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Abstract
Introduction Measurement of physiological parameters in extreme environments is essential to advancing knowledge, prophylaxis and treatment of altitude sickness. Point-of-care testing facilitates investigation in non-specialist and remote settings, as well as becoming increasingly popular at the bedside for real-time results in the clinical environment. Arterialised capillary earlobe blood gases are recommended as a valid alternative to arterial sampling in research. This study aimed to test the feasibility of obtaining and analysing daily earlobe samples at high altitude.
Methods From 17 to 24 January 2016, 24 participants on a research expedition to Ecuador underwent daily earlobe blood gas measurements including pH, partial pressure of oxygen and partial pressure of carbon dioxide to 5043 m. Samples were analysed using an Abbott i-STAT blood gas analyser and G3+ cartridges.
Results Daily measurements were successfully obtained and analysed at the point of care in 23/24 participants and were well tolerated with no adverse events. 12% (27/220) cartridges failed and required repeat sampling.
Conclusions Daily earlobe blood gas analysis using the Abbott i-STAT is feasible in a protected environment at high altitude. Participants and equipment should be kept warm before and during testing. Spare cartridges should be available. This methodology may be useful for both research and therapeutic measurements in remote, rural and wilderness medicine.
- altitude medicine
- respiratory physiology
- physiology
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Key messages
The Abbott i-STAT and G3+ cartridges performed well in a high altitude environment within the manufacturer specified temperature range of 16°C–30°C.
We report a cartridge failure rate of 12% in these conditions, with unconfirmed aetiology.
Blood gas trends were consistent with arterialised capillary samples and reflected increasing altitude, although data were not confirmed by arterial gas measurements.
Daily earlobe arterialised capillary sampling was well tolerated by subjects, with no adverse events.
This methodology may be useful for both research and therapeutic measurements in remote, rural and wilderness settings.
Introduction
Measurement of physiological parameters in extreme environments is essential to advancing understanding, prophylaxis and treatment of high altitude illness. Naturally, these environments are not conducive to traditional laboratory testing. Point-of-care devices allow investigations to be conducted in specialist, non-specialist and remote settings and can offer considerable benefits to rural and remote healthcare provision.1 2 The Abbott i-STAT is a portable, handheld blood gas analyser which gives comparable results with conventional laboratory equipment3 and has been used successfully in austere environments.4
Arterial blood gas measurement offers useful insight into pulmonary gas exchange and physiological compensation for hypoxaemia.5 However, arterial puncture is associated with complications including vasospasm, intraluminal clotting, bleeding, aneurysm and haematoma formation and transient obstruction of blood flow.6 Patients often report the procedure as painful and unpleasant.7
Earlobe arterialised capillary blood gas sampling has been described as a feasible alternative to arterial measurement8 with acceptable correlation between pH, partial pressure of oxygen (pO2) and partial pressure of carbondioxide (pCO2) measurements from earlobe and arterial sampling.9 10 Earlobe sampling is generally well tolerated and associated with fewer side effects.
This study aimed to test the feasibility of daily earlobe blood gas sampling and analysis using the Abbott i-STAT at high altitude.
Methods
From 17 to 24 January 2016, daily measurements were obtained from 24 subjects (18 male, six female, aged 23–78 years) on a high altitude research expedition. Samples were taken each day in Quito (2800 m); Chuquiragua Lodge, Chaupi (3400 m); Lodge Estrella del Chimborazo (3950 m) and the Whymper Hut (5043 m). Equipment and procedures were tested at sea level in the UK prior to departure.
Equipment
The i-STAT system manual specifies an operating temperature range of 16°C to 30°C for i-STAT cartridge testing, with transport temperatures of −10°C to 46°C acceptable. The analyser contains a solid-state barometric pressure sensor which determines the ambient atmospheric pressure used for the pO2 sensor calibration and tolerates a barometric pressure of 300–850 mm Hg, with a maximum relative humidity of 90% (Abbott Point of Care. i-STAT®1: System Manual, 2013).
A range of cartridges are available which allow measurement of a multitude of parameters such as electrolytes, blood gases and acid-base status, clotting studies, beta-human chorionic gonadotropin and cardiac assays including troponin. We limited study to G3+ cartridges measuring blood gases, given their relevance to altitude physiology, but feasibly other assays could have been tested in this setting.
The i-STAT and cartridges were maintained above minimum operating temperature simply by being kept inside a habited sleeping bag overnight in preparation for morning sampling. Equipments were placed in an open insulated box with bottles containing warm water throughout testing, similar to a previously described technique.11 The i-STAT was docked to a generator-driven power supply throughout testing, and thus, battery performance was not assessed.
The i-STAT user manual states that users should use a puncture device that provides free-flowing blood, as inadequate blood flow may produce erroneous results. Prior to departure, clicker-type lancets were found to yield inadequate blood samples without forcefully squeezing the ear, thus increasing risk of tissue fluid contamination. Therefore, standard single-use stainless steel 3.2 mm lancets were employed.
Data collection
All testing took place indoors in non-temperature controlled environments. Participants had rested at the altitude of testing overnight and samples were obtained in the morning before any significant exertion had occurred, except on day 6 when samples were taken in the morning after a short walking ascent from 4800 m to 5043 m.
A minimum of two investigators were required to obtain each sample effectively. Participants were prepared by wearing a warm hat with ear cover for at least 5 min prior to sampling. The inferior aspect of the preferred earlobe was cleaned sparingly using an alcohol wipe and allowed to air dry. Liberal application of alcohol-based cleaning solution was avoided to minimise cooling by vaporisation. With the earlobe held securely, a lancet was inserted to the hilt, rotated through 180° in each direction and then withdrawn. In some subjects, a second adjacent puncture was made. The first drop of blood was wiped with gauze to remove tissue thrombin contamination. A heparinised capillary tube was held horizontal to the sampling area and allowed to fill using capillary action with blood flowing freely. If blood did not flow freely, bleeding was encouraged by gently holding the anterior and posterior aspects of the helix/scapha and slowly massaging downwards towards the puncture site. If blood spread and ceased to coalesce, the area was re-wiped with gauze and sampling continued until the capillary tube was filled. After a complete sample had been obtained, participants were asked to apply firm pressure to the sampling site to promote haemostasis.
Samples were immediately transferred from heparinised capillary tube to i-STAT cartridge at the point of care. The cartridge was then immediately inserted into the i-STAT machine. The i-STAT machine was removed from the warmed box only to input subject identification data and facilitate cartridge insertion, then promptly returned. Approximately 3 min were required for the machine to analyse and deliver each result, which included measured pO2, pCO2 and pH. Analysis was undertaken at the point of care and sampling repeated if no result was obtainable. At each altitude step, the machine and cartridges underwent a calibration and verification process, as recommended by the manufacturer, to provide quality assurance.
Data input
Data were manually recorded on a paper grid at the point of testing and later inputted into an Microsoft Excel spreadsheet by two investigators. A third investigator verified the data on the electronic spreadsheet against records using the i-STAT’s memory function.
Clinical waste
Sharps were deposited in sharps boxes, packed and returned to the UK for safe disposal. Clinical waste was incinerated responsibly in Ecuador.
Results
We successfully obtained, analysed and recorded daily samples in 23/24 participants. One subject was withdrawn on day 2 due to challenges obtaining samples. pH, pO2 and pCO2 trends are shown in Figure 1. All participants denied considerable discomfort and none experienced significant bruising. Where minor bruising was evident, an alternative (typically contralateral) puncture site was used. There were no cases of major haemorrhage or wound infection.
Measured pH, partial pressure of oxygen (pO2) and partial pressure of carbondioxide (pCO2) from earlobe samples using Abbott i-STAT. Days 1 and 2: Quito. Day 3: Chaupi. Days 4 and 5: Lodge Estrella del Chimborazo. Days 6–8: Whymper Hut. Participants slept at the height of sampling each day except day 6, when a short walking ascent from 4800 m to 5043 m was undertaken immediately before sampling. Participants were started on acetazolamide on either day 6 or day 7 as part of an associated study, corresponding with the recorded fall in pH. Error bars denote SD.
Repair
Partway through operation on day 6, investigators became unable to fully insert cartridges into the i-STAT cartridge port. Close inspection revealed that a small catch had clicked shut inside the cartridge port. The mechanism by which this occurred is unknown. The catch was carefully released with a hair clip, and the contacts were cleaned using a cotton bud and isopropyl alcohol solvent. The i-STAT resumed to function without further incident.
Discussion
Data suggested that arterialised capillary samples were successfully obtained. The machine did not function below its stated lower temperature limit (16°C) and also appeared sensitive to humidity.
Twelve per cent (27/220) of cartridges failed and required repeat sampling. The i-STAT gives generic error codes which are detailed in the user manual (Abbott Point of Care, i-STAT 1: System Manual, 2013). These include ‘insufficient sample’ which may indicate the well being empty or air bubbles present, ‘unable to position sample’ which may indicate clotting, overfilling or failure to seal cartridge and ‘sample positioned beyond/short of fill mark’. We found that the pressure required to close the well chamber caused blood to advance within the cartridge, requiring a slight compensatory underfilling to prevent movement beyond the fill mark.
This study is limited by its lack of correlation with arterial blood sampling or pulse oximetry to confirm reliability. The absence of data pertaining to the precise nature of cartridge failure events precludes verification of exact failure mechanism and as such this report is purely descriptive.
Conclusions
With some improvisation, we find earlobe blood gas measurement and analysis using the Abbott i-STAT to be feasible in a protected or indoor environment at high altitude. Careful consideration must be made to maintain the i-STAT within its working temperature range during use and transport range when not in use. This can be achieved using simple and readily available methods. We recommend that an adequate supply of spare cartridges should be available due to the moderate observed failure rate and inability to obtain new stock in this setting. As with any novel equipment or process taken to such remote areas, we suggest that investigators are familiar with procedures prior to departure. This methodology may be useful for both research and therapeutic measurements in remote and rural settings.
Acknowledgments
The authors would like to thank the Jabbs Foundation for providing charitable funding to purchase the i-STAT machine and other members of the BMRES for their willing participation. CTL is grateful to the Sir Arthur Thomson Charitable Trust for funding his participation as a medical student in this research expedition.
Footnotes
Contributors All authors contributed to the study design, data collection, manuscript preparation, revision and approval of the final manuscript and agreed to be accountable for the integrity of this work.
Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
Disclaimer The authors have no past or current commercial interest in Abbott Healthcare Pvt and received no commerical grant or donated equipment. All equipment used in this study was purchased by the BMRES using charitable funding from The Jabbs Foundation.
Competing interests None declared.
Patient consent Not required.
Ethics approval Ethical approval for this study was granted by the Chichester University Research Ethics Committee.
Provenance and peer review Not commissioned; externally peer reviewed.
Collaborators Ashdown KM, Bradwell AR, Bradwell SB, Cadigan P, Campbell CA, Clarke A, Delamere J, Edsell ME, Imray CH, Johnson BG, Ladha C, Letchford A, Lock HE, Lucas SJ, MacLennan I, Myers SD, Newman C, Rue CA, Simmons J, Talks BJ, Thomas O, Wright AD
Correction notice Since this paper was published the author name I Chesner has been updated so just the first initial is listed and Figure 1 has been replaced to a version with higher resolution.