Metabolic Cart Technique

Updated: Dec 09, 2020
  • Author: Agustina D Saenz, MD; Chief Editor: Michael R Pinsky, MD, CM, Dr(HC), FCCP, FAPS, MCCM  more...
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Approach Considerations

For appropriate measurements, the system needs to be calibrated prior to measurement and every 8 hours when used continuously. Calibration is performed after an analyzer warmup period of 30 minutes and after being moved or reenergized. The gas analyzers and flow sensor are calibrated per manufacturer recommendations with precision compressed gases. Usually, they contain 15%-16% of oxygen and 3%-5% of carbon dioxide. Some devices use room air for calibration. It is important to ensure that no oxygen source is flowing near the cart at the time of calibration, as this will affect calibration and subsequent analyses.

To calibrate the flow sensor, a certified 3-liter calibration syringe is used to push air through the system, simulating the breathing pattern of an adult. A smaller syringe is used to calibrate the sensor in pediatric patients as per manufacturer recommendations. It is also important to follow the recommendations of the ATS/ERS for calibration of devices. [3, 4]


Metabolic Cart in Patients on Ventilatory Support

For metabolic carts that use a flow generator, the inlet port needs to be open and unobstructed.

For metabolic carts that use a mixing chamber or the breath-by-breath mode, the inlet port of the flow sensor is connected to the expiratory outlet of the ventilator. This is accomplished with a disposable 22-mm tube or an adapter connected to a corrugated extension tube. It is important to ensure that there are no leaks between the ventilator and metabolic cart. This attachment should be done first to wash out any room air before starting the test.

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The inspiratory FiO2 is measured by inserting a 22-mm adapter with the inspiratory FiO2 line attached. This needs to be placed in the inspiratory tubing of the ventilator after the humidifier or on the patient side of the humidifier.

The expiratory air sampling collection point must be at least 50 cm away from the patient Y piece to prevent expired air from entering the sample line. Most of the newer-generation ventilators have expiratory ports to which an adapter can be attached, eliminating this problem.

Metabolic cart Metabolic cart

Canopy and Mask Method for Nonintubated Patients Breathing Room Air Only

The canopy can be used in spontaneously breathing patients to avoid the need to hold a mouthpiece, therefore demanding less energy.

The canopy or full–face mask method allows the metabolic cart to draw air through the canopy or mask at approximately 30 liters per minute. If the “Power Disconnect alarm” or the “No-breathing alarm” is activated, the canopy needs to be removed immediately, as no airflow is passing through the canopy or mask.

The hose of the canopy and mask need to be connected to the flow generator.

The inspiratory sample line is placed at the air inlet of the canopy to have contact only with ambient air. The head of the patient is covered by the canopy, and the skirt of the canopy is tucked in to prevent room air from diluting the measured expiratory gases.

During the procedure, the patient remains awake and at rest in a quiet thermoneutral room.

The metabolic procedure is complete once the average minute oxygen consumption (VdotO2) and carbon dioxide (VdotCO2) production change is less than 10% during a minimum 15- to 20-minute interval, at the same time that the RQ varies less than 5%.

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Patients on Supplemental Oxygen Therapy

Patients who are on continuous oxygen therapy can undergo metabolic studies using a reservoir or blow-by system. The two most important items to remember is a stable FiO2 and isolation of the inspired and expired concentration gases to prevent dilution of the sample. A one-way valve with a mouthpiece and nose clip or a one-way valve that is integrated into the mask is used.

The mouthpiece is placed in the patient’s mouth and a nose clip on his or her nose to prevent gas leakage. Alternatively, a mask is used, being careful to avoid leaks around the mask. The technique requires matching the FiO2 delivered through the device such as a cannula. The patients is allowed an extended period to acclimate to the process of breathing on or through the device.


Setup of Elevated FiO System

The metabolic cart used must be able to measure elevated oxygen levels; not all carts have this function. If the metabolic device can measure ventilator studies, it can probably be adapted for use in nonventilated patients receiving elevated oxygen therapy.

When using a blow-by system, the inspired high flow must be isolated from the subject inspiratory limb of the circuit to prevent dilution of the exhaled breath sample. The inspiratory FiO2 must also be carefully monitored to ensure it is not fluctuating with the patient’s breathing pattern. A typical setup is illustrated in the diagram below and can also be used with dynamic exercise testing with elevated oxygen levels.

Elevated Oxygen Study for Patients on Routine Oxyg Elevated Oxygen Study for Patients on Routine Oxygen Therapy
Metabolic cart screen Metabolic cart screen