Gas Exchange

Gas Exchange Quick Guide

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What is Gas Exchange? Pulmonary gas exchange measurement, known also as indirect calorimetry, allows continuous and non-invasive measurement of oxygen consumption (VO2), carbon dioxide production (VCO2), and calculation of respiratory quotient (RQ) and energy expenditure (EE).

How is Gas Exchange measured?

Indirect calorimetry is now available, for the first time, as a module which is integrated to airway gas and lung mechanic measurements.

The gas exchange measurement is used with the Datex-Ohmeda modular patient monitors both in anesthesia and critical care.

The D-lite flow sensor and gas sampler is located at the patient’s airway. The flow measurement is based on the pressure drop across a special property turbulent flow restrictor.

Inside the gas module the Datex-Ohmeda 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.

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.

1)Takala J.Meriläinen P:"Handbook of gas exchange and indirect calorimetry "p.30,Datex-Ohmeda Document 876710

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, 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.

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 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 de-convolution algorithm.

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.

Respiratory Quotient is calculated from VCO2 and VO2 once every minute and updated to the display also once every minute.

Energy Expenditure cannot be directly measured, but is calculated from VCO2, VO2 and urinary nitrogen (UN) once per minute.

Why use Gas Exchange measurement?

Gas exchange measurement provides a valuable clinical tool for diagnostic and therapeutic purposes.

Clinical applications vary from assessment of energy requirements and response to nutrition to analysis of ventilation and oxygen transport in critically ill patients.

The modern technology enables an easy, maintenance free and continuous measurement of gas exchange. Modularity allows both hemodynamic and respiratory parameters to be presented on a single screen, which makes the diagnosing of changes faster and easier.

The Datex-Ohmeda bedside module provides fast set-up without any precalibrations and is thus immediately operational when needed.

The modularity allows the switching the module between the patients and brings flexibility and cost efficacy to the treatment.

The visible number field shows the actual values continuously and trend history allows the observation of changes in patient’s metabolic status.

Clinical benefits

Follows changes in patient’s metabolic status

• Hypermetabolism caused by injuries or septic reactions is easily detected

Helps in nutritional assessment

• Measurements of energy expenditure and respiratory quotient help to avoid under-or overnutrition

Provides clear treatment indications

• Helps to define sufficient oxygen delivery
• Guidance for ventilator settings
• Adequate nutrition

Helps in weaning

• Change in VO2 in weaning trials indicates the work of breathing
• Feeding can be optimized

Clinical examples

In critical care

CO2 production acutely increase 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.
Both VO2 and VCO2 increased as an indicator of increased work of breathing when ventilation mode was changed to pressure support mode (PSV) after controlled ventilation.

In anesthesia

VO2 decreases rapidly during massive bleeding. VO2 measurement during surgery can be used to estimate names the effect of therapeutic interventions.

Last updated: 1 March 2001Created
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