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 VC02

T 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 anRESPIRATOR endotracheal tube to a closed rebreathing circuit that includes

/ g/ /co _ - a volume cycled respirator. 02 and N20 are added to thet , 9 SCRUB P $ SENSOR closed circuit at exactly the same rate at which they are

extracted by the patient. Two valves force air to moveF< \ g I} 0 counter-clockwise around the circuit. CO2 is removed from

the exhaled air by a soda lime absorber. Dry 02 is supplied byPATIENT a pump from a source at ambient temperature and pressure

N20 PUMP t ,before the gas returns to the patient. An air circulatorROTOR 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. The

SOURCE 20 FEEDBACK LOOP VOLUE feedback loop that controls the N20 flow consists of aFigure 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 periods

KENT 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 to

PAT: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 production duringmechanical ventilation of anesthetized patients with a modi-

It9;1 : Efied Mapleson D circuit and a mass spectrometer. Since the' 'I '.t___TO CO2 concentration in gas emerging from the expiratory valve

is nearly constant, as is the fresh gas inflow from theL

anesthesia machine, the patient's VC02 can be determined asPIPES IN WALLS-----AIR TO PATIENT the product of these two quantities. Changes in C02 output

AIR FROM PATIENT measured by this technique allow for continuous monitoringFigure 4. Diagram of a gas flow system with a head canopy to be of the patient's metabolic activity.used in any one of four rooms of a hospital unit. The air ventilating thecanopy is returned to the gas analyser in an adjacent room forcontinuous analysis. From Kinney [191, with kind permission of the PROBLEMS OF METABOLIC RATE MEASUREMENTSauthor and Ross Laboratories, Columbus, Ohio. There are many problems that may be encountered when

measuring metabolic rate during mechanical ventilation. Oneconcern is the components used in the gas exchange

X-----* rAPL UPmeasurement system: CO2 and 02 analyzers and flow me-IFILTERED 1 g2: | PATIENT LS FLOW | X SAMPLE FLOW ters. Accurate, reproducible, clinically useful, and relevantOUTSIDE AIR CANOPY MEASUREMENT metabolic rate measurements require well calibrated instru-

_ j | | | l [FMAIN FLOW ments. Even after initial calibration, accuracy of the instru-FOR CALIBRATIONJ EDAUST ANALYZER ments can be affected by moisture, temperature, and particu-

N2, 02- CO? FOR CALISFILT JEXHAUST C 02 ANALYZE late matter. Humidity and temperature changes can alter gasTO lOUTSIDflow viscosity and solubility in blood, leading to unsteady

I~~~ ~ ~ ~ ~~~~~~O ANALYZER inspired and expired 02 and CO2 concentrations, and hence

|FLOW MEAsURINGI i l j ~~artifacts in gas exchange measurements. This problem isI| I ~ ~ ~~~~T °2~0 ANALYZER Ff usually corrected by using hydroscopic filters or dryingI~~ ~ ~ ~~~CR PUNCH agents that absorb the water vapor from the expired gas.|-- I ~RECORDERS,CR UNH-- Variations in atmospheric pressure, temperature, and humid-

Figure 5. Block diagram of the steps in the air flow system indicated ity are corrected by converting all gas volumes to standardin Figure 4. From Kinney [19], with kind permission of the author and pressure (760 mm Hg), temperature (0°C), and dry valuesRoss Laboratories, Columbus, Ohio. (STPD), or to atmospheric conditions (ATPD). The solubility

32 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE JUNE 1986

of N20 in blood can also cause unsteady inspired 02 (respiratory quotient of 0.67) to validate the accuracy of aconcentration (F102), leading to errors in measurements [141. respiration chamber for human subjects.

Precise metabolic measurements require an accurate oxy- Engstrom et al. [9] used an artificial lung with a spirometergen fuel cell that is linear over the entire range of 02 to validate the accuracy of their closed circuit 02 consump-concentration. Linearity may decrease with use of the fuel tion system. An in vitro validation of the open circuit methodcell, and high 02 concentrations can markedly reduce its life was accomplished by comparing the results from the instru-[161. ment with those obtained by a nitrogen (N2) dilution tech-An important consideration in performing metabolic mea- nique [281. In this technique, N2 is injected into a stream of air

surement during mechanical ventilation is the difficulty of drawn from a canopy and collected in a Tissot spirometer.maintaining a stable F102. Fairley and Britt [231 examined this The N2 displaces an equal volume of air from the stream,phenomenon and showed a wide range of mean pressures which yields a V02 equal to the amount of 02 contained in theand oxygen gas flows throughout the ventilatory cycle. With displaced volume of air.one ventilator, the F102 varied from 51-96%, being highest at The validation of the 02 replenishment technique forlow gas flow and high cycling pressure. Another ventilator measuring V02 incorporates both in vitro and in vivo methodsdelivered a more constant F102 in the range of 61.5-72.3%. 129, 301. The in vitro validation includes a respirator set at aThese variations in Fl02s were attributed to the cleanliness fixed tidal volume, a ventilatory rate to simulate a patient'sof the air-entrainment filter and the ease with which the breathing pattern, and a breathing bag to simulate thepneumatic clutch moved forward as inflation began. High patient. 02 is added to the ventilator through a mass flowtension on the clutch reduced the quantity of air entrained. In controller and then removed to simulate a known V02. Thisorder to decrease the wide range of Fl02s at different cycling system was validated over a wide range of FI02s and VO2s.pressures, these ventilators should be driven by compressed The in vivo validation used volunteers whose exhaled air wasair, with 02 added as required by the patient. collected by a Tissot spirometer and analyzed for 02 partialBosomworth and Spencer [24] also observed this unsteady pressure with a mass spectrometer.

F102 during mechanical ventilation, noting that a higher air To validate various metabolic measurement systems, Dam-dilution setting on the ventilator caused a wide range of ask et al. [161 constructed a lung model with precise inflowsdelivered Fl02s. For example, if a particular ventilator was of C02 and N2 to simulate VC02 and V02. The modelset to deliver room air, the measured F102was 20.8%. If the consisted of a 13.5 litre glass jar and an attached 1 litreair mixer was set at 40% 02, the delivered F102 ranged from breathing bag. The lung model was ventilated at preset tidal55-60%, and at a setting of 60% 02, the delivered F102 volumes and frequencies. Further validation was acquired byvaried from 70-83%. These ranges in F102 were magnified comparing the measured RQ against that of burning methanolwith increasing gas flow into the ventilator. in the jar, which also provided a convenient method for theUltman and Burszstein [251 showed that greater inspired simultaneous validation of both the 02 and C02 analyzers.

oxygen concentration results in a considerable amplification Since the measurement of RQ is independent of inspired andof error in the measurement of V02. Browning et al. [261 expired gas flows, this simple technique is useful not only inemphasized the clinical significance of a stable F102 deliverythroughout the entire breathing cycle in order to accurately 282-_-measure V02. The absolute limit in the stability of F102becomes narrower with decreases in the inspired-expired°2 P02difference [261. For example, if the inspired-expired differ- 1 1 1 1 1ence is 0.02%, and the fluctuation in the F102 is as large as0.5%, then a 25% error occurs in measuring V02- In a series 227-of experiments, the above investigators observed that the 33measurement of V02 in mechanically ventilated patients witha Beckman Metabolic Measurement Cart was unsuccessful 2because of the variation in F102. There was much less Tbw U Z4f2Iicfluctuation in F102 when both high pressure inlets to the 0_ventilator were supplied by the same premixed air and oxygen -source using an external blender. Figure 6a shows 02 and Scor 6CO2 wave forms for a ventilator in standard use. The 02 wave (a)form is not constant during inspiration. In Figure 6b, F102variation is reduced by attaching an external blender to the air 328-and 02 sources, and using a high pressure converter to feed 702the blended gas mixture into the ventilator. The 02 and CO2 P02wave forms are mirror images of one another during inhala- TO"tion and exhalation.

276-VALIDATION OF INSTRUMENTS 36-

Clinical work with commercially available metabolic ratemeasuring systems requires an independent calibration pro- PCO2cedure capable of simulating a wide range of clinical condi- Torrtions [161. If precise investigations are being considered, it is n_inadequate simply to follow the manufacturer's instructions 8 1216for operation and calibration. An independent validation of Secondsthe instrument is needed, not only to test the accuracy of the (b)instrument, but also to serve as a means of diagnosingoperational problems. Together with reporting on the design, Figure 6. Airway P02 and pCO2. Ventilation with Bourns Bear Ifunction, and cilnical use of various instruments, investiga- ventilator, a. No external blender yields unsteady results. b. Use oftors have described how systems were validated. Newburgh external blender smooths results. From Browning et al., with kindet al. [27] described the complete combustion of alcohol permission of the authors and the publisher of Crit Care Medl[26J.

JUNE 1986 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE 33

validation but also in trouble-shooting operational problems of children undergoing corrective surgery for various congenitalthe entire system. heart defects. Seelye et al. [411 and Abbott et al. [421

observed that V02 substantially increased while rewarmingTHE OPERATING ROOM during CPB. This possibly reflects repayment of an 02 debtBy measuring V02 with an open circuit method, that developed during hypothermic CPB and circulatory

Severinghaus and Cullen [311 demonstrated that increasing arrest. Another study [431 demonstrated that normalizationconcentrations of halothane in spontaneously breathing pa- of arterial C02 tension reduced V02, thus improving 02tients depresses not only the myocardium, but also total body delivery to tissues during CPB. A recent study by the authorV02. Theye and Touhy [32] showed that during halothane (unpublished data) found a rise in post-CPB V02 that is aboveanesthesia with controlled ventilation, V02 is greatly affected pre-CPB levels, which also suggests that an 02 debt mayby premedication, thiopental, and muscle relaxants, and only have developed during CPB. This debt is related to hypother-slightly by halothane concentration. The same authors attrib- mia and the non-pulsatile blood flow used during CPB. C02uted decreases in V02 during light methoxyflurane anesthesia accumulates during cooling and is released after rewarming.to anesthetic concentration and to a low body core tempera- These observations are supported by an increase in serumture [33]. Theye and Touhy [34] also reported that V02 with lactate during and after CPB. A rise in VCO2 after CPB is dueether anesthesia is greater than that with halothane, which to the rapid elimination of C02 from the body stores.may be attributed to the stimulatory effect of ether on thesympathetic nervous system. Westenskow and Jordan [29] POSTOPERATIVE MEASUREMENTSreported the effects of a balanced anesthetic technique with The measurement of metabolic rate following major sur-controlled ventilation on V02. The cumulative effects of gery may be of value in three ways [441: (1) reduction of thefentanyl and thiopental in decreasing V02 were attributed to metabolic rate by maintaining normal body temperature mayreduced cerebral and myocardial oxygen consumption. In aid recovery; (2) significant conservation of the mechanicalanother study using a gas exchange measuring system work of breathing can be achieved by appropriate use ofadapted to a computer [351, a concurrent elevation in V02 mechanical ventilation; (3) measurement of metabolic rateand reduction in VC02 were seen. These metabolic changes monitors organ deterioration by detecting limited oxygenwere probably due to a combination of light anesthesia and availability seen in cases of hypovolemia and unstable cardiacrespiratory alkalosis caused by hyperventilation. function. An early report [45] showed that, in both thoracicThe effects of surgical stress and body temperature on and non-thoracic surgical procedures, the postoperative

metabolic rate have been investigated during many clinical metabolic rate was not significantly different from theconditions in the operating room. Waxman et al. [361 showed preoperative rate. In both groups, the V02, VCO2, andthat total body V02 may be an important parameter in alveolar ventilation were related linearly, suggesting thatmonitoring the surgical patient, as the maintenance of a postoperative ventilation was regulated relative to the meta-stable circulatory status does not necessarily correspond to a bolic demands of the body.stable metabolic rate. During surgical procedures, decreases In observing the metabolic rate in postoperative open heartin V02 were attributed to low body temperature, reduced patients, Raison et al. [44] showed an increase in V02 whichmetabolic demand due to anesthesia, and redistribution of diminished as the patient's body core temperature increasedblood flow due to hemorrhage and shock. Postoperatively, in the recovery room. The effects of shivering, mechanicalthe observed increases in V02 could be attributed to repay- ventilation, and 02 debt repayment are offered as possiblement of the intraoperative oxygen debt. Despite the protect- explanations for these changes in metabolic rate. In order toive effects of anesthesia and hypothermia, the low in- further define the role and importance of monitoring meta-traoperative V02 seen in critically ill patients was inadequate bolic rate, Wilson et al. [46] measured V02 in postoperativeto meet the metabolic requirements caused by surgical surgical patients. They found a reduced V02 in those patientsstress. These authors [37] also observed that by monitoring with sepsis or shock and in those who eventually died. TheV02, it may be possible to predict morbidity and mortality in V02 measurements, along with the arterial blood gases,surgical patients who have sustained massive intraoperative provided a useful parameter for evaluating the prognosis ofblood loss. In those patients who eventually died, a decrease critically ill patients.in V02 was seen during hemorrhage, and there was no The effects of body temperature on metabolic rate haveincrease during resuscitation. This finding reflects a cellular been studied by many investigators. It is well known thatoxidative deficit contributing to postoperative organ failure shivering in the postoperative period will cause large in-and mortality. creases in V02 and VC02. The importance of a warm ambient

Recently, we have reported the importance of monitoring temperature in the operating room, ensuring a normothermicmetabolic rate in patients undergoing major vascular surgery temperature for the patient in the recovery room, has been[38]. Following cross clamping of the abdominal aorta, there reported [47]. Roe et al. [48] observed that if cooling haswere significant reductions in V02 and VC02. When the occurred during anesthesia, postoperative V02 increasesclamp was released, V02 and VCO2 were greater than during during rewarming. An intraoperative fall in body temperaturethe control period before the aorta was clamped. The of 0.3-1.20C was correlated with postoperative shiveringincrease in V02 may reflect repayment of an oxygen debt during rewarming and caused a 92% increase in V02.developed during cross clamping. The increase in VCO2 is due Rodriguez et al. [49, 50] observed that, in patients who hadto the recirculation of serum lactate accumulated during the major surgical procedures complicated by intraoperativecross clamp period. hypothermia, and who were mechanically ventilated duringWith the evolution of cardiac surgery over the past 25 the postoperative period, shivering could be controlled by a

years, many investigators have studied the various metabolic non-depolarizing muscle relaxant or a narcotic. This treat-consequences of cardiopulmonary bypass (CPB). Special ment controlled the rapid rise in metabolic rate and circulatoryattention was paid to the effects of flow rate, perfusion demands seen with shivering, while preserving cardiac func-pressure, and temperature on gas exchange during CPB. One tion. In critically ill patients, especially those with underlyingof the first reports, by Starr [391, showed a direct relationship cardiovascular disease, the rapid increases in metabolic andbetween the CPB flow rate and V02. Bjork and Holmdahl [40] circulatory demands that occur following major surgery maynot only examined the effect of hypothermia on V02 during not be well tolerated. An attempt to stabilize the metabolicCPB, but also showed that V02 and blood gas measurements rate with muscle relaxants or narcotics may ameliorate themay be used to monitor the period of circulatory arrest in stress of surgery and improve the care of patients in the ICU.

34 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE JUNE 1986

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patients: oxygen consumption and blood flow. JSurg Res, 10:613-627,1970. tients. Crit Care Med, 10:82-85, 1982.2. McDaniel D, Stone JG, and Fatas AN et al.: Catheter-induced pulmonary 27. Newburgh LH, Johnson MW, WHey FH et al.: A respiration chamber for

artery hemorrhages: Diagnosis and management in cardiac operations. J Thorac use with human subjects. J Nutrit, 13:193-201, 1937.Cardiov Surg, 82:1-4, 1981. 28. Kappagoda CT and Linden RJ: A critical assessment of an open circuit

3. Van Slyke DD and Neill JM: The determination of blood gases in blood technique for measuring oxygen consumption. Cardiovasc Res, 6:589-598,and other solutions by vacuum extraction and monometric measurement. J Biol 1972.Chem, 61:573, 1924. 29. Westenskow DR and Jordan WS: Changes in oxygen consumption

4. Gump FE, Price JB, and Kinney JM: Whole body and splanchnic flow and induced by fentanyl and thiopentane during balanced anaesthesia. Can Anaesthoxygen consumption measurement in patients with intraperitoneal infection. Soc J, 25:18-21, 1978.Ann. Surg, 171:321-328, 1970. 30. Westenskow DR, Raeiner DB, Gehmlich DK, and Johnson CC: Instru-

5. Gump FE, Martin P, and Kinney JM: Oxygen consumption and caloric mentation for monitoring oxygen consumption using a replenishment tech-expenditure in surgical patients. Surg Gynecol Obstet, 137:499-513, 1973. nique. J Bioengng, 2:219-227, 1978.

6. Shackman R. Braber GL, and Redwood C: Oxygen consumption and 31. Severinghaus JW and Cullen SC: Depression of myocardium and bodyanesthesia. Clin Sci, 10:219-228, 1951. oxygen consumption with flurothane. Anesthesiology, 19:165-177, 1958.

7. Margaria G, Maschia G, and Marvo F: Determination of 02 consumption 32. Theye RA and Touhy GF: Oxygen uptake during light halothanewith Pauling oxygen meter. J App Physiol, 6:776-780, 1954. anesthesia in man. Anesthesiology, 25:627-634, 1964.

8. Webb P and Troutman SJ: An instrument for the continuous measure- 33. Theye RA and Touhy GF: Considerations in the determination of oxygenment of oxygen consumption. J Appl Physiol, 26:867-871, 1970. uptake and ventilatory performance during methoxy-flurane anesthesia in man.

9. Engstrom CG, Herzog P, and Norlander 0: A method for the continuous Anesth. Analg, 43:306-312, 1964.measurement of oxygen consumption. Acta Anaesth Scand, 5:115-128, 34. Theye RA and Touhy GF: Oxygen uptake during ether anesthesia in1961. man. Anesth Analg, 43:483-489, 1964.

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