Question: How is StO₂ affected by hypothermia?

Note: Hypothermia is a core body temperature of 35°C (95°F) or less. Average body temperature is 37°C (98.6°F).^1,4,5

Summary Response:

Measurement of StO₂ is unaffected by hypothermia. That is, the InSpectra StO₂System reports an accurate value for StO₂ regardless of body temperature.

Patients with hypothermia may exhibit modestly decreased StO₂, but this does not interfere with the ability of StO₂ to distinguish hypoperfusion (manifested by mortality and MODS) in trauma patients.

StO₂ does better than:

• base deficit in identifying which hypothermic patients will develop MODS.
• systolic blood pressure in identifying which hypothermic patients will develop MODS or die.

Unlike pulse oximetry which depends on pulsatile flow to generate a measurement, StO₂ is measured in the microcirculation and can be determined despite diminished pulsatile flow during hypothermia and hypotension. A pulse oximeter reading may not be available in a hypothermic and/or hypotensive patient because of diminished pulsatile signal in the finger.

Background: HTI’s StO₂ Trauma Study

Among 359 severely injured patients, the mean (±SD) lowest body temperature within the first 6 hours after ED arrival was 35.0°C (±1.6), range = 21.0° – 38.3°C.³ In 48 patients who died or developed MODS and were hypothermic within 6 hours after ED arrival, hypothermia did not interfere with the ability of StO₂ to predict these adverse outcomes.

In contrast, the ability of base deficit and systolic blood pressure to predict MODS was diminished by hypothermia. This also was true for systolic blood pressure’s ability to predict death.^4,5

EU investigator-initiated study

In six healthy volunteers cooled from a core temperature of approximately 36.0° to 34.0°C for one hour and then rewarmed to baseline, StO₂ changed from 73% ±8 at baseline to 67% ±14 at the end of cooling and back to 74% ±7 at the end of rewarming.²

HTI’s porcine study

In eight porcine hind limbs (4 animals x 2 limbs), there was no effect on mean StO₂ with core temperature between 32.0° – 38.0°C. Mean ±SD StO₂ at 32.0°C was 67% ±5.2 and at 38.0°C, it was 71% ±5.9, 95% CI, p-value=0.31.⁶

References

Published

¹Martin RS, Kilgo PD, Miller PR, Hoth JJ, Meredith JW, Chang MC. Injury-associated hypothermia: an analysis of the 2004 National Trauma Data Bank. Shock. 2005 Aug:24(2):114-118.

²Ali SZ, Taniguchi Y, Zmoos S, Kurz A. Influence of mild hypothermia and rewarming on near infrared spectroscopy derived tissue oxygen saturation. Poster presentation at 2006 European Society of Anaesthesiology Abstract # A-837, Page 216 in meeting program.

Unpublished

³Final Report: An Evaluation of the InSpectra^TM Tissue Spectrometer during Traumatic Shock Resuscitation (IP# TS-0017).

⁴Nelson, T. The effect of hypothermia in StO₂ trauma study group. Internal analysis: August 16, 2006. (IP #TS-0017).

⁵Beilman G, Nelson T, Nathens A, Moore F, Rhee P, Puyana J, Moore E, Cohn S. Early Hypothermia in Severely Injured Trauma Patients is a Significant Risk Factor for Multiple Organ Dysfunction Syndrome but not Mortality. Abstract: December 14, 2006. Submitted to ISICEM. (IP #TS-0017).

⁶Myers D Hutchinson Technology unpublished data 2006.