Improvised Method for Increasing the Temperature of an i-STAT Analyzer and Cartridge in Cold Environments

To the Editor:

The i-STAT point-of-care analyzers (Abbott, East Windsor, NJ, USA) have become increasingly common for clinical care and research in ultramarathons. ¹ However, there are implicit limitations when one attempts to implement a high-technology diagnostic tool in the low-technology wilderness settings where these events are commonly held, as there is a narrow temperature operating range of 16° to 30°C (61° to 86°F). Like hot weather, ² cold conditions can adversely affect the performance of the i-STAT analyzer and introduce unique challenges with not only the analyzer but also the cartridges and subject from whom blood is drawn.

During a recent ultraendurance research study in September at Bear Valley, CA (18.4° N, 120° W; 2018 m [6621 feet]), the nighttime temperature dropped to 2°C (35°F) from 2:00 to 4:30 am. When we attempted to collect blood samples, the analyzer displayed a message that indicated the i-STAT was recording an ambient temperature outside its operating range. To warm the analyzer, a 2-L hydration bladder was filled with near-boiling water, wrapped in a towel, and placed in the bottom of a 7.0-quart cooler. A wool shirt was rolled into the shape of a cradle to bolster and insulate the analyzer (Figure 1). Initially, the cooler lid was left open to prevent condensation and subsequent heating over its operating range, and the entire apparatus was covered with a down sleeping bag. The hydration bladder did not need to be refilled during the 2.5 hours of drawing samples. However, after approximately 1.5 hours, an error message appeared indicating the i-STAT analyzer was again out of temperature range. At this point, closing the cooler lid kept the i-STAT within operating range for another hour without overheating.

Figure 1

Hydration bladder filled with near-boiling water (left panel); hydration bladder wrapped in a towel (middle panel); i-STAT analyzer placed on top of hydration bladder, cradled with wool shirt (right panel).

Cold temperatures lead to peripheral vasoconstriction that often requires multiple attempts to express fingertip blood after pricking with a lancet. This is uncomfortable for the athlete and often leads to noncompliance in elective blood testing for research purposes. Before the blood draw, we had study subjects hold a 1-L Nalgene water bottle filled with hot water for approximately 2 minutes, which yielded ample blood with a single prick to fill a cartridge (approximately 65 μL). The cold ambient temperatures appeared to increase the viscosity of blood, which limited the filling of the capillary tubes used to transfer blood from the fingertip to the cartridges, as well as the loading of the cartridge itself. Multiple transfers of blood with a capillary tube were often required to fill a single cartridge. This prolonged filling time promoted blood clotting in the cartridge and caused an error message to appear. To circumvent this, the cartridge was placed on an inverted aluminum lid on top of a pot of hot water (Figure 2). The i-STAT cartridge was left sitting on the overturned aluminum lid before and during the loading of the blood sample, which kept the cartridge warm and allowed successful filling. The hot water had to be replaced when the lid was no longer warm to the touch, approximately every 15 minutes.

Figure 2

i-STAT cartridge on top of aluminum cooking pot filled with hot water with overturned lid.

The techniques we describe do not require new technology; however, these methods have proven effective for using equipment typically found in wilderness race settings. Cartridges and analyzers are temperature sensitive, and these should all be kept out of direct sun, heat, and cold in a small cooler. As these events typically occur throughout the night in diverse and often challenging conditions, such improvisation may be necessary to ensure that diagnostic equipment is functional. Like the recently reported cooling of i-STAT analyzers, ³ these techniques described may lead to malfunction of the equipment. And although further testing is required before applying these methods to increase the temperature of an i-STAT analyzer and cartridges in cold temperatures to patient care, the alternative may be to have no diagnostic testing equipment at all.