Metabolic Measurements During Mechanical Ventilation

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  • Metabolic MeasurementsDuring Mechanical Ventilation

    Michael C. Damask Benedict-Roth spirometer with a CO2 absorber included in theDepartment of Anesthesiology anesthesia circuit. Historically, the most frequently usedColumbia University College of Physicians and Surgeons indirect methods for metabolic measurement during mechani-

    cal ventilation are the Douglas bag technique [7], the openINVASIVE METHODS circuit flow-through technique [8], and closed circuit spiro-M ETABOLIC rate can be clinically measured using the Fick metry [9].M principle to calculate oxygen consumption (VO2) and In the most common method for measuring metabolic ratecarbon dioxide production (VCO2) [1]. Oxygen consumed (or during mechanical ventilation, expired gas is collected in acarbon dioxide produced) is calculated as the product of the Douglas bag or large spirometer. Concentrations of 02 anddifference in gas concentration between mixed venous and CO2 are then measured using either gas meters, chemical gasarterial blood (cm3 gas/cm3 blood), and the cardiac output analyzers, mass spectrometry, Haldane techniques, or the(cm3 blood/min). This method requires a pulmonary artery Scholander technique. Once the expired minute volume andcatheter to sample mixed venous blood. Cardiac output is the inspired 02 and CO2 concentrations are known, V02 andmeasured by either thermodilution or indicator-dilution VCO2 are calculated. It is essential to obtain very accuratemethod. The problems associated with invasive methods are measurements of both gas concentrations and gas volume.numerous, which makes their clinical application difficult. However, Douglas bags are notoriously cumbersome andThe insertion of a pulmonary catheter requires skill and proper subject to leakage, and can collect gas for only a limited timemonitoring facilities, and carries definite risks of complication period (2-6 min).[2]. To measure blood oxygen and carbon dioxide concentra-tions accurately, the Van Slyke method should be employed OPEN CIRCUIT FLOW-THROUGH TECHNIQUE[3]. The common practice of calculating blood gas content An open circuit flow-through method has been used forfrom partial pressure and hemoglobin concentration mea- continuous metabolic measurements mainly on spontane-sured with clinical analyzers may be a source of error. ously breathing patients, but also for patients on mechanicalIncomplete mixing of the indicator, dye, or cold solution in the ventilators [8, 10]. With the open circuit method, the patientcardiopulmonary circulation is the most common problem Of breathes through a valve that separates inspired air fromdirect cardiac output measurements, and therefore of indirect expired air, which is collected and analyzed. Lister et al. [1 1 ]metabolic rate measurements. Changes in blood flow during used this method to measure oxygen uptake in spontaneouslymeasurement is another source of error [4]. Also, with the breathing infants and children. In this system (Fig. 1), thedirect method, careful attention must be paid to gas tempera- head of the patient is placed in a chamber through which ature and pressure. stream of room air flows. This arrangement allows for the

    continuous entry of room air into the system, while expiredNON-INVASIVE METHODS gas diluted with room air is removed and collected. The

    Direct calorimetry uses physical methods for measurement mixture is sampled, and the 02 concentration is measured byof free-body heat production, whereas indirect calorimetry a paramagnetic oxygen analyzer. The V02 is calculated bymeasures energy released as heat from chemical reactions in multiplying the gas flow rate through the system by thethe body [5]. Metabolic measurement by indirect means is difference in oxygen concentrations between room air andpreferable because indirect methods are non-invasive and can exhaled air mixed with room air. The total gas flow throughbe adapted to flow-through techniques, making it possible to the system is measured before sampling mixed expired gas.monitor V02 and VCO2 continuously. However, the major Recently, a more sophisticated open circuit flow-throughproblem with these techniques is that they require many technique has been reported, which measures the metaboliccomponents, thus making them too cumbersome for use in an rate in premature infants by using more accurate CO2 and 02operating room or intensive care unit. Also, the time and cost analyzers [121. With this technique, two dependent variablesof assembling these systems make them impractical in must be considered: the flow of gas through the system, androutine clinical use. For the most part, these systems have the oxygen concentration in the expired gas/room air mixture.been used as research tools on patients spontaneously As the flow rate increases, the difference between thebreathing room air, but not on those who require mechanical oxygen concentration of room air and that of mixed expiredventilation. gas decreases. Since, in order to prevent the loss of expiredShackman et al. [6] performed one of the first studies air from the system, flow rates are usually five or ten times

    measuring V02 of mechanically ventilated patients under the minute volume, the sampled mixed expired 02 and CO2anesthesia. The device used for these measurements was a concentrations approximate room air. Therefore, the instru-

    ments used must be capable of accurately measuring smalldifferences in 02 concentration [11]. The accuracy of the

    Michael C. Damask, MD is Assistant Professor open circuit method is decreased because of the high andof Anesthesiology at Columbia University Col- often variable inspired oxygen concentration that is typicallylege of Physicians and Surgeons, and Attend- required by seriously ill patients [131. Under anesthesia, theIng Anesthesiologist at Columbia-Presbyterian measurement of inspired and expired volumes with the openMedical Center, Anesthesia Service, New circuit technique becomes inaccurate if nitrous oxide (N20) isYork, NY 10032. used [141.

    CLOSED CIRCUIT SPIROMETRY METHODIn the closed circuit spirometry technique, the patient is

    connected to either a mask, an endotracheal tube, or acanopy, and breathes into and out of a spirometer that is

    30 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE JUNE 1986 0739-5175/86/0600-0030$01.00 1986 IEEE

  • attached to a kymograph. The patient consumes 02 and E A M 2 U IN A m 5a.7EMexcretes CO2. The expired CO2 and water vapor are ab-sorbed, so that gas volume changes in the system are dueonly to the amount Of 02 consumed. The 02 uptake by thelungs is measured by the amount added during the experi-ment to keep the total gas volume of the system unchanged LJ(5, 1 51. \Engstrom et al. [9] described a closed circuit method that

    .permits continuous metabolic measurements during mechani-cal ventilation and anesthesia (Fig. 2). In this system, thereare two conditions which must be satisfied in order tomeasure 02 consumption: (1) constant volume in the ventila-tory system, which includes the patient's lungs, the anesthe-sia machine, and the gas concentration measuring device;and (2) constant partial pressures of gases in the system.These conditions are fulfilled with the use of a ventilatorconnected to a volume-controlling device that adjusts theflow of oxygen into the system in such a way as to keep the Figure 2. Closed circuit method for continuous gas exchange mea-volume constant. surements during mechanical ventilation. Major components of theThe closed circuit technique is subject to a number of errors system: Engstrom ventilator to regulate minute volume and absorb

    that may lead to inaccurate results. Leaks related to mask, C02; (A) bellows; (B) reservoir bag; (C) spirometer measuring volumemouthpiece, hoses, and the system itself are clearly major change of gas; and (D) oxygen analyser. From Engstrom, Herzog, andproblems. Leaks in the inspiratory side will add gas and cause Norlander [91, with kind permission of the authors and publisher offalse low values for V02; leaks in the expiratory side will Acta Anaesth. Scand.cause false high values for V02. Other problems relate to alimited collection time (usually less than 10 minutes), whichis not long enough for the body's CO2 and 02 stores to reacha steady-state. Changes in lung volume can also be a source COMMERCIAL INSTRUMENTSof error [5]. The closed circuit technique presents problems in Recently, a new generation of commercial instrumentsmetabolic measurements during N20 anesthesia [14]. Be- (Siemens-Elema Servo Ventilator 900B with CO2 and 02cause it takes many hours for the body's different gas analyzers; Beckman Metabolic Measurement Cart) has beencompartments to reach equilibrium, inert gas tension must developed to measure V02 and VCO2 of patients in theremain constant during the measurement period. The descent operating room and intensive care unit. These instrumentsof the spirometer indicates the net gas exchange as the CO2 have sufficient accuracy and compactness to be used rou-output, and the measured V02 will include any exchange of tinely during many clinical situations [161.inert gas. During the period of closed circuit breathing, N20 The Siemans-Elema system is composed of the Servowill pass from the blood into those body compartments that Ventilator 900B with CO2 Analyzer 930 and 02 Consumptionare still unsaturated. This fall in blood N20 will lower alveolar Calculator 980 (Seimans-Elema AB, Ventilator Division,N20 and thus cause a loss of N20 from the spirometer. The Solna, Sweden). This sytem is designed to continuouslyresult will be an error in the measurement of V02. The closed measure V02 and VCO2 in mechanically ventilated patients. Acircuit method also requires considerable modification for flowtransducer in the ventilatorproduces an expiratoryflowmeasurement of VCO2 because CO2 is normally absorbed by signal [1 7, 181. The CO2 analyzer uses an infrared source andsoda lime in the system. detector near the endotracheal tube to measure expired CO2

    FLOW METER concentration. The oxygen consumption calculator includesan oxygen fuel cell (Teledyne, Inc., Los Angeles, CA, class B-

    THENq | 1 1), a mixing chamber, and analog electronics. Inspired gas issampled from the mixing chamber. VCO2 is calculated as the

    12) g product of instantaneous expiratory flow rate and expiredis | ,CO2 concentration. V02 is calculated as the product of the|MIlXED U [ instantaneous expiratory flow rate and the difference be-EXPIRED BLOWER ATMOS tween inspired and expired oxygen concentrations. TheGAX. X 1 {1> respiratory quotient (RQ) is computed every minute as VC02T I FrLOW METER | divided by V02.

    O filNG | o |The Beckman metabolic measurement cart (MMC) (Beck-IAt-[}wn LltNAL - man Instruments, Anaheim, CA) is also designed to measureV02, VCO2, and RQ during spontaneous and controlled

    [Yl VE I H1C^L C lINTER- tbreathing. Its components are a Beckman OM-Il polaro-. 1 t | uc^ .-FACE FS C02graphic 02 analyzer, an LB-2 infrared CO2 analyzer, a turbine

    ,---I Fs flow transducer, a mixing chamber, and a Monroe program-0, mable calculator. The expiratory port of the ventilator isconnected to the mixing chamber. A continuous sample ofELECTRICAL FLOW -- expired gs(500 cm3lmin) tknfrom temixingchmeflows through a dessication tube containing Dryerite, andFigure 1. Open circuit flow-through technique for continuous gas into the CO2 and 02 analyzers. This gas sample is then

    exchange measurements during spontaneous ventilation. Major com- returned to the mixing chamber, and the total expired gasponents of the system: canopy with blower providing continuous passes through the flow transducer. This transducer has anflow of room air; paramagnetic oxygen analyser; flow meters. From accuracy of 2% over a range of 6-600 liters/mmn. TheLister, Hoffman, and Rudolph [111, with kind permission of the programmable calculator continuously processes signalsauthors and publisher of Pediatrics, from the gas analyzers and flow transducer.

    JUNE 1986 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE 31

  • iFEEDBACK LOOP TECHNICAL AND THEORETICAL PROBLEMSPERCENT 0 cnieig tcncl tertcloREFERENCE FEEDBACK MOTOR After considering the technical and theoretical problems of

    COPARATOR CONTROL SPEED accurately measuring metabolic rate in the clinical environ-ment with the closed and open circuit techniques, Wes-

    OXYGEN U ClRCU.ATOR [ P tenskow et al. [13] developed the replenishment method forSOORCE use during mechanical ventilation with high oxygen concen-trations. In this system (Fig. 3), the patient is connected by an

    RESPIRATOR endotracheal tube to a closed rebreathing circuit that includes/ g/ /co _ - a volume cycled respirator. 02 and N20 are added to the

    t , 9 SCRUB P $ SENSOR closed circuit at exactly the same rate at which they areextracted by the patient. Two valves force air to moveF< \ g I} 0 counter-clockwise around the circuit. CO2 is removed fromthe exhaled air by a soda lime absorber. Dry 02 is supplied by

    PATIENT a pump from a source at ambient temperature and pressureN20 PUMP t ,before the gas returns to the patient. An air circulator

    ROTOR LOW PASS COMPARATOR between the 02 inlet port and the 02 sensor mixes the gases.SPEED FILTERN20 REFERENCE Partial pressure of 02 iS measured by a Clark electrode. TheSOURCE 20 FEEDBACK LOOP VOLUE feedback loop that controls the N20 flow consists of a

    Figure 3. Replenishment method for gas exchange measurements volume sensor, a feedback control circuit, and a pump. Theduring mechanical ventilation with oxygen-enriched air. Major compo- advantages of the replenishment technique are as follows: (1)nents of the system: oxygen sensor; oxygen pump; volume sensor; measurement of inspired and expired flow is not required; (2)bellows; N20 pump; CO2 absorber. From Westenskow et al. [131, inspired 02 concentration is held constant; (3) uptake ofwith kind permission of the authors and publisher of IEEE Transac- anesthesic gases is compensated for; and (4) this system cantions on Biomedical Engineering. be used with many current ventilators without major modifi-

    cations.Kinney [191 developed a closed system that encloses the

    patient's head in a rigid, transparent canopy with a neck seal.I ; , ____> ,A continuous 40 litre/min gas stream flows through the

    rX n h, OUTSIDE AIR system. Gas concentrations are measured with CO2 and 02, : analyzers (Figs. 4 and 5). This non-invasive system is

    -CANOPY l I practical for investigations of metabolic rate over long periodsKENT ROOM # I AIR SUPPL and in sick patients, but it is limited for patients breathing

    -BED high 02 concentrations [4]. It has been used clinically toPAT:ENT ROOM *1 #2 2 #1 c12022 determine the metabolic state of septic, injured, and nutrition-

    ROOM O ally depleted patients [20, 21]. When applied to spontane-SEiCTOZ ously breathing patients, this method is quite reliable. How-*4

    _3 *4 C 02 ever, it cannot be readily adapted for use on mechanicallyl Fl'lANALYZER ventilated patients.

    Bain and Spoerel [22] measured CO2 produc...