MethodsAfter obtained informed consent from patients, we measured the RMR (m-RMR) of two patients (two female; mean age 76) with complete tetraplegia (C2 AIS: A and C4 AIS: A) twice during a week interval using QUARK RMR indirect calorimeter (Cosmed, Rome, Italy) with two sampling flow setting in September 2017.
Mechanical ventilation (Elisée 150, Resmed, Didcot, United Kingdom) was carried which the inspiratory gases were conducted via an inlet tube to the tracheostomy cannula and the expiratory gases were released to room air from a valve near the tracheostomy cannula. VO2, VCO2, respiratory quotient (RQ = VO2 / VCO2), and mRMR were determined with Quark RMR by two observers (SW and PS). The Quark RMR is a breath-by-breath system for gas exchange measurements. It measures CO2 and O2 concentrations over the respiratory cycle through a sampling line connected close to the endotracheal tube.
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Flow is measured using a turbine flowmeter attached to the expiratory port of the ventilator. There is a software application to compensate for bias flow in the ventilator circuit, which has to be set by the user before the start of every measurement. It uses a paramagnetic O2 analyser and an infrared CO2 analyser.
Before each measurement the Quark RMR warmed up and calibrated according to manufacturer instructions before each test session. The Quark RMR was calibrated with a gas mixture of 16% oxygen, 5% carbon dioxide, and balance nitrogen. FiO2, FeO2, FiCO2 and FeCO2 are the measured gas concentrations from inspired and expired ventilation gases. FO2 is the consumed oxygen calculated as FiO2 – FeO2 and the FCO2 is the produced carbon dioxide FeCO2-FiCO2.
As normal room air contains less than 0.03% of CO2, FCO2 can be considered to be equal to FeCO2.10 The mRMR was calculated from measured VCo2 and VO2 values using the Weir’s equation11 as follows. Urinary nitrogen (UN) is the amount of nitrogen excreted in urine during 24 hours. The error in mRMR if UN is not measured is less than 2%.9mRMR (kcal/day) = 3.
9141 x VO2 (L/min) +1.106 x VCO2 (L/min) -2.17x UN (g/day)The RMR measurements were carried out in the SCI centre in the afternoon between (4-6pm) twice within a 1 week interval. Both patients were continuously treated with invasive mechanical ventilation support via tracheostomy throughout the day. mRMR was measured while the patients were awake and ventilated using ambient air (FiO2: 21%). No additional oxygen was used during any measurements. The mRMR was measured during a fasting state (artificial nutrition support was switched off 4 hours prior to measurement). The mean total measurement time was 20 minutes for each measurement session.
A stable period of a minimum of five minutes in mRMR was considered as a valid measurement. The stability was determined as <10% and <5% deviations from the mean VO2 and RQ values, respectively. RQ values were required to be between 0.7 and 1.0. The patient's peripheral blood oxygen saturation was recorded constantly during the metabolic measurements with a portable non-invasive pulse oxymeter.
(Beneview T5, Huntingdon, Cambridgeshire, United Kingdom).Blood samples for determination of haemoglobin, serum albumin, serum C – reactive protein were measured. Increase in serum C – reactive protein >10g/L and body temperature >37oC were considered for signs of infection and m-RMR will be excluded. The mRMR values were compared to the eRMR values calculated using four different predictive equations. (Table 1).