The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Joseph Orr1, Lara Brewer1, Michael Sherman1

Introduction: Indirect calorimetry systems calculate metabolic rate and calorie consumption from directly measured oxygen uptake (VO2) and carbon dioxide production (VCO2). Indirect calorimetry is useful for determining patient feeding requirements and for assessing the status of patients with sepsis and other disease states such as malignant hyperthermia. Contemporary indirect calorimeters make us of side-stream gas sampling, which can also be problematic because of difficulties with signal alignment in the presence of ventilator settings such as positive end-expiratory pressure (PEEP) and potential for fluid buildup, which leads to obstructions in the sampling tube.

We evaluated a prototype system that uses mainstream sensors to acquire the breath-by-breath O2, CO2 and flow signals needed for indirect calorimetry (Philips Medical Systems, Wallingford, CT). The on-airway oxygen sensor uses the principle of photo-luminescence quenching to optically measure oxygen partial pressure 100 times per second. We compared the VO2 measurements from the on-airway sensor against VO2 measurements from a metabolic cart (Deltatrac, Datex, Helsinki, Finland) in intensive care unit patients.

Oxygen consumption measurements were calculated as the integral of the airway flow and oxygen signals over the entire breath (inspiration and expiration). After eliminating extraneous measurements, data from all of the breaths in each minute were summed and reported. We compared the resulting minute-by-minute VO2 measurements in 14 intensive care unit patients using the new mainstream sensor system against simultaneously reported VO2 measurements from the metabolic cart. The mainstream sensors were placed in the breathing circuit between the wye and the endotracheal tube. The average VO2 measurements reported by the two systems for each patient were compared.

The average difference between the two systems was 6.423.4 ml/min, (1.76.9%). The plots below shows the difference

Our system has been successfully evaluated in healthy volunteers and using a bench simulation. An advantage of an on-airway system is that the inspired oxygen signal is analyzed continuously over the complete breath. The reference system (Deltatrac) only monitors the inspired oxygen for a brief period once each minute and assumes that the inspired oxygen signal does not chang