Assessing the Use of Portable Blood Gas Measurements (iSTAT Instrument) in Emergency Air Transport

Abstract

Abstract Hyper and hypoventilation of patients on ventilators leads to poor outcomes. Traditionally, mechanical ventilation protocols in air medical transport are guided by pulse oximetry (SpO2) and continuous end tidal CO2 (EtCO2), which can lead to over-ventilation of patients. The use of portable blood gas measurements is a desirable adjunct in the air medical transport setting due to the complexity and duration of management of critical patients with only limited, noninvasive monitoring options. Previously, protocols relied on SpO2 and EtCO2 to control ventilator settings during in-flight patient management. The iSTAT is a hand-held point of care device often used in acute hospital setting to obtain arterial blood gas results within minutes using a disposable test cartridge and small specimen size. However, iSTAT use in air medical transport has been limited due to analytical and durability concerns in the uncontrolled helicopter environment. The purpose of this study was to test these concerns including the effects of vibrations on the microfluidic transport of blood in the cartridge, pressure changes at altitude on the processing of the cartridge, and temperature changes on the iSTAT instrument between readings during in-flight use. To ensure accurate inflight testing, we assessed precision and accuracy of inflight blood gas measurements compared to pre- and post- flight measurements on quality control material. Precision of initial instrument verification met the manufacturer’s coefficient of variation (CV) claims (lactate = 3.59%, pH = 0.059%, pCO2 = 4%, pO2 = 3%). Initial accuracy was assessed by instrument comparison. Bias of iSTAT compared to the laboratory radiometer ABL800 instrument was acceptable for clinical use (lactate = -7.95%, pH = -0.041%, pCO2 = 1%, pO2 = -2%). For the majority of tests (pH, pCO2, pO2, HCO3, BE, and SO2) we found no significant differences between inflight absolute values compared to pre- and post- flight measurements by one-way anova (p>0.05), and no significant difference in precision (CV) between in air and pre- and post- flight measurements for both low and high quality control samples. In measuring lactate levels, we found significant differences between inflight absolute values compared to pre- and post- flight measurements (p<0.0001) but these were determined to be clinically insignificant (mean (mmol/L): preflight = 6.87, inflight = 6.77, postflight = 6.69). Vibration and pressure differences in air compared to on land were therefore considered clinically insignificant. To keep the instruments at an operable temperature in flight between readings, we found utility in using an insulated lunch box with additional styrofoam placed in the bottom to prevent heat transfer. Most importantly, improved clinical outcomes from proper ventilation of patients were achieved. The results of this study demonstrate that the iSTAT instrument provides clinically accurate blood gas measurements in air as compared to standard in-hospital use.