Metabolic Cart

Critical illness can significantly affect metabolism, so an accurate measurement of the resting energy expenditure (REE) can help determine the energy requirements in ICU patients. REE (usually 70% of the total energy expenditure) can increase after burns, sepsis, trauma, and surgery. A precise calculation of energy expenditure may prevent overfeeding or underfeeding.

REE can be measured with indirect calorimetry using a metabolic cart, which is used to measure the oxygen consumption (VO2) and carbon dioxide production (VCO2). Every liter of oxygen consumed is equivalent to the energy cost of 5 kcal. A metabolic cart can also be used to assess the energy requirements during exercise and to determine work capacity.

In 1949, Weir introduced an equation to facilitate the calculation of REE, as follows:

REE = [VO2 (3.941) + VCO2 (1.11)] 1440 min/day

Metabolic cart is indicated for the following:

• To guide appropriate nutritional support

• To determine the oxygen cost of work of breathing and to help select appropriate ventilator mode and settings[1]

• To determine the causes of increased ventilatory requirements (high glucose intake can increase carbon dioxide production, stimulating ventilation and complicating weaning)

• To measure cardiac output[2]

• Exercise physiology

In general, metabolic cart has no contraindications unless transient disconnection from mechanical ventilation cannot be tolerated.

The following are relative contraindications to indirect calorimetry:

• Leaks around endotracheal or tracheostomy tube, including cuffless tubes

• Chest tube to suction and leaks around the chest tube

• Subcutaneous emphysema and communicating tracheal esophageal fistula

• Ventilatory modes that use bias flow or leak compensation

Although not representing a contraindication, measurements may be inaccurate in patients who require high levels of oxygen (FiO2 ≥60%), high PEEP (>10 cm H2 O), air leaks, peritoneal or hemodialysis up to 4 hours after (the latter due to elimination of carbon dioxide across the artificial dialysis membrane).

The key to indirect calorimetry is that all inspired and expired air must be collected; any potential leaks in the closed ventilatory system leads to errors in the readings or uninterpretable results.

Patient Instructions

Thoroughly explaining the procedure to the patient is important, and preprocedure preparation is important. Outpatients are asked to fast overnight, and studies should be performed first thing in the morning. Patients should be instructed to avoid any drinks with caffeine or sugar and to avoid strenuous activity or exercise until the test is completed. They should also be instructed to wake and dress in comfortable clothes and to be driven to the facility with minimal effort, if possible.

Upon arrival, the patient should be placed on a stretcher or recliner for 30-45 minutes before the test. The patient needs to be in a resting state. There should be no interruptions during measurement and no nursing care for at least 1 hour before the test (including suctioning) in an environmentally neutral room.

In ventilator-dependent patients, ventilator settings should not be changed for at least 90 minutes before the procedure.

All sources of supplemental oxygen (eg, nasal cannula, masks, aerosol delivery) are turned off, if possible, to avoid a falsely low metabolic rate.

Diet or tube feedings should remain unchanged for 48 hours prior to measuring the REE.

Patient must take nothing by mouth (NPO) for at least 2 hours before testing owing to the increased metabolic rate caused by the thermogenesis effect following meals. The exception is for patients receiving continuous tube feedings, as this does not affect the testing.

Equipment

Many commercial devices for this type of testing are available. It is advisable that the types of patients to be studied reflect the needs of the institution. Patients on ventilators require a device that measures elevated oxygen levels. Some institutions use a metabolic cart for cardiopulmonary stress testing and/or research-level data collection. It is with these classes of advanced systems that a canopy system is also available. These devices also have robust databases and can perform advanced data collection and analysis, such as body-fat percentage or breakdown of substrate utilization.

Another group of devices is easier to operate but can be used only in individuals who do not require oxygen therapy and cannot be used for exercise testing. These devices usually do not have extensive databases or trending capabilities.

Other equipment includes the following:

• Disposable 22-mm ventilator tubing
• Precision calibration gases and volume calibration syringe
• Thermometer, barometer, and hygrometer (built into most metabolic systems)
• Canopy, valve with mouthpiece, or facemask for nonintubated patients
• Heart rate monitors (only for exercise testing)
• Treadmill or ergometer (for exercise measurements)
• Anthropometric equipment for measurement, when necessary (eg, calibrated skinfold calipers, measuring tape, bioimpedance)

Anesthesia

Anesthesia is not required, as this would alter the patient’s metabolic rate.

Positioning

The patient should be awake throughout the procedure in a supine or semi-Fowler position and in an environmentally neutral comfortable area or quiet room.

Complications

Measurement of REE via metabolic cart is a safe procedure, but some potential complications or limitations include the following:

• Reduction in alveolar ventilation

• Increased work of breathing

• Hypoxemia or bradycardia secondary to disconnection of the patient from the ventilator

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]

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

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 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.

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 Oxygen Therapy

Metabolic cart screen