GE Healthcare Quick Guide
Gas Exchange
What is gas exchange?
How is gas exchange measured?
Pulmonary gas exchange measurement, known also as indirect calorimetry, allows continuous and . non-invasive measurement of oxygen consumption (V O2), carbon dioxide . production (VCO2), and calculation of respiratory quotient (RQ) and energy expenditure (EE).
Gas exchange measurement is captured through a gas module and is integrated with both the airway gas and lung mechanic measurements.
The gas exchange measurement is used with modular monitors in both anesthesia and critical care areas.
Gas exchange information is sampled through a single non-invasive D-lite* flow sensor and gas sampler at the patient’s airway. Inside the gas module a paramagnetic sensor is used to measure the O2 curve and the infrared bench for the CO2 curve. Both measurements are based on the side-stream principle. The breath-by-breath measurement of pulmonary gas exchange is technically very demanding and requires sophisticated compensation and data processing algorithms to achieve the accuracy required in the clinical use.
Clinical value Tool to detect changes in patient’s metabolic status
Synchronization of concentrations and flow
• Hypermetabolism caused by injuries or septic reactions
Tool for nutritional assesment • Measurements of energy expenditure and respiratory quotient to avoid under- or over-nutrition
Compensation of signal deformations
Support in treatment decisions
pressure difference Haldane transformation
• Helps to define sufficient oxygen delivery • Guidance for ventilator settings • Adequate nutrition
Because of the side-stream principle the measurements of gas concentrations and flow are not simultaneous. When a gas sample passes through the D-lite sensor, the flow signal is recorded with a negligible delay (less than 10 ms) since the pressure difference propagates to the module through the spirometry tubing with the speed of sound. In contrast, it takes on the order of 1.5 s for the gas sample to travel through the sampling line to the module where first its O2 and then CO2 concentrations are measured.
Support in weaning process
The transport time delay is not constant; it must compensate for the fluctuations in the sample flow, variations in the pressure at the D-lite sensor and changes of the gas concentration. In addition, the finite rise times of the O2 and the CO2 sensors need to be compensated utilizing a deconvolution algorithm.
• Measurement results are valid only when the respiration rate is below 35/min
After reconstructing the original waveforms of the gas concentrations and shifting the curves to match the flow signal, the final calculations are based on mathematical integration operation of the product of the flow and each gas signal.
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The respiratory quotient is calculated from VCO2 and VO2 once every minute and updated to the display also once every minute. (1)
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RQ = VCO2/VO2
The energy expenditure . . cannot be directly measured, but is calculated from VCO2, VO2 and urinary nitrogen (UN) once per minute. (1)
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EE = 5.5*VO2+1.7*VCO2-2*UN
1
Takala, J. et al. Handbook of gas exchange and indirect calorimetry, p 30, GE Healthcare Datex-Ohmeda Document 876710.
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• Change in VO2 in weaning trials may indicate work of breathing • Optimal feeding during weaning
Points to note
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• Adequate VO2 / VCO2 values cannot be measured with a leaking airway, with FiO2 higher than 85%, or when N2O+O2 mixtures are used in ventilation
• Measurements cannot be taken during the following ventilation modes: high by-pass flow, HFV, BiPAP • Unstable oxygen delivery may cause erroneous . VO2 readings • Disconnect the D-lite sensor during nebulization of medications to avoid damaging the sensor and contaminating the gas sensors in the module
Simple, easy design Today’s technology enables an easy, continuous measurement of gas exchange. The bedside module provides for fast set-up without any pre-calibrations, which makes it immediately operational when needed. The modular design allows for both the hemodynamic and respiratory parameters to be presented on a single screen, which makes the diagnosing of changes fast and easy. Modularity also allows the switching of the module between the patients and brings flexibility and cost efficacy to the treatment. The visible number field continuously shows the actual values, and trend history enables the clinicians to observe and identify changes in patient’s metabolic status.
Clinical examples In Critical Care CO2 production acutely increased from 200 ml/min to more than 300 ml/min. This increase, related to an acute onset of bacteremia, was the sole cause of the increase in ventilatory demand.
An acute increase in ventilatory demand
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Both VO2 and VCO2 increased as an indicator of increased work of breathing when the ventilation mode was changed to pressure support mode (PSV) after controlled ventilation.
Work of breathing
In Anesthesia
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VO2 decreases rapidly during massive bleeding. VO2 measurement during surgery can be used to estimate the effect of therapeutic interventions.
Additional resources
Severe bleeding during surgery
For white papers, guides and other instructive materials about GE Healthcare’s clinical measurements, technologies and applications, please visit http://clinicalview.gehealthcare.com/
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