenation was recorded during takeoff and during landing, and positioning (HTB or HTF) was also recorded (Table 2). Decreased cerebral oxygenation was not significantly associated with patient positioning during takeoff (P ¼ .11) or during landing (P ¼ .13). There were no significant associations between z-axis peak acceleration and rSO2 drop during takeoff (rpb ¼ 0.01, P ¼ .97) or landing (rpb ¼ 0.33, P ¼ .15).
Because all but 2 patients in the study sample were less than 2 years old, a reanalysis was performed that omitted the 2 older patients (7 and 11 years old, both transported by ground ambulance). The reanalysis did not significantly improve associations of rSO2 drop with patient positioning or with z-axis peak acceleration. Therefore, the variability of age was not considered a concern in this study. Finally, over the course of interfacility transport (excluding takeoff and landing), the percentages of patients (n ¼ 22) who were observed as experiencing a 20% drop or greater from pretransport baseline rSO2 were 55% ground ambulance (6/11), 50% helicopter (3/6), and 40% fixed wing aircraft (2/5). Discussion Commonly discussed areas of risk regarding medical transport include the effects of hypoxia at altitude, gas expansion at altitude, noise, vibration, temperature instability, decreased humidity, inherent decrease in level of care, and the potential for complications related to the movement of patients such as the dislodging or failure of medical equipment as well as the physiologic effects of the movement per se.4,13,14 There exists research on the exposure of neonates to sound and vibration during ground and air transport without an investigation into how neonates are clinically affected by these factors.15 Although there is little evidence to support the practice, it has been advocated that certain patient head positions should be used in patient transport to minimize the negative effects of head-to-toe acceleration forces on blood pooling and cerebral oxygenation. As previously mentioned, this pilot study was conducted in order to measure peak acceleration forces during interfacility transport; examine whether drops in cerebral rSO2 occurred; and test the associations between rSO2 drops, acceleration, and patient positioning during takeoff and landing. Our study compliments others in this field in measuring the cerebral oxygenation of pediatric and neonatal patients during interfacility transport. For example, Stroud et al7 used NIRS technology to examine the effect of altitude on cerebral oxygenation of pediatric patients who were transported by helicopter and found that cerebral oxygenation may change with acute changes in altitude. Moreover, Stroud et al concluded that during interfacility helicopter transports, cerebral oxygenation monitoring could be conducted using NIRS technology.
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Of interest in our study was the z-axis acceleration (aligned with the spine) across different transport vehicles because this force is postulated to lead to fluid redistribution into the lower extremities and a reduction in cerebral oxygenation resulting from decreased venous return and cardiac output. In this study, the z-axis peak accelerations (absolute values of g) during takeoff and landing of ground ambulance, helicopter, and fixed wing aircraft were small and comparable with each other. Furthermore, the z-axis peak accelerations were not significantly associated with decreased cerebral oxygenation during takeoff or landing. The results of our study thereby suggest that z-axis peak accelerations of small magnitude may not affect cerebral oxygenation. However, it should be noted that accelerations in our study were smaller in comparison with those reported by Silbergleit et al3 in which the acceleration forces for ground ambulance and helicopter ranged from 0.32 to 0.83 g. Nevertheless, both our study and the study by Silbergleit et al observed acceleration forces that were generally smaller than those reported by Kobayashi et al6 (4-7 g) in the context of adult aircraft pilots. From the adult aircraft pilot literature, there is an association between z-axis acceleration and decreased cerebral oxygenation.6,11,12 This association is potentially caused by different forces subjected based on the location within the aircraft (eg, pilot in the cockpit vs. patient in fuselage) and the resultant incline as well as the speed of the aircraft (eg, commercial vs. fighter jet). Of additional interest in our study was the expected relationship between cerebral oxygenation and patient positioning.5 We found that decreased cerebral oxygenation was not significantly associated with patient positioning during takeoff or during landing. However, it should be noted that patients who experienced a drop in rSO2 during takeoff or landing were positioned such that the force of acceleration was in the head-to-toe direction (ie, 2 patients HTF during takeoff and 4 patients HTB during landing all experienced decreased cerebral oxygenation). Because the supine position is parallel to the long axis of a fixed wing aircraft, gravitational forces from acceleration and deceleration theoretically may have pathological effects on patients. During these critical times, a patient may be at risk of a transient redistribution of blood and shift of body organs.16 Because takeoff acceleration is the strongest force, the presumption is that the faster the acceleration rate, the more severe the effects. Orthostatic intolerance as a possible severe effect may cause a temporary reduction in cardiac output and concomitant reduction in intracranial pressure during takeoff with the head in the forward position.17 Patients with marginal cardiac reserve or those with significant intracranial pathology would be at greatest risk. Further investigation with a larger sample is warranted in order to clarify the association between cerebral oxygenation and patient positioning during transport. Finally, we were concerned about the issue of patient deterioration during transport. Decreases in rSO2 have been found to be early markers of clinical deterioration.9 In our study using NIRS technology, we detected a drop in rSO2 of at least 20% in 11 of 22 patients (55% ground ambulance, 50% helicopter, and 40% fixed wing aircraft). Yet, no clinical deteriorations were observed among our study patients during their transport, and, therefore, no clinical interventions occurred. Nevertheless, we appreciated the usefulness of NIRS technology. The detection of clinical deterioration through physical examinations during transport can be challenging under the conditions of noise, limited space, and movement of the transport vehicle. Therefore, using NIRS technology to continuously monitor rSO2 may facilitate the early detection of changes in clinical status.18 Our pilot study should be interpreted in light of several limitations. To begin with, our sample size was small, which precludes
us from making definitive interpretations from the results. Next, we acknowledge that ground transport entails multiple start and stops because of the nature of traffic. Cerebral oxygenation was not logged continuously en route to obtain rSO2 values during all starts and stops, only during the initial moment of forward movement and final braking in ground transport. Related to this limitation, the cerebral oximeter used in this study was not specifically manufactured for the medical transport setting. Its battery life was limited, which constrained the data collection. Cerebral oxygenation could be continuously monitored, but the caveat was that it could be monitored only for short durations. Future research examining rSO2 on a continuous basis to span the entire duration of transport should be conducted once equipment efficiency advancements have been made. Another limitation of this study is that time measurements spent on peak accelerations and decelerations were not logged because they were short in duration, lasting only seconds. Future research on z-axis peak accelerations of transport vehicles might include formal time measurements. Additionally, clinical details about transport patients and not simply their diagnoses should be obtained to guide interpretations about their positioning during transport (eg, HTF is avoided for patients with poor cardiac output or elevated intracranial pressure). Furthermore, sudden drops in altitude or changes in direction (eg, ground ambulance swerving to avoid obstacles) are inevitable during transport. These details, including lateral and vertical forces in helicopter transport, were not examined in this study, and they may have confounded the results. Finally, we acknowledge that although previous research10 has defined a significant decrease in rSO2 as a 20% or higher reduction from baseline, the standard values of abnormal cerebral oxygenation have yet to be established for pediatric and neonatal patients. Conclusions During the transport of pediatric and neonatal patients between facilities, z-axis peak acceleration forces generated by ground ambulance, helicopter, and fixed wing aircraft are small and comparable in magnitude. Further investigation with a larger sample is warranted to clarify the association between cerebral oxygenation and patient positioning during transport. References 1. Edge WE, Kanter RK, Weigle CG, Walsh RF. Reduction of morbidity in interhospital transport by specialized pediatric staff. Crit Care Med. 1994;22: 1186e1191. 2. Orr RA, Felmet KA, Han Y, et al. Pediatric specialized transport teams are associated with improved outcomes. Pediatrics. 2009;124:40e48. 3. Silbergleit R, Dedrick DK, Pape J, Burney RE. Forces acting during air and ground transport on patients stabilized by standard immobilization techniques. Ann Emerg Med. 1991;20:875e877. 4. Woodward GA, Insoft RM, Kleinman ME, eds. American Academy of Pediatrics Section on Transport Medicine. Guidelines for Air and Ground Transport of Neonatal and Pediatric Patients. 3rd ed. Elk Grove, IL: American Academy of Pediatrics; 2007. 5. Hurd WW, Jernigan JG. Aeromedical Evacuation: Management of Acute and Stabilized Patients. New York, NY: Springer; 2003. 6. Kobayashi A, Kikukawa A, Onozawa A. Effect of muscle tensing on cerebral oxygen status during sustained high þGz. Aviat Space Environ Med. 2002;73: 597e600. 7. Stroud MH, Gupta P, Prodhan P. Effect of altitude on cerebral oxygenation during pediatric interfacility transport. Pediatr Emerg Care. 2012;28: 329e332. 8. Watzman HM, Kurth CD, Montenegro LM, Rome J, Steven JM, Nicolson SC. Arterial and venous contributions to near-infrared cerebral oximetry. Anesthesiology. 2000;93:947e953. 9. Murkin JM, Arango M. Near-infrared spectroscopy as an index of brain and tissue oxygenation. Br J Anaesth. 2009;103(Suppl 1):i3ei13. 10. Austin EH 3rd, Edmonds HI Jr, Auden SM, et al. Benefit of neurophysiologic monitoring for pediatric cardiac surgery. J Thorac Cardiovasc Surg. 1997;114: 707e715, 717; discussion 715-706.
M.E. Valente et al. / Air Medical Journal xxx (2016) 1e5 11. McKinley RA, Tripp LD Jr, Bolia SD, Roark MR. Computer modeling of acceleration effects on cerebral oxygen saturation. Aviat Space Environ Med. 2005;76:733e738. 12. Ryoo HC, Sun HH, Shender BS, Hrebien L. Consciousness monitoring using near-infrared spectroscopy (NIRS) during high þGz exposures. Med Eng Phys. 2004;26:745e753. 13. Schierholz E. Flight physiology: science of air travel with neonatal transport considerations. Adv Neonatal Care. 2010;10:196e199. 14. Woodward GA, Insoft RM, Pearson-Shaver AL, et al. The state of pediatric interfacility transport: consensus of the second National Pediatric and Neonatal Interfacility Transport Medicine Leadership Conference. Pediatr Emerg Care. 2002;18:38e43.
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15. Campbell AN, Lightstone AD, Smith JM, Kirpalani H, Perlman M. Mechanical vibration and sound levels experienced in neonatal transport. Am J Dis Child. 1984;138:967e970. 16. Martin T, Rodenberg HD. Aeromedical Transportation: A Clinical Guide. 2nd ed. Burlington, VT: Ashgate; 2006. 17. The Naval Aerospace Medical Institute. United States Naval Flight Surgeon's Manual. 3rd ed. Washington, DC: The Bureau of Medicine and Surgery, Department of the Navy; 1991. 18. Denault A, Deschamps A, Murkin JM. A proposed algorithm for the intraoperative use of cerebral near-infrared spectroscopy. Semin Cardiothorac Vasc Anesth. 2007;11:274e281.