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*Vienna Red Cross, Van Swieten and the Research Institute of the Vienna Red Cross, Vienna, Austria;
Hungarian National Emergency Service, Hungary; and the
Department of Anesthesiology and General Intensive Care, University Hospital of Vienna, Vienna, Austria
Address correspondence and reprint requests to Dr. Klaus Hoerauf, Department of Anesthesiology and General Intensive Care, University Hospital of Vienna, Waehringer Guertel 18–20, A-1090 Vienna, Austria, Europe. Address email to klaus.hoerauf{at}univie.ac.at
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Abstract |
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IMPLICATIONS: Capnography is a useful tool to improve respiratory monitoring in nonintubated trauma victims on emergency transport and an easy to use supplement to pulse oximetry.
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Introduction |
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The continuous monitoring of oxygen saturation in emergency patients is especially important, as many of these patients suffer from various coexisting medical and surgical problems that may lead to respiratory depression.
Because pulse oximeters provide instantaneous, noninvasive, in vivo measurement of arterial oxygenation by determining the absorption light of different wavelengths of the blood, pulse oximetry has rapidly become a standard in prehospital emergency medicine in the last 10 yr.
Pulse oximeters require adequate plethysmographic pulsations to distinguish arterial blood light absorption from background venous blood and tissue light absorption (3). Studies in clinical and ambulance settings have demonstrated that hypothermia and vasoconstriction might critically impair peripheral perfusion and thus plethysmographic pulsation and the portion of total signal used to detect hemoglobin saturation (4). Another problem in reading accuracy is associated with the conditions of prehospital emergency care, especially the rapid car movements during rescue transport (5,6). Recently, a volunteer study demonstrated a wide difference in pulse oximeter performance under motion conditions (7). In a clinical study, we could show that time of malfunction was more than 20% of the total transport time, even though the pulse oximeter used was classified as motion-resistant (8).
Another option for continuous monitoring would be an online recording of exhaled CO2. Exhaled air is usually sampled by means of an infrared analyzer. Although this method is accurate in most clinical settings, cardiovascular abnormalities (e.g., congenital heart diseases) and technical sampling malfunctions (e.g., line obstructions) are known potential error sources (9). However, determination of CO2 is standard for ventilated patients (10), and some efforts have been made to monitor spontaneously breathing patients in clinical settings (9,11,12). Currently, there are no data regarding the prehospital monitoring of nonintubated emergency patients by noninvasive capnography.
OxyArmTM represents a new technology with the appearance of a telephone headset, delivering oxygen without the constraints of a conventional oxygen delivery device (Fig. 1) (13,14). A new feature of a newly redesigned Oxyarm device is the possibility to perform end-expiratory capnography in an open system with spontaneously breathing patients. Exhaled CO2 can therefore be measured by a portable infrared analyzer attached to the Oxyarm, as demonstrated recently by Paul et al. (15).
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Therefore, we designed this study to test the hypothesis that ETCO2measured through Oxyarm would give us additional information regarding the frequency of respiration. Second, it was our aim to show that the new Oxyarm device is easy for paramedics to handle and does not impair patients’ comfort. Finally, we wanted to find out if supplemental monitoring with capnography reduces periods of interruption in emergency monitoring.
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Methods |
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At the injury scene, one investigator who was not involved in the study as an author of the publication determined whether the patients were suitable for this study and obtained consent for participation. Routine care, such as bandaging of injuries and of splinting fractures, was administered. Every patient received a pulse oximeter (8500A; Nonin, Plymouth, MN) on the second finger of his/her right hand and an Oxyarm device (Oxyarm CapnoPlus; Southmedic, Barrie, ON) on his/her head. The Oxyarm was connected to an oxygen delivery system (4 L oxygen per minute) and to a capnography device (NPB-75; Tyco Healthcare, Vienna, Austria). With the capnometer we did not measure exact values of ETCO2. It was used only to get information about the frequency of respiration.
As previous studies demonstrated that the quality of pulse oximeter readings was dependent on patients’ temperature, tympanic temperatures were recorded from Mon-a-Therm digital thermometers (Mallinckrodt Anesthesiology Products, St. Louis, MO). Another thermocouple positioned nearby the patient’s head recorded the ambient car temperature. Skin sensors (Mallinckrodt Anesthesiology Products) were placed on the forearm and the finger to register indirect signs of vasodilation (finger warmer than forearm) or vasoconstriction (forearm warmer than finger).
Morphometric characteristics and the type of injury were recorded. Noninvasive blood pressure and heart rate measurements were performed on entering the ambulance and again on arrival at the destination hospital. All measurements were recorded by the same independent investigator.
During transport, the number and duration of the alerts of the pulse oximeter and of the capnometer were recorded. An alert was defined as a period of time where the monitoring device could not show any value resulting in the "technical error" alert sound of the device for more than 15 s. All alerts were recorded automatically by an IBM-compatible laptop via serial port and directly transferred in a datasheet file (Excel XP; Microsoft, Redmond, WA). The recorded alert data could not be seen by the paramedics as their information was password saved. The extraction of the file was performed by an investigator not otherwise involved in the trial.
Patients were asked to rate their satisfaction with the Oxyarm device and with the finger clip on a 100 mm VAS (0 = maximum comfort and 100 = maximum discomfort). Moreover, the patients were asked to use the same VAS to rate their subjective feeling of oppression by the device. Finally, the patients were asked if they thought that providing additional oxygen supply and monitoring simultaneously in one device, as done by the Oxyarm, was positive for them. This question was answered "yes" or "no".
The paramedics themselves rated the practicability of the new device on the same subjective score.
We calculated that we needed 70 patients to detect a 25% difference in time of malfunction with a common standard deviation of the twofold difference at the P < 0.05 level and at least 80% power. Normally distributed, continuous data were compared with analysis of variance, and nonparametric data were compared by the use of Mann-Whitney U-test. Data are presented as mean ± SD; P < 0.05 was considered statistically significant.
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Results |
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The total number of alerts, alerts per patient, total time of malfunction, time of malfunction per alarm, and the percentage of malfunction time during transport were significantly (P < 0.01) less with capnography compared with oximetry (Table 2).
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Discussion |
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Clinical trials of respiratory monitoring researched many benefits of capnography in nonintubated patients, as it provides information about each breath; under relatively stable conditions there is a correlation of ETCO2 measurements to arterial PaCO2 in nonintubated patients (19). Even if this relation can be questioned in trauma rescue, capnography gives additional information on the patient’s respiratory status. If the patient stops breathing, the ETCO2 concentration decreases rapidly to the baseline level (20). This allows a rapid detection of respiratory failure on transport even before SpO2 decreases.
Consequently, capnography increases the safety margins in prehospital care (21,22). Usually, pharyngeal catheters are necessary to obtain reliable results (23,24). The major advantage of the Oxyarm used in the present study was that it provided the option of noninvasive ETCO2 online measurement to detect respiratory frequency, while providing simultaneous oxygen delivery.
This study shows three major findings: first, we showed that this technique using the Oxyarm is well accepted by the patients and easy to use by paramedics. Patients feel no oppression by the device and have a positive subjective feeling with the combination of oxygen inhalation and monitoring. Second, measuring respiratory frequency was possible in all patients. Finally, from a technical point of view capnography is a more reliable measurement than pulse oxymetry in the emergency setting, with many fewer false alerts, and guarantees constant monitoring with almost no interruptions.
Our data fit in well with results of clinical studies investigating capnography in spontaneously breathing patients. These studies showed that capnography can be used in the hospital setting to monitor the adequacy of spontaneous ventilation not only during general anesthesia and recovery but also in the awake unintubated patient, either in the intensive care unit or during regional anesthesia. ETCO2 monitoring can serve as an apnea monitor. However, the major limiting factor is the admixture of end-tidal gas with air or insufflated oxygen, resulting in a falsely low PETCO2, particularly in patients breathing through the mouth (9,11,12,20,23,25–28).
However, the main focus in this trial was to determine the practicability of capnography under the difficult circumstances of prehospital rescue. Our data show that this technique on transport reduces the number of monitoring-free periods in a significant and clinically important way. Paramedics and emergency physicians actively involved in prehospital rescue are aware that frequent alerts of the pulse oximeter have certain negative effects on the quality of daily patient care. Frequent "malfunction" alerts might affect concentration, drawing the paramedic’s attention away from the clinical aspect to the monitor. This might lead to misinterpretation and underestimation of the importance of these alerts. Consequently, there might be a risk of underestimation of a potentially dangerous situation such as hypoxemia. This risk is further increased when a patient is not transported by a physician but by paramedics, who might not be as quick to detect hypoxemia from clinical signs.
We are convinced that pulse oximetry is an important addition to the security of our patients and should be used in as many cases as possible; however, because of its limitations in difficult surroundings, it should be supplemented by noninvasive capnography to reduce monitoring-free periods to a minimum.
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