Rebreathing improves accuracy of ventilatory monitoring

Objective. Our objective was to determine if rebreathing would reduce the gradient between arterial and end-tidal CO₂ tension during positive-pressure ventilation.Methods. Design: Experimental investigation.Setting: Anesthesiology laboratory.Subjects: A total of 10 dogs of either sex.Interventions: Anesthesia (sodium pentobarbital) and muscle relaxation (pancuronium) were induced and animals were tracheally intubated and ventilated with a standard anesthesia ventilator and breathing circuit with CO₂ absorber and then with a Mapleson D circuit with a fresh gas flow rate ( f) equal to alveolar ventilation plus the sampling flow rate of two capnometers. Rebreathing was varied by adjusting the respiratory rate (RR) so that minute ventilation ( e) to f ratio was 1:1, 2:1, 3:1, and 4:1.Results. CO₂ production (ATPD) was determined as the product of expired concentration of CO₂ and e (BTPS). Alveolar ventilation ( a) was calculated by dividing the product of CO₂ production and barometric pressure corrected for ambient temperature and water vapor pressure at body temperature by PaCO ₂. Tidal volume, RR, airway gas temperature, concentration of CO₂ in gas at the tracheal tube and inlet/outlet of the mechanical ventilator, body temperature, arterial gas tensions and pH, heart rate, arterial blood pressure, and cardiac output were measured. Minute ventilation, mean arterial blood pressure and end-expiratory CO₂ tension (Péco ₂)(BTPS) were calculated. During positive-pressure ventilation, concentration of inspired CO₂ was zero with standard circuitry, and significantly increased with Mapleson D when e: f ratio was 1:1 (0.56±0.19%), 2:1 (1.97±1.30%), 3:1 (2.56±1.05%), and 4:1 (3.01±1.45%) (p<0.05).Péco ₂ was 34.8±3.2 mm Hg during ventilation with the standard circuit, and significantly increased during ventilation with Mapleson D when e: f ratio was increased from 1:1 (35.4±2.5 mm Hg) to 2:1 (40.2±3.6 mm Hg) and was not further increased at a e: f ratio of 3:1 (41.8±2.7 mm Hg) or 4:1 (41.3±2.4 mm Hg). The selected fresh gas flow rate was appropriate, because PaCO ₂ remained unchanged regardless of e: f ratio, indicating PaCO ₂ was dependent on f, not on e. The gradient between PaCO ₂ andPéco ₂ during ventilation with the standard circuit was 6.6±3.0 mm Hg; during ventilation with Mapleson D, it decreased significantly when e: f ratio was increased from 1:1 (6.5±3.6 mm Hg) to 2:1 (2.9±1.5 mm Hg), but was not significantly reduced further at 3:1 (1.7±1.1 mm Hg) or 4:1 (1.8±0.5 mm Hg) (p<0.05).Conclusions. Rebreathing with a Mapleson D circuit and a f equal to a permitted normal CO₂ elimination. ArterialPCO ₂ toPéco ₂ gradient decreased significantly during rebreathing, thus improving the reliability of capnography for estimating arterialPCO ₂. Consideration should be given to using the Mapleson D as a rebreathing circuit.Address correspondence to Editorial Office, Department of Anesthesiology, University of South Florida College of Medicine, MDC Box 59, 12901 Bruce B. Downs Boulevard, Tampa, FL 33612.Presented, in part, at the American Society of Anesthesiologists' Annual Meeting in New Orleans, October 1992.     

Mixed venous O2 saturation: Measured by co-oximetry versus calculated from P\bar v o 2

Objective. The objectives of our study were (1) to compare mixed venous saturations calculated by a blood gas machine with those measured directly by a co-oximeter; and (2) to compare the sensitivities and specificities of o ₂s derived from these values.Methods. Charts were retrospectively reviewed of all MICU patients [n=16] between December 1, 1991 and January 31, 1992, who required pulmonary artery catheters for their usual care and who had hemoglobin saturations of mixed venous blood concurrently measured by both a co-oximeter (Co-Ox Model 482, Instrumentation Lab, Lexington, MA) and a blood gas analyzer (Nova Biomedical StatLab5, Waltham, MA) which uses a variant of the Severinghaus equation to calculate S O ₂ from P O ₂). Data used at the time of each S O ₂ measurement to calculate oxygen consumption ( O ₂) further was collected.Results. Available for analysis were 118 mixed venous blood samples. Although the S O ₂ values had a correlation coefficient of 0.807 (95% confidence interval [CI] 0.736 to 0.861, Fisher's z-transform), when O ₂s were calculated, the blood gas analyzer calculated saturations had a sensitivity of only 58.3% and a specificity of 89%, when compared with those calculated using the saturations measured by the co-oximeter. Attempts to mathematically improve upon the Severinghaus equation and upon an additional four regression equations used by other blood gas analyzers resulted in universally worse sensitivity.Conclusion. If S O ₂s calculated by a blood gas machine—rather than those co-oximetrically measured—are used to calculate O ₂s, 42% of patients with low O₂s will be misclassified as normal and 11% of normals will be misclassified as low. This total error appears to be the result of measurement error by the Po₂ electrode of the blood gas analyzer and shifts of the oxyhemoglobin dissociation curve, which are not accounted for in the equation that is used to calculate saturation from measuredPO ₂. We were not able to improve mathematically the sensitivity of any of the available regression equations used by blood gas analyzers to calculate S O ₂ from P O ₂. Therefore, it remains necessary to use co-oximetrically measured saturations when calculating O ₂.This study was presented in abstract form at the American College of Chest Physicians 58th Annual Scientific Assembly, Chicago, IL, October 28, 1992.     

Decreased critical mixed venous oxygen tension and critical oxygen transport during induced hypothermia in pigs

The effects of hypothermia on oxygen delivery and tolerance to hypoxia were studied in 8 normothermic (36.8°C) and 10 hypothermic (29.3°C) pigs that had been anesthetized and surgically implanted with instruments. Cardiac output ( t), o ₂ [oxygen consumption, or t × , where is arteriovenous oxygen content difference], arterial and mixed venous blood gas values, and lactate concentrations were measured as the animals were made progressively hypoxic. Under control, normoxic conditions, mixed venous oxygen tension ( ) was 41.4 ± 2.1 mm Hg (mean ± SE) in the normothermic animals and 26.1 ± 1.6 mm Hg in the hypothermic animals; these values are close to those predicted in our previous theoretical analysis. To study tolerance to hypoxia during hypothermia, critical and critical total oxygen transport (TOT = t × CaO₂, where CaO₂ is oxygen content of arterial blood) were determined by decreasing the inspired oxygen concentration (FiO₂) in steps and measuring the point where o ₂ and blood lactate levels becamePo ₂ or TOT dependent. Again as predicted, the critical was lower in the hypothermic animals (15.5 ± 1.0 mm Hg at 29.3°C compared with 22.0 ± 1.4 mm Hg at 36.8°C), but critical venous oxyhemoglobin saturation values were not statistically different at the two temperatures. Critical TOT was also decreased during hypothermia, as was the margin of reserve in both and TOT (the difference between the normoxic and the critical values).Supported by Grants HL 17731 and HL 07212 from the National Institutes of Health, and by grants from the California Lung Association and the Veterans Administration.The authors acknowledge the valuable technical assistance of Eric Merhoff and J. J. Wright.This work is a portion of: Willford DC. Oxygen transport and utilization during induced hypothermia, PhD Dissertation, University of California, San Diego, 1985. Some of this work was presented in preliminary form at meetings of the American Physiological Society in 1982, 1983, and 1985 [38–40], and at the 1985 symposium, Swine in Biomedical Research [2,41].     

Superior vena caval and mixed venous oxyhemoglobin saturations in children recovering from open heart surgery

Simultaneous superior vena caval (SCvo ₂) and mixed venous ( ) oxyhemoglobin saturation values in 15 children recovering from open heart surgery were compared to assess the value of superior vena caval blood samples in monitoring systemic oxygen supply/demand balance. Samples were obtained immediately following the operation and postoperatively every morning for 4 days. During the 4-day study period, the patients' cardiopulmonary functions improved, allowing partial weaning from respiratory and cardiovascular support. The lowest values of superior vena caval (46.7±8.4%) and mixed venous (63.7±10.9%) oxyhemoglobin saturation were measured immediately after the operation. At this time, 6 patients had abnormally lowSCvo ₂ values, but normal values. BothSCvo ₂ and increased; the difference between them decreasing significantly during the study period (P<0.001). The results show thatSCvo ₂ is consistently lower than in children recovering from open heart surgery. This difference may be secondary to residual intracardiac left-to-right shunting of blood or to altered distribution of systemic blood flow. The saturation difference between the two venous samples decreases during postoperative recovery, making a superior vena caval blood sample an inadequate substitute for a mixed venous blood sample in calculating derived cardiopulmonary variables intended to reflect the function of the body as a whole. BecauseSCvo ₂ was frequently subnormal while was in the normal range, monitoring of could not be reliably used to rule out oxygen supply/demand imbalance during the early postoperative period in these patients.     

Theoretical analysis of oxygen transport during hypothermia

Oxygen transport and delivery to peripheral tissues during hypothermia are analyzed theoretically, taking into consideration various conditions observed both in nature and clinically. With decreasing temperature, P₅₀ (the oxygen tension [Po ₂] at 50% hemoglobin saturation with oxygen) decreases, thereby leading to low mixed venous oxygen tension ( ) and thus low tissuePo ₂ values. On cooling from 37°C to 25°C at pH 7.4, the P₅₀ decreases from a normal 26.8 mm Hg to 13.2 mm Hg. In the intact animal, as well as in a patient on cardiopulmonary bypass, oxygen consumption ( ) and cardiac output ( , or recommended pump flow rate) decrease. If the ratio of remains constant, then the arteriovenous O₂ content difference, , must remain constant. If is 5 ml/dl, we calculate that the must decrease from a normal 40 mm Hg to 26.8 mm Hg at 25°C. Clinically induced hypothermia is usually accompanied by hemodilution of the patient's blood to 50% normal hematocrit, which would reduce to 13.7 mm Hg. Use of constant relative alkalinity (pH=7.58 at 25°C) further reduces the P₅₀ to 10.8 mm Hg and the to 10.9 mm Hg. Other clinical situations are also discussed. Sensitivity analysis predicts that during hypothermia (and thus tissuePo ₂) is very dependent on P₅₀, hemoglobin concentration, and , and less dependent on oxygen solubility and arterialPo ₂. We conclude that monitoring of mixed venous or tissuePo ₂ might be advisable, and that blood flow is the component of oxygen transport most amenable to manipulation by the clinician to ensure adequate tissue oxygenation during induced hypothermia. Supported by Grants HL17731 and HL07212 from the National Institutes of Health, and by a grant from the Veterans Administration.     

Oxygen consumption after cardiac surgery —a comparison between calculation by Fick's principle and measurement by indirect calorimetry

Oxygen consumption calculated by Fick's principle (c ₂) was compared to oxygen consumption measured (m ₂) by indirect calorimetry (Deltatrac Metabolic Computer) in 10 patients in the post-operative period after cardiac surgery. For 50 pairs of measurements the mean difference (m ₂–c ₂) was 34±27ml/min·m². The limits of agreement were –20ml/min·m² to 88ml/min·m². These results showed that c ₂ and m ₂ were not interchangeable in this study.     

Effect of measurement errors on cardiac output calculated with O2 and modified CO2 Fick methods

We have investigated the effect of measurement errors on cardiac output, calculated via three different Fick methods. In method 1, the classic O₂ Fick equation is expressed in terms of oxygen uptake ( ), arterial pulse (Sao₂) and venous oximetry (Svo₂) saturations. The second method, a modified CO₂ Fick method, is obtained by replacing in method 1 with carbon dioxide production ( ) divided by the respiratory quotient. In method 3, cardiac output is expressed as divided by the product of the Sao₂-Svo₂ difference and a constant. This constant is determined from initial measurements of , Sao₂, Svo₂, and thermodilution cardiac output (Q_th). This determination of the constant results in equality of the initial cardiac output of method 3 with the simultaneously determined Q_th and, therefore, is similar to performing an autocalibration. For each of the three preceding Fick methods, we derive general expressions that explicitly show how measurement errors (random and systematic) in the Fick variables ( , , Sao₂, and Svo₂) propagate into errors in calculated cardiac output. The errors in theoretically calculated cardiac output decrease as the Sao₂-Svo₂ difference increases, except for the systematic error in method 3. The systematic error of method 3 is constant and depends only upon the accuracy of the initial Q_th. Analytic expressions for the sensitivity of calculated cardiac output to errors in individual Fick variables are also obtained. Using estimates from the literature for typical systematic and random measurement errors in the Fick variables, the resultant errors in cardiac output are numerically calculated. The effect of random measurement errors on errors in calculated cardiac output was comparable among the three methods. However, the systematic error was least with method 3. Total errors (random and systematic) were comparable among the three methods. Using these numerical measurement errors, we conclude that continuous cardiac output may be calculated with comparable accuracy with each of these methods.     

Oxygen cost of breathing for assisted spontaneous breathing modes: Investigation into three states of pulmonary function

Objective We investigated the effects of continuous positive airway pressure (CPAP) and pressure support ventilation (PSV) on the oxygen cost of breathing ( O₂resp) for different states of pulmonary function. Additionally O₂resp was measured during spontaneous breathing.Design This was done in a controlled and prospective study. Ventilatory modes were applied randomly.Setting Measurements were performed in a quiet room on volunteers (VOL) and inpatients treated for chronic obstructive pulmonary disease (COPD). Post-operative patients after aortocoronary bypass surgery (ACB) were studied on the cardio-thoracic intensive care unit just before and after extubation.Patients Healthy volunteers (n=14), postoperative patients after aorto-coronary bypass surgery (n=15) and patients with COPD (n=9), xFEV₁ 47.7%) were the objects of study.Interventions Demand flow CPAP (5 mbar) and PSV (7 mbar, PEEP 5 mbar), using the Hamilton Veolar ventilator, were investigated in comparison to spontaneous breathing.Measurements and results O₂ measured by a Datex Deltatrac metabolic monitor. O₂resp was calculated by subtraction of total oxygen uptake O₂tot) in controlled mode ventilation (CMV) from that in the respective spontaneous breathing mode. For VOL and COPD patients who were not intubated, a CPAP facemask connected to a short 7.5 mm tube was used as connection to the ventilator. Breathing spontaneously under a canopy system VOL showed a VO₂resp of 4.5±4.0% compared to 9.2±3.5% for ACB and 15.4±7.7% for COPD. CPAP changed the VO₂resp to 7.8±3.9%, 12.0±4.0% and 9.1±3.6% respectively. PSV reduced the O₂resp to 7.9±3.8% in ACB and 7.7±5.5% in COPD.Conclusions This investigation confirms findings that postoperative patients have a mild increase in O₂resp. COPD exhibit the highest increase in VO₂resp. Tracheal tubes, masks and CPAP on a demand flow apparatus increases O₂resp in volunteers and postoperative patients after cardiac surgery. The same amount of CPAP in contrary reduces O₂resp in patients with COPD. Pressure support ventilation can offset the additional O₂resp induced by CPAP but at the same level does not further reduce O₂resp in COPD patients.     

Measurement and interpretation of maximal oxygen uptake in patients with chronic cardiac or circulatory failure

Rapidly responding gas.analyzers have simplified the monitoring of oxygen uptake in the clinical exercise laboratory. An incremental, exhaustive, upright exercise test can be safely used to determine the, plateau in oxygen uptake during exercise, ormaximal , in patients with chronic cardiac or circulatory failure. We define in these patients as an increase in of less than 1 ml/min/kg despite an increment in work load. The value for indicates the patient's aerobic capacity; it also predicts the maximal cardiac output during exercise and therefore serves as an estimate of cardiac reserve and of the severity of cardiac or circulatory failure. Symptom-limited during exercise, termedmaximum oxygen uptake but more appropriatelypeak , bears no relationship to . The two terms should not be used interchangeably.     

Can Capnography Detect Bronchial Flap-Valve Expiratory Obstruction?

Objective. We have previously shown in a mechanical lung model [1] that bronchial flap-valve expiratory obstruction results in sequential lung expiration, best detected by prolonged and low magnitude tracheal expired flow ( ) from the obstructed lung. However, the normal expiratory resistance of clinical ventilation circuits might also generate prolonged, low value exhaled , that could be confused with bronchial flap-valve obstruction. We reasoned that bronchial flap-valve obstruction would also cause sequential CO₂ unloading from each lung and result in a biphasic tracheal capnogram. Methods. To test this hypothesis, we ventilated (V_T, 650 ml; f, 10 br/min) a dual mechanical test lung, with each side connected to a separate alcohol-burning chamber. An airway adapter-monitor system measured airway , P, PCO₂, and FO₂. The circumference of the diaphragm in a respiratory one-way valve was trimmed to generate unidirectional resistance to expiratory . Measurement sequences were repeated after this flap-valve was interposed in the left main-stem bronchus. Results and Discussion. During moderate or severe left bronchial flap-valve obstruction, left bronchial was delayed so that the left lung anatomical dead space (devoid of CO₂) mixed with normal right exhalate to depress the expiratory upstroke or early plateau of the tracheal capnogram. During severe obstruction, decreased perfusion of the left lung caused lower alveolar PCO₂. Then, prolonged low from the left bronchus also resulted in depression of the end of the tracheal alveolar plateau. In general, the low magnitude of bronchial from the obstructed lung limited its effect on the tracheal capnogram and the best marker of sequential lung emptying during bronchial flap-valve obstruction may be late exhaled without reduction in total tidal volume.     

Measurement of tissue perfusion by oxygen transport patterns in experimental shock and in high-risk surgical patients

Survivors of high-risk general (noncardiac) surgery were observed to have cardiac index (CI) values averaging 4.5 l/min·m², oxygen delivery ( O₂) of >600 ml/min·m², and oxygen consumption ( O₂) of 170 ml/min·m². In contrast, these values were relatively normal in patients who subsequently died. A very early predictive index based on these observations was found to predict outcome in 94% of high-risk patients. The hypotheses that increased- O₂ and O₂ in the survivors represent compensatory physiologic responses and that these values were appropriate therapeutic goals were tested in prospective randomized clinical trials and found to reduce mortality and morbidity significantly. The optimal goals were more easily attained with colloids, red cells, dobutamine, and vasodilators, according to their capacity to improve tissue perfusion, as reflected by increased flow and oxygen transport. The extremely complex interactions between- O₂ and O₂ are reviewed.     

Experimental and clinical tests of the oxyconsumeter: A new oxygen uptake monitor

Summary The prototype of a microprocessor controlled oxygen uptake monitor oxyconsumeter developed by Draeger-werk AG, Luebeck, FRG, has been tested. The measuring accuracy of this device was assessed with laboratory bench experiments utilizing both the nitrogen dilution technique and the hydrogen combustion technique to simulate oxygen uptake ( O2). The correlation coefficient between the simulated and the measured O2 values was 0.9989 (p<0.05, n=115). The average relative error of the O2 values was –3.32%±3.88% when breathing 21 vol% oxygen and –5.58%±4.53% for 70 vol% oxygen (percent of reading). This was within the range given by the manufacturer (±5% for 21 vol% to <40 vol%, ±10% for 40 vol% to <70 vol%) with few exceptions. Furthermore the oxyconsumeter was used in clinical experiments to determine oxygen uptake during general anaesthesia. Oxygen uptake was monitored using a non-rebreathing system with an externally triggered expiratory valve. The difference between preanaesthetic reference values and values determined during anaesthesia averaged –24.8±20.1 ml/min/m2 oxygen. This average relative change of –16.0±11.5% was statistically significant in 11 of 15 cases (p<0.05).     

Combined effects of NO inhalation and intravenous PGF2α on pulmonary circulation and gas exchange in an ovine ARDS model

Objectives Inhalation of nitric oxide (NO) selectively dilates pulmonary vessels in well-ventilated regions. Prostaglandin F₂ (PGF₂) is a vasoconstrictor and is reported to enhance hypoxic pulmonary vasoconstriction. The objective of this study was to examine whether the combination of intravenous PGF₂ and inhaled NO in ARDS lungs has a beneficial effect on oxygenation.Design We investigated the effect of intravenous PGF₂ infusion (0.05–10.0 g/kg per min) with and without NO inhalation (60 ppm) on the hemodynamics and gas exchange in an ovine ARDS model, examining the pulmonary artery pressure versure the flow plot by varying cardiac output.Measurements and results After lung lavage, NO inhalation reduced the mean pulmonary arterial pressure (MPAP) by decreasing the zero-flow pressure intercept from 10.6±3.8 (mean±SD) to 8.5±3.8 mmHg (p<0.05) with no significant change in slope. NO inhalation improved PaO₂ from 56±12 to 84±38 mmHg (p<0.005) and reduced pulmonary shunt from 65±5 to 53±8% ( ) (p<0.001). The dose-dependent effects of PGF₂ infusion were: (1) increased MPAP attributed to an increased slope in pulmonary artery pressure-flow plot; (2) decreased cardiac index; (3) decreased with unchanged PaO₂. The dose-dependent decrease in after PGF₂ infusion was attributed to the decreased cardiac output.Conclusions It is suggested that inhalation of NO reduced the critical vascular pressure near alveoli without affecting upstream vessels, while infused PGF₂ constricted the larger upstream pulmonary artery vessels without appreciably affecting the critical pressure. Inhalation of NO into well-ventilated lung areas shifted perfusion to well-oxygenated areas, and there was no supplemental shift in blood flow by adding an infusion of PGF₂.This study was supported by USPHS grant HL 42391 to W.M.Z. and a Kitasato Research Foundation grant to H.K.     

Oxygen consumption and mixed venous oxygen saturation monitoring during orthotopic liver transplantation

Mixed venous oxygen saturation monitoring has been advocated for some critically ill patients. Patients with end-stage hepatic failure have oxygen consumption rates that are lower than normal. Using the Fick equation, oxygen consumption may be calculated if mixed venous and arterial oxygen tensions (and saturations), hemoglobin, and cardiac output are determined simultaneously. This report describes a unique pattern of changes in and oxygen consumption in 7 patients undergoing liver transplantation. A previous study correlated plasma carbohydrate (glucose) levels with early hepatic graft survival. After induction, the 7 patients reported here had low oxygen consumption and high values. The oxygen consumption rates decreased to the lowest point during the anhepatic phase and rose above baseline by the end of the case. The and oxygen consumption data reported here follow the presence of presumed hepatic metabolic activity (increased CO₂ and ionized calcium). Further research must be completed to determine whether these measurements indicate early hepatic nonfunction.     

Oxygen consumption and mixed venous oxygen saturation monitoring during orthotopic liver transplantation

Mixed venous oxygen saturation monitoring has been advocated for some critically ill patients. Patients with end-stage hepatic failure have oxygen consumption rates that are lower than normal. Using the Fick equation, oxygen consumption may be calculated if mixed venous and arterial oxygen tensions (and saturations), hemoglobin, and cardiac output are determined simultaneously. This report describes a unique pattern of changes in and oxygen consumption in 7 patients undergoing liver transplantation. A previous study correlated plasma carbohydrate (glucose) levels with early hepatic graft survival. After induction, the 7 patients reported here had low oxygen consumption and high values. The oxygen consumption rates decreased to the lowest point during the anhepatic phase and rose above baseline by the end of the case. The and oxygen consumption data reported here follow the presence of presumed hepatic metabolic activity (increased CO₂ and ionized calcium). Further research must be completed to determine whether these measurements indicate early hepatic nonfunction.     

Changes in renal vein,renal surface,and urine oxygen tension during hypoxia in pigs

To determine whether ureteral urine oxygen tension could serve as a monitor of renal hypoxia and its relationship to other renal O₂ tension parameters, we simultaneously measured femoral artery (PaO₂), renal vein (Pr O₂), renal surface (PrsO₂), and ureteral urine (PuO₂) oxygen tensions in 8 anesthetized pigs while incrementally decreasing the inspired oxygen concentration (FiO₂) from 21% to 12%. Renal artery blood flow, measured by transit time ultrasound, renal oxygen consumption, and thermodilution cardiac output, was constant. Changes in PaO₂, Pr O₂, PrsO₂, and PuO₂ caused by decreasing FiO₂ were evaluated by one-way analysis of variance. The relationships between PuO₂ and the other O₂ tension parameters were evaluated by correlation coefficient and linear regression statistics. Of six possible O₂ decrements (combinations of 3, 6, and 9%), only Pr O₂ significantly decreased with all six decrements. PuO₂ decreased when FiO₂ decreased 6% or more. PuO₂ is not a sensitive indicator of systemic hypoxia. Under constant renal perfusion and oxygen consumption, PuO₂ had a correlation coefficient of 0.80 and a regression equation of PuO₂=0.84 (Pr O₂)+11.6, with Pr O₂. PuO₂ is related to Pr O₂ when renal perfusion is constant.     

Importance and Interpretation of Fast-Response Airway Hygrometry During Ventilation of Anesthetized Patients

Background Measurement of oxygen uptake () should help detect non-steady state critical events and metabolic derangement during anesthesia. requires measurement of respiratory relative humidity (RH) and temperature (T). We have developed a fast response T and humidity sensor (HS), which uses tiny wet and dry thermometers to determine RH by psychrometry, where low RH causes evaporation to decrease wet T below dry T. In laboratory bench studies, we determined that ≥5 l/min gas flow through the HS is required for valid psychrometry function. This study demonstrates that monitoring of flow through the HS enhances the accuracy of RH measurement and interpretation. Methods Phase One: Laboratory bench validation; We designed a special bench setup for the validation of metabolic gas exchange compared to precise ethanol combustion. Phase 2: Clinical study; During mechanical ventilation of 6 anesthetized surgical patients, airway flow was used to successfully select valid wet T and dry T during inspiration and expiration, from which respective RH’s were calculated using principles of psychrometry. Results The average (±SD) percent error for airway (compared to the stoichiometric value) was −1.84 ± 2.69% (Table 2). The average (±SD) percent error for airway was 0.91 ± 3.10%. Average RQ was 0.649 ± 0.017. For all patients, average inspired RH was 36.1 ± 11.8% (range of 17–52%), which differed significantly from expiration (103 ± 9%). Among the 6–8 consecutive breaths for each patient, average standard deviations of expired RH were only 0.6%. Conclusion We conclude that airway flow monitoring enhances the interpretation and accuracy of the fast-response HS measurements during inspiration and expiration, allowing for the determination of in patients during anesthesia. Rosenbaum A, Breen PH. Importance and interpretation of fast-response airway hygrometry during ventilation of anesthetized patients.     

Inspiratory work imposed by continuous positive airway pressure (CPAP) machines: the effect of CPAP level and endotracheal tube size

Inspiratory work imposed by Continuous Positive Airway Pressure (CPAP) machines has been a matter of concern. The imposed inspiratory work of CPAP machine circuits (Wcir) and the effect of the total breathing apparatus with endotracheal tube (ETT) and connector included in the circuit (Wapp), were measured in three continuous flow (CF) and various configurations of three demand flow (DF) CPAP machines. The performance was assessed at 0, 5, 10 and 15 cmH₂O CPAP using a Michigan Instruments Test Lung Model 1600, internal compliance set at 50 ml/cmH₂O, driven at square wave inspiratory flows ( I) of 20, 40 and 60 l/min at a tidal volume of 500 ml. Work, expressed in mJ/l, was calculated from the area of pressure-volume loops. Inspiratory work, Wcir and Wapp, was dependent upon the particular CPAP machine, I and ETT size, but not upon CPAP level, being maximum at I 601/min and with ETT 7.0 mm i.d. Work values (Wcir) varied from 50 to 325 mJ/l with both CF and DF machines and up to 1100 mJ/l with ETT and connector (Wapp). No consistent advantage of CF over DF machines was demonstrated. There was little advantage of high gas flows (>5 l/min) in various DF circuits. Within an individual machine maximum negative pressures generated during inspiration correlated with both Wcir and Wapp.     

Estimation of cardiac index by means of the arterial and the mixed venous oxygen content and pulmonary oxygen uptake determination in the early post-operative period following surgery of congenital heart disease

Objective To assess the reliability of estimation of cardiac index based on the mixed venous oxygen saturation and methods of improving the estimation of cardiac index.Setting PICU in an university hospital.Design In the post-operative period following complete repair of congenital heart disease we carried out 55 measurements of blood gases in 25 infants and children (mean age 16.1 months, mean body surface 0.43 m²) from a systemic artery (arterial) and the pulmonary artery (mixed venous). We also determined the pulmonary oxygen uptake and calculated the cardiac index (CI) using Fick's principle. In the analysis we compared the CI with the mixed venous oxygen saturation and with the quotient of the arterial oxygen content (C_aO₂) and the oxygen extraction . This quotient is equal to arterial oxygen delivery (DO₂) divided by the oxygen consumption (VO₂).Results Pearson's correlation coefficient was 0.77 when was compared to CI in a linear regression model. Assuming an inverse relationship between and CI the correlation was much better (r=0.90). However, the best estimation of CI provides the quotient .Conclusions correlates much better with CI than the , therefore CI could be better estimated based on . Furthermore provides good information about the oxygen supply situation of the body.     

Continuous intravascular monitoring of PO2 and PCO2. A comparativein vitro-in vivo study

Two electrodes placed at the tip of catheters forin vivo determinations of and respectively, were tested in dogs. Results were satisfactory when compared to a highly accurate reference method, correlation coefficients were close to 1 (P < 10^–9). Means of the differences were respectively –1.74 ± 1.14 torr for the probe (P < 0.01) and –1.62 ± 0.72 torr for the sensor (P < 0.0001). While no drift was detected in the electrode that of the was significant but negligible compared to the variability of measurements. Thus, for values between 20 and 85 torr, and values between 20 and 140 torr,in vivo monitoring is sufficiently reliable for clinical use.This study was supported by a grant-in-aid from I.N.S.E.R.M., C.N.R.S. and Paris VII University.