JOURNAL HOME CME HOME THIS MONTH PAST ISSUES ETOC COLLECTIONS
AUTHORS REVIEWERS EDITORIAL BOARD FEEDBACK RSS HELP
A&A International Anesthesia Research Society
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a colleague
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kober, A.
Right arrow Articles by Hoerauf, K.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Kober, A.
Right arrow Articles by Hoerauf, K.
Related Collections
Right arrow Trauma
Right arrow Equipment

Anesth Analg 2002;95:961-966
© 2002 International Anesthesia Research Society

TECHNOLOGY, COMPUTING, AND SIMULATION

The Influence of Active Warming on Signal Quality of Pulse Oximetry in Prehospital Trauma Care

Alexander Kober, MD*{dagger}{ddagger}, Thomas Scheck, MD*{dagger}{ddagger}, Frank Lieba, BS{dagger}{ddagger}, Renate Barker, MD*{dagger}{ddagger}, Wolfgang Vlach, MD*{dagger}{ddagger}, Wolfgang Schramm, MD*, and Klaus Hoerauf, MD*

*Department of Anesthesia and Intensive Care, University of Vienna; {dagger}Vienna Red Cross, Van Swieten; and {ddagger}Research Institute of the Vienna Red Cross, Vienna, Austria

Address correspondence to Dr. Klaus Hoerauf, Department of Anesthesiology and General Intensive Care, University Hospital of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria. Address e-mail to klaus.hoerauf{at}univie.ac.at There are no reprints available.


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Victims of trauma such as contusions and simple fractures are usually transported by paramedics. Because many victims are intoxicated with alcohol or other drugs, they are vulnerable to some risk of inadequate respiration. Thus, their oxygenation is monitored by noninvasive pulse oximetry. We tested the hypothesis that active warming of the whole body during transport to the hospital can improve the reliability of arterial oxygen saturation (SpO2) monitoring. Twenty-four trauma patients transported to hospital were included in the study and randomly assigned to two groups: one group (n = 12) was covered with normal wool blankets, and the other group (n = 12) was treated with resistive heating blankets during transport. We recorded core temperature, shivering, skin temperature at the forearm and finger, SpO2, and hemodynamic variables. Before randomization, both groups were comparable. On arrival at the hospital, the actively warmed patients had significantly warmer core (36.1 ± 0.3°C versus 35.5 ± 0.3°C; P < 0.001) and skin (34.1 ± 1.5°C versus 24.9 ± 1.4°C; P < 0.001) temperatures. In the actively warmed group, the pulse oximeter had significantly fewer alerts (31 versus 58) and a significantly less time of malfunction (146 ± 42 s versus 420 ± 256 s) and provided more constant measurements in the actively warmed group (P < 0.001). In this study we showed that active warming improves pulse oximeter monitoring quality in trauma patients during transport to the hospital.

IMPLICATIONS: Clinical trials show that pulse oximeter signal quality is limited by hypothermia. In this study we show that active whole-body warming of trauma victims improves monitoring quality during transport to the hospital.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Continuous monitoring of arterial oxygen saturation (SpO2) with a pulse oximeter has become a standard in prehospital emergency medicine in the last 10 yr. Pulse oximeters provide instantaneous, noninvasive, in vivo measurement of arterial oxygenation by determining the absorption of light of different wavelengths in the blood. Consequently, pulse oximeters require adequate plethysmographic pulsations to allow them to distinguish arterial blood light absorption from background venous blood and tissue light absorption. Because the technique is simple and often used, it is important to examine circumstances wherein its reliability may be questioned. Several studies in a clinical setting show that hypothermia and vasoconstriction critically impair peripheral perfusion and thus plethysmographic pulsation and the portion of total signal used to detect hemoglobin saturation (1). In addition to patient temperature-related problems, technical problems may also occur because cool ambient temperatures. Pulse oximeter sensors contain two light-emitting diodes (LEDs), which typically emit red and infrared light (2). The peak wavelength of the emitted radiation from a LED changes with the ambient temperature. Consequently, studies suggest that accuracy problems may occur in a cold setting (3).

Body temperature is normally highly regulated (4). Hypothermia can nonetheless result after extreme or prolonged environmental exposure (5) or when thermoregulatory defenses are impaired by drugs, especially alcohol. Injuries are often associated with natural disasters or sports activity in remote and cold locations. Even minor trauma in urban regions is associated with prolonged exposure to cold environments and is also frequently associated with alcohol intoxication (5). We recently evaluated the incidence and severity of hypothermia in patients with minor trauma in an urban setting, finding that 72% of urban trauma victims experienced hypothermia, defined as a core temperature <35.9°C (5).

There are numerous options for treating hypothermia victims once they reach a hospital (6). However, methods for treating victims of hypothermia during transport are limited. The recent development of a resistive heating system based on carbon fiber offers a convenient method of actively treating hypothermia in the field and during transport (5).

The carbon-fiber heating blanket is a new product (ThermaMed, Bad Oeynhausen, Germany). The entire cover measures 80 x 200 cm, with the actively heated section being 40 x 148 cm. Resistive heating is provided by passing a 7- to 8-ampere current through the carbon fiber. The batteries weigh 0.5 kg, with each lasting 30–40 min.

From our recent studies we know that this warming system effectively reduced hypothermia and that it reduced shivering, pain and fear; we therefore hypothesized that it might also increase the reliability of pulse oximetry under prehospital transport conditions. We designed this study to test the hypothesis that active warming produces vasodilatation of the arms and fingers, resulting in fewer technical alerts of pulse oximetry because of bad signal quality in trauma patients.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
This study was conducted with approval of the Ethics Committee of the Research Institute of the Vienna Red Cross. It was conducted in December 2000 and January 2001. Written consent was waived, but a verbal consent was obtained from the participating patients. We enrolled 24 patients older than 19 yr.

The Austrian Red Cross is divided into two parallel rescue systems. Minor trauma and illnesses are handled by paramedics who are permitted neither to give either any drugs or fluids nor to perform any invasive procedures. Patients with more serious injuries or illnesses are transported to a hospital by ambulance with a physician in attendance. In this study, all patients were transported by paramedics and thus had minor trauma, e.g., small fractures or contusions. Patients were excluded if they were not fully conscious or if they could not easily communicate with the paramedics.

At the injury scene, one investigator determined whether the patients were suitable for this study and obtained verbal and written documented consent for participation. Patients in both groups were covered with a carbon-fiber electric heating blanket that was itself covered by a single wool blanket.

Patients were randomly assigned to two groups: passive warming or resistive heating. Randomization was based on computer-generated codes that were sealed in sequentially numbered opaque envelopes. The blankets were set to 42°C in the patients assigned to active warming. In contrast, the blanket electrical system was not activated in patients assigned to passive warming. The electrical system was set by an unblinded investigator on the basis of the randomization, and the control unit was subsequently locked in a metal box. Once positioned, the covers were left in place until the patients arrived at the hospital. The designated treatment began at the accident site even before the patients were transferred into the ambulance.

Routine care was administered, which included bandaging injuries and splinting fractures. Every patient was monitored with a tympanic thermocouple (Mallinckrodt Anesthesiology Products, Inc., St. Louis, MO) and a noninvasive infrared-based pulse oximeter (8500A; Nonin, Plymouth, MN).

Patients then were transported to the hospital, and the time of transport was recorded. The patient’s choice of hospital was seriously considered because the cases were nonurgent; consequently, they were not necessarily taken to the nearest facility.

Morphometric characteristics and the type of injury were recorded. Measurements were performed on entering the ambulance and again on arrival at the destination hospital. The first set of measurements was made before opening the randomization envelope. All measurements were recorded by the same independent investigator (AK), who was blinded to the randomization and whether the carbon-fiber blanket electrical system had been activated. He was not permitted to touch the patient or blanket, but he did talk with the patient during the rescue and transport.

Measurements included oscillometric blood pressure and heart rate, and we also monitored patients’ shivering. Air temperature was recorded from a thermocouple positioned at the level of the patient’s head. Core temperatures were recorded from the tympanic membrane, a site that correlates well with distal esophageal and pulmonary artery temperatures even during the extreme thermal perturbations of cardiopulmonary bypass.

As in previous studies, the aural probe was inserted until the patients felt the thermocouple touch the tympanic membrane; appropriate placement was confirmed when they easily detected a gentle rubbing of the attached wire. The aural canal was occluded with cotton, the probe securely taped in place, and a gauze bandage positioned over the external ear. The first reading was performed at least 5 min after the tympanic membrane probe was inserted. Temperatures were recorded from Mon-a-Therm digital thermometers (Mallinckrodt Anesthesiology Products, Inc.), which have an accuracy and precision near 0.1°C. To determine skin temperatures, we used an infrared-based temperature-monitoring device. The independent investigator counted the alerts of a motion-resistant pulse oximeter (Nonin 8500A) during transport. Alerts were defined as "bad signal quality," which means that the SpO2 value was questionable, and "total malfunction," which means that the device did not show any SpO2 value. All time intervals of malfunction were recorded.

Tympanic membrane temperatures <36°C were considered hypothermic. Normally distributed, continuous data were compared with two-tailed, unpaired Student’s t-tests. Nonparametric, continuous data were compared by use of Mann-Whitney U-tests. Data are presented as mean ± SD; P < 0.05 was considered statistically significant. Tympanic membrane temperatures were considered to be core values. Unless otherwise specified, all temperatures are reported as tympanic membrane values.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
An internal audit confirmed that patients were properly assigned to passive warming or active heating. None of the enrolled patients dropped out of the study. Our analysis is thus based on 12 patients in each treatment group.

Hypothermia (defined as tympanic membrane temperature <36°C) was observed in all patients at the time of rescue; the mean ± SD temperature was 35.6°C ± 0.3°C. One patient had core temperatures <35°C (34.8°C).

Patient characteristics in each treatment group were similar. None of the potential confounding factors differed significantly in the two groups. The duration of transport averaged roughly 30 min in each group (Table 1). Shivering was comparable in both groups (Table 2)


View this table:
[in this window]
[in a new window]
  		Table 1. Demographic and Morphometric Characteristics and Potential Confounding Factors
 

View this table:
[in this window]
[in a new window]
  		Table 2. Table 2.
 
The initial average core temperature was 35.6°C ± 0.3°C in all patients. From the time of rescue until arrival at the hospital, core temperature in all of the passively warmed patients remained at 35.5°C ± 0.3°C. In contrast, temperature increased in most actively warmed patients to approximately 36.1°C ± 0.3°C. One patient out of the 12 actively warmed patients remained hypothermic, whereas none of the patients assigned to passive warming reached the hospital normothermic (Fig. 1).



View larger version (23K):
[in this window]
[in a new window]
  		Figure 1. Core temperature profiles of the individual patients in both groups. {blacksquare} indicates mean; *P < 0.001.

 
Before randomization, all patients in both groups had a peripheral temperature difference of 4.4°C ± 1.6°C, which means colder fingers than forearms. On arrival at the hospital, all patients in the actively warmed group showed an increased finger temperature, resulting in a decreased difference of -1.2°C ± 0.6°C. The difference in the passively warmed patients increased slightly, to 5.1°C ± 1.3°C. Similar results were found when comparing core temperature and forearm temperature and when comparing core and finger temperatures (Table 2, Figs. 1 and 2). Patients passively warmed showed statistically significantly higher SpO2 (98.5% versus 96.5%) when they arrived at the hospital, but this was without clinical relevance. The lowest observed SpO2 was 94%. Consequently, no patient was given oxygen to avoid hypoxemia. In the group of passively warmed patients, 58 alerts ("total malfunction" and "bad signal quality") were detected, versus 38 in those assigned to active warming (P < 0.04). The oximeter was working properly 77% of the time during transport in the passively warmed group versus 92% in the active warming group (P < 0.002). Consequently, the absolute and relative duration of failures was significantly higher (P < 0.001) in the passive warming group compared with the actively warmed patients (Table 2).



View larger version (23K):
[in this window]
[in a new window]
  		Figure 2. Finger temperature profiles of the individual patients in both groups. {blacksquare} indicates mean; *P < 0.001.

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Many studies question the reliability of pulse oximetry, mostly in a perioperative or intensive care unit setting (7–10). In these studies, hypothermia was one major limitation to the quality of noninvasive saturation monitoring. Reasons for this include vasoconstriction and nonoptimal function of the sensor LEDs in a cold environment. There are no published data on hypothermia and pulse oximetry in the prehospital setting. We knew from our daily routine that pulse oximetry often fails when the patient is hypothermic. This failure poses a risk for trauma patients because many of them are intoxicated with alcohol or other drugs and thus are at risk of hypoventilation and hypoxemia.

This study had the following findings: First, we showed that active warming during prehospital emergency transport leads to increased peripheral temperatures, which we demonstrated by recording warmer fingers than forearms in the actively warmed group. Second, we showed that this rewarming increases the reliability of oximetry under the difficult environmental conditions of prehospital emergency care in trauma victims. Our data fit well with results of clinical studies that investigated the signal quality of pulse oximeters: vasodilatation is an effect of local warming (11–14). Local warming leads to a relaxation of vasomotor muscles and thus increases peripheral blood flow. Victims of trauma are known to be massively vasoconstricted because of pain (15), fear (16), sympathetic reactions, and an increase of catecholamines in their circulation (17,18). Additionally, they are often exposed to a cold and humid environment. All of our patients showed evidence of peripheral vasoconstriction when we approached them before randomization. Known effects of the reduction of tissue perfusion near a peripheral wound include a decrease of subcutaneous oxygen tension and thus an increased risk of wound infection (19,20). From our own published data, we knew that active warming during emergency transport leads to a significant reduction of fear and pain (5). This reduction of stress compared with the local application of warmth apparently leads to effective vasodilation even under difficult transport conditions. Active warming of trauma victims on transport to hospital is the first nonpharmacologic approach to increased tissue perfusion in a prehospital setting. Referring to recently published clinical data, this might be an effective way to reduce the infection rate after rescue. Further prospective research in this field is needed.

The patients in the passively warmed group showed significantly higher SpO2, but the difference does not seem to have any clinical relevance (Table 2), and although it was statistically significant, there were no therapeutic consequences. However, it is a known phenomenon from clinical and experimental trials that warming of the LED of the pulse oximeter leads to a change in the emitted light wavelength, and consequently higher values are shown on the display (3). Compared with blood gas analysis, warmed diodes show results that correspond better with the SpO2 measured from the blood sample than cold diodes. In our case, we could not perform such a comparison, because paramedics in Austria are not allowed to take blood samples.

The main focus of our interest in this trial was the reliability of pulse oximetry under the difficult circumstances of prehospital rescue. Our data show that active warming on transport significantly reduces the number of alerts and the duration of malfunction of the pulse oximeter. Paramedics and emergency physicians who are actively involved in prehospital rescue know from their daily routine that frequent alerts of the pulse oximeter have negative effects on the quality of care for our patients. Frequent "malfunction" alerts might distract attention away from the patient to the monitor, which could, in turn, lead to misinterpretation and underestimation of the importance of these alerts. Consequently, during daily routine there could be a risk of underestimating a potentially dangerous situation, such as hypoxemia. This risk is increased when the patient is not transported by a physician, but by paramedics. The authors of this study are convinced that pulse oximetry is an important factor for the security of our patients and should be used in as many cases as possible. Consequently, we suggest that our system of active warming will make pulse oximeters more reliable, more practical, better accepted by paramedics, and more secure for the patient during transport.


   Acknowledgments
 
Supported by a unrestricted grant of ThermaMed, GmbH (Bad Oeynhausen, Germany). Mallinckrodt Anesthesiology Products, Inc. (St. Louis, MO) donated the thermocouples.

We would like to thank the Research Institute of the Vienna Red Cross (Vienna, Austria) for making it possible for us to conduct the study.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

  1. Lawson D, Norley I, Korbon G, et al. Blood flow limits and pulse oximeter signal detection. Anesthesiology 1987; 67: 599–603.[ISI][Medline]
  2. Wukitsch MW, Petterson MT, Tobler DR, Pologe JA. Pulse oximetry: analysis of theory, technology, and practice. J Clin Monit 1988; 4: 290–301.[ISI][Medline]
  3. Falconer RJ, Robinson BJ. Comparison of pulse oximeters: accuracy at low arterial pressure in volunteers. Br J Anaesth 1990; 65: 552–7.[Abstract/Free Full Text]
  4. Sessler DI. Perioperative heat balance. Anesthesiology 2000; 92: 578–96.[ISI][Medline]
  5. Kober A, Scheck T, Fulesdi B, et al. Effectiveness of resistive heating compared with passive warming in treating hypothermia associated with minor trauma: a randomized trial. Mayo Clin Proc 2001; 76: 369–75.[ISI][Medline]
  6. Bernardo LM, Gardner MJ, Lucke J, Ford H. The effects of core and peripheral warming methods on temperature and physiologic variables in injured children. Pediatr Emerg Care 2001; 17: 138–42.[ISI][Medline]
  7. Silverston P. Pulse oximetry at the roadside: a study of pulse oximetry in immediate care. BMJ 1989; 298: 711–3.
  8. Trivedi NS, Ghouri AF, Lai E, et al. Pulse oximeter performance during desaturation and resaturation: a comparison of seven models. J Clin Anesth 1997; 9: 184–8.[ISI][Medline]
  9. Trivedi NS, Ghouri AF, Shah NK, et al. Effects of motion, ambient light, and hypoperfusion on pulse oximeter function. J Clin Anesth 1997; 9: 179–83.[ISI][Medline]
  10. Barker SJ, Shah NK. The effects of motion on the performance of pulse oximeters in volunteers. Anesthesiology 1997; 86: 101–8.[ISI][Medline]
  11. Pergola PE, Kellogg DL Jr, Johnson JM, Kosiba WA. Reflex control of active cutaneous vasodilation by skin temperature in humans. Am J Physiol 1994; 266: H1979–84.[Abstract/Free Full Text]
  12. Padilla J, Garcia-Villalon AL, Fernandez N, et al. Effects of hyperthermia on contraction and dilatation of rabbit femoral arteries. J Appl Physiol 1998; 85: 2205–12.[Abstract/Free Full Text]
  13. Saumet JL, Abraham P, Jardel A. Cutaneous vasodilation induced by local warming, sodium nitroprusside, and bretylium iontophoresis on the hand. Microvasc Res 1998; 56: 212–7.[ISI][Medline]
  14. Dollberg S, Atherton H, Hoath S. Changes in skin blood flow over the foot with warming of the contralateral heel. Acta Paediatr 1998; 87: 416–8.[ISI][Medline]
  15. Blessing WW, Nalivaiko E. Regional blood flow and nociceptive stimuli in rabbits: patterning by medullary raphe, not ventrolateral medulla. J Physiol 2000; 524 (Pt 1):279–92.[Abstract/Free Full Text]
  16. Hare RD, Blevings G. Defensive responses to phobic stimuli. Biol Psychol 1975; 3: 1–13.[ISI][Medline]
  17. Burnstock G. Local mechanisms of blood flow control by perivascular nerves and endothelium. J Hypertens Suppl 1990; 8: S95–106.[Medline]
  18. Heneghan JB. Response of germfree animals to shock. J Med 1990; 21: 51–66.[ISI][Medline]
  19. Greif R, Akca O, Horn EP, et al. Supplemental perioperative oxygen to reduce the incidence of surgical-wound infection: Outcomes Research Group. N Engl J Med 2000; 342: 161–7.[Abstract/Free Full Text]
  20. Akca O, Melischek M, Scheck T, et al. Postoperative pain and subcutaneous oxygen tension. Lancet 1999; 354: 41–2.[Medline]
Accepted for publication May 24, 2002.




This article has been cited by other articles:


Home page
 Anesth. Analg.Home page
A. Kober, B. Schubert, P. Bertalanffy, L. Gorove, T. Puskas, B. Gustorff, A. Joldzo, and K. Hoerauf
Capnography in Non-Tracheally Intubated Emergency Patients as an Additional Tool in Pulse Oximetry for Prehospital Monitoring of Respiration
Anesth. Analg., January 1, 2004; 98(1): 206 - 210.
[Abstract] [Full Text] [PDF]

Home page
 Anesth. Analg.Home page
C. S. Weenig, A. Kober, and K. H. Hoerauf
Improving Pulse Oximetry Signal Quality * Response
Anesth. Analg., April 1, 2003; 96(4): 1240 - 1240.
[Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a colleague
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kober, A.
Right arrow Articles by Hoerauf, K.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Kober, A.
Right arrow Articles by Hoerauf, K.
Related Collections
Right arrow Trauma
Right arrow Equipment

Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press