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TECHNOLOGY, COMPUTING, AND SIMULATION

The Influence of Stellate Ganglion Transcutaneous Electrical Nerve Stimulation on Signal Quality of Pulse Oximetry in Prehospital Trauma Care

From the Department of Anesthesia and General Intensive Care, Medical University of Vienna, Vienna, Austria.

Address correspondence to Dr. Thomas Lang, Department of Anesthesiology and General Intensive Care, Medical University Hospital of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria, Europe. Address e-mail to thomas.lang{at}meduniwien.ac.at.

	Abstract

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BACKGROUND: Accurate monitoring of the peripheral arterial oxygen saturation has become an important tool in the prehospital emergency medicine. This monitoring requires an adequate plethysmographic pulsation. Signal quality is diminished by cold ambient temperature due to vasoconstriction. Blockade of the stellate ganglion can improve peripheral vascular perfusion and can be achieved by direct injection or transcutaneous electrical nerve stimulation (TENS) stimulation. We evaluated whether TENS on the stellate ganglion would reduce vasoconstriction and thereby improve signal detection quality of peripheral pulse oximetry.

METHODS: In our study, 53 patients with minor trauma who required transport to the hospital were enrolled. We recorded vital signs, including core and skin temperature before and after transport to the hospital. Pulse oximetry sensors were attached to the patient’s second finger on both hands. TENS of the stellate ganglion was started on one side after the beginning of the transport. Pulse oximeter alerts, due to poor signal detection, were recorded for each side separately.

RESULTS: On the hand treated with TENS we detected a significant reduction of alerts compared to the other side (mean alerts TENS 3.1 [1–15] versus control side 8.8 [1–28] P < 0.05). The duration of dropouts was shorter as well (mean duration TENS 77 [16–239] s versus control side 333 [78–1002] s).

CONCLUSION: The data indicate that blockade of the stellate ganglion with TENS improves signal quality of pulse oximeters in the prehospital setting.

	Introduction

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Continuous monitoring of peripheral arterial oxygen saturation (Spo₂) with a pulse oximeter has become an important tool in prehospital emergency medicine. Pulse oximetry is widely used to monitor Spo₂, although its reliability might be compromised under certain circumstances. Several studies performed in the clinical setting have shown that hypothermia and vasoconstriction critically impair peripheral perfusion, and thus diminish plethysmographic pulsation (1–4). We (5) evaluated the incidence and severity of hypothermia in patients with minor trauma in an urban setting. The results show that nearly 75% of trauma victims in this setting suffer from hypothermia, defined as a core temperature below 35.9°C. The principal mechanism of vasoconstriction in the upper extremity in the presence of mild hypothermia is sympathetic activity mediated by the stellate ganglion of the upper paravertebral sympathetic chain.

The stellate ganglion can be selectively blocked to temporarily eliminate unilateral sympathetic innervation of the head, neck, and upper extremity. Stellate ganglion blockade improves bloodflow by up to 50% in patients with peripheral vascular disease (6). Dilation of the arteries increases the skin temperature in the upper extremity on the side of the blockade (7–9). Transcutaneous electrical nerve stimulation (TENS) can be applied to the stellate ganglion in order to reduce sympathetic outflow (9). We hypothesized that blockade of the stellate ganglion using TENS may be an effective means of improving the quality of monitoring under prehospital transport conditions.

	METHODS

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The study was approved by the local IRB. Verbal and written consent was obtained from the patients at the scene of injury.

In this prospective clinical trial, we compared the quality and reliability of Spo₂ with and without stellate ganglion blockade under out-of-hospital emergency medicine transport conditions. Inclusion criteria were minor trauma, ambulance staffed only by paramedics, and an anticipated transportation time of >20 min from the scene of injury to the hospital. All paramedics participating in the study were specifically trained in the application of the TENS stimulator, pulse oximeter, and the data acquisition equipment.

Exclusion criteria were impaired consciousness (Glasgow coma scale <15), inability to communicate with the paramedic because of insufficient language skills, and an anticipated transportation time of <20 min.

At the scene of injury, the investigator established the patient’s eligibility for the study. After appropriate care had been administered, the investigator asked the patients whether they were willing to participate in a clinical trial. The patient’s verbal and written consent was obtained and documented. The patient’s core temperature was monitored with a tympanic thermocouple (Mallinckrodt Anesthesiology Products, St. Louis, MO). On the second finger of both hands we placed a pulse oximetry sensor featuring motion resistance technology (Nonin 8500A, Plymouth, MN). On one side of the patient’s neck (the side was determined on a random basis using nontransparent sealed envelopes), at the location of the stellate ganglion, we positioned two adhesive 25 mm x 25 mm electrodes to administer the TENS treatment, while the other side remained untreated. Of the two adhesive electrodes, one was placed near to the paravertebral location in the region of the trapezius muscle on its base near the sixth cervical transverse process, and the other in the region of the stellate ganglion in the supraclavicular aspect (Fig. 1). To obtain a stellate ganglion block, we used a dual-channel TENS stimulator (TENStem eco®, Pierenkemper GMBH, Ehringhausen, Germany). This portable device weighs 170 g, measures 114 mm x 59 mm x 27 mm, and is powered by a commercially available 9 V battery (technical data: output voltage, 70 mA; frequency range, 0.5–120 Hz; pulse width, 60–300 µs; power input, 15 mA). The device uses the software TENStem eco Program 1. It applies impulses generated by two channels with a frequency of 100 Hz and a pulse width of 200 µs. Patients were transported to the hospital with the TENS device activated.

View larger version (9K): [in this window] [in a new window]   Figure 1. The graphics in (a) and (b) show the two adhesive electrodes placed in correct position on the patient’s skin.

Morphometric characteristics (gender, weight, height) and the type of injury were recorded. All measurements were performed by the same investigator, and included oscillometric arterial blood pressure and heart rate.

Air temperature was recorded from a thermocouple positioned at the level of the patient’s head. Core temperatures were recorded from the tympanic membrane, which has been shown to correlate strongly with the distal esophageal and pulmonary artery temperatures (10,11). The aural probe was inserted until the patient felt the thermocouple touch the tympanic membrane. Appropriate placement was confirmed when the patient could easily perceive a gentle rubbing of the attached wire. The auditory canal was occluded with cotton, the probe securely taped in place, and a gauze bandage fixed above the ear. The first reading was performed after the tympanic membrane probe had been inserted for at least 5 min. Temperatures were recorded from Mon-a-Therm digital thermometers (Mallinckrodt Anesthesiology Products), whose accuracy and precision are nearly 0.1°C. Skin temperatures were recorded with Mon-A-Therm® Skin Temperature Probes (Nellcor, Temperature Management, TYCO-Healthcare, St. Louis, MO) attached to the forearm and the third finger on each side of the patient. As described in previous studies, we recorded temperature differences between the forearm and the fingertip (12–17). Differences of more than 4°C were interpreted as an indirect sign of vasoconstriction. All measurements were made while the patient was transported to the hospital. Dropout alerts of the pulse oximeters, defined as an alarm due to poor signal detection, were also recorded during transport. The alerts from the two pulse oximeters on both arms were recorded separately. The investigator pushed predefined keys on a notebook computer for loss and regaining of signal in each of the devices. Data were exported into excel-format and reorganized to a resolution of 1 s for analysis. A proprietary software that was capable of recording certain keystrokes over time in a resolution of a tenth of seconds had been written by one of our technicians. Only alerts longer than 30 s were considered to be of clinical significance. Shorter alerts were counted but not used for the analysis. Once the patients had arrived at the hospital, the measurements were stopped.

A medical expert who was not involved in the study ensured that the monitoring equipment (pulse oximeters, temperature probes, and TENS) was properly connected to the patients.

In the a priori study plan for a per-protocol analysis, our intention was to detect a 30% reduction in alarms with a common standard deviation equal to the difference at the P = 0.05 level with a power of 85% using an ANOVA. The calculated sample size required 20 patients treated with TENS on one side. Considering the uncertain dropout rate, we decided to screen at least 50 patients. A two-way repeated measures ANOVA was used to test for a trial effect and a group effect (SPSS 11 for Macintosh). Data were presented as means ± sd; P < 0.05 was considered statistically significant.

	RESULTS

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Fifty-three patients were enrolled in, and completed the study. Demographic data are shown in Table 1. All patients had a Glasgow coma score of 15. Twenty-one patients were men and 32 were women. Thirty percent had an ASA physical status of 2, 70% an ASA physical status of 3.

Hypothermia (defined as tympanic membrane temperature <36°C) was observed in 50 of 53 patients (94.3%) at the time of rescue as well as upon arrival at the hospital (50 of 53; 94.3%). The mean ± sd temperature was 35.6 ± 0.3°C at the time of rescue and 35.6 ± 1.5°C upon arrival at the hospital (P = 0.98; see Table 2).

Peripheral vasoconstriction (defined as a difference of 4°C in skin temperature between the ipsilateral finger and the forearm) was present at the time of initial rescue in 50 of the 53 (94.3%) patients, both in the non-TENS and the TENS upper limbs. Of these 50 patients, peripheral vasoconstriction persisted at the time of arrival in the hospital in all of the non-TENS upper limbs. In contrast, only eight of the TENS-treated upper limbs showed persistent peripheral vasoconstriction (P < 0.001).

Demographic characteristics are shown in Table 1. No patient had signs of hemodynamic instability. The mean duration of transport was 24 ± 3.8 min. Shivering was observed in all patients.

The median number of alerts in the limbs not treated with TENS was 8.8 [1–28] and the median duration, 333 [78–1002] s. The median number of alerts in the limbs treated with TENS was 3.1 [1–15] and the median duration, 77 [16–239] s. TENS significantly reduced the number as well as the duration of alerts (P < 0.05).

	DISCUSSION

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The purpose of this trial was to determine whether TENS stimulation of the stellate ganglion improves the reliability of pulse oximetry during prehospital transport. The reliability of pulse oximeters in the perioperative or intensive care unit setting and the difficulty obtaining reliable signals has been discussed in several studies (3,4,18,19). Hypothermia proved to be a major factor limiting the quality of noninvasive Spo₂ monitoring (20). No data concerning the hypothesized relationship between hypothermia and the quality of pulse oximetry in the prehospital setting have been reported thus far. The study demonstrated that TENS can produce a measurable change in the signal quality of the pulse oximeter, measured in terms of the number and the duration of alerts. However, our results may have been influenced by factors other than TENS treatment. The latter was administered by placing the electrodes on one side of the patient’s neck. The patient was told that a stimulator would transmit electric energy and that this might improve his/her condition as registered in the monitoring procedure. The placement of the electrodes might have caused a physiologic reaction that altered the quality of the pulse signal. As our measurements were started after activation of the TENS stimulator, it cannot be proven that the changes were due to electrical stimulation alone. To eliminate this weakness of the study design, it would have been necessary to place electrodes connected to one stimulator each on either side of the patient’s neck, and to use one stimulator for active treatment and a second for sham treatment. As we know from experience, the patient’s cooperation in a study is also dependent on the complexity of the setting. Being connected to several devices is more uncomfortable than being attached to one; we therefore refrained from the double-version.

A further weakness of the study design is the location of the electrodes. As stimulation of the stellate ganglion with TENS does not cause any mydriasis, there was, in fact, no proof that the electrodes were placed in the correct position. The position of the electrodes depended on the skill of the investigator, which introduced an element of bias.

After the data for this study had been collected, advancements were made in signal artifact rejection technology of pulse oximeters. It would be meaningful to compare the findings of the present study with those achieved in an equivalent setting using the new Massimo® pulse oximeter technology.

In this study, TENS applied to the stellate ganglion was effective for improving pulse oximetry monitoring in hypothermic vasoconstricted patients prior to hospital admission. This noninvasive technique might be applicable in similar situations, such as in hypothermic vasoconstricted patients being transported from the operating room to the intensive care unit after surgery.

	Footnotes

 
Accepted for publication January 25, 2007.

Support for the work was provided solely from the departmental sources (Medical University of Vienna).

Reprints will not be available from the author.

	REFERENCES

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