A Small Bypass Mixing Chamber for Monitoring Metabolic Rate and Anesthetic Uptake: The Bymixer

We developed a miniature mixing chamber, the bymixer, that aids in the measurement of respiratory gas metabolism and inhalation anesthetic uptake. A small fraction of total respiratory gas flow is bypassed from the inspiratory or expiratory limb of the breathing circuit to the bymixer. We tested the relationship between total flow and bypass flow. To analyze the error and response time of the system, we compared the mixed expired carbon dioxide from the bymixer (capacity, 0.3 L, ratio of bypass flow to total flow, 1/4.5) with that from a conventional 2.0-L mixing chamber in 15 volunteers and 12 anesthetized patients during spontaneous or controlled ventilation. Bypass flow correlated well with total flow (r2 = 0.999 to 1.000) when total flow ranged from 0 to 70 L/ min and the ratio of bypass flow to total flow ranged from 1/3 to 1/20. The difference between the values of mixed expired carbon dioxide from the two mixing chambers was small, ranging from?0.03 to 0.01 vol% in both of the ventilatory modes. The relative error was within 2.3% of the carbon dioxide values obtained from the conventional chamber. We observed the error at the lowest minute volume (4 L/min). The 90% response time of the bymixer (24.8 seconds) was similar to that of the conventional chamber (19.1 seconds) at a minute volume of 7.5 L/min. For clinical use, we combined a conventional breathing circuit with two bymixers, one for mixing inspired gas and the other for mixing expired gas. We found this device accurate and easy to use.     

A nomogram for rapid calculation of metabolic requirements on intubated patients

There is a need for estimating the calorific requirements of patients undergoing or about to undergo total parenteral nutrition (TPN) other than by complicated direct calorimetry or by guesswork. We describe a simple, cheap, indirect calorimetric method for determining energy requirements from the measurement of mixed expired carbon dioxide tension (P_ĒCO₂) in patients who are intubated, and in whom the breathing circuit characteristics allow collection of pure expired gas. This can be achieved by collection of expired gas from ventilators where an on-demand fresh gas flow rather than a continuous flow occurs during spontaneous or intermittent positive pressure ventilation, such as with the Siemens Servo 900C.     

Functional evaluation of a CPAP circuit with a high compliance reservoir bag

Continuous positive airway pressure is widely used in the treatment of ARF and an evaluation of the systems is important. The authors used an artificial model to test a continuous flow system with a high compliance reservoir bag. The results confirm that the system is effective in maintaining positive pressure stability within a wide range of inspiratory peak flow rates, even when a low fresh gas flow rate is employed. Nevertheless, rebreathing of expired gases is possible and may be noticeable at high expiratory flow rates, caused by the high compliance of the reservoir bag.Partially presented at the XXXVth National SIAARTI Congress, Venice, 22–25 September 1983     

Optimizing fresh gas flow and circuit design for the delivery of continuous positive airway pressure

OBJECTIVE: To examine the effect of varying circuit design and the fresh gas flow rate on the circuit work imposed by a continuous positive airway pressure (CPAP) circuit. DESIGN: Circuit work was measured during simulated inspiration (500 mL) with a lung model at inspiratory flow rates (V) of 40, 60, and 80 L/min during the administration of 10 cm H2O CPAP through either a modified Mapleson-A or modified Mapleson-D circuit, both alone and when connected to a face mask (i.e., simulating an intubated and nonintubated patient). Fresh gas flow was varied from 10 to 250 L/min. RESULTS: The minimum circuit work occurred at a fresh gas flow rate approximating V; however, circuit work was consistently lower for the modified Mapleson-A circuit compared with the modified Mapleson-D circuit. As the fresh gas flow rate was increased sequentially to 250 L/min, circuit work remained close to the minimum value for the modified Mapleson-A, but increased gradually with the modified Mapleson-D, e.g., from 0.017 kg.m/L at a fresh gas flow rate and V of 80 L/min to 0.035 kg.m/L at a fresh gas flow rate of 250 L/min and a V of 80 L/min. Rotation of the fresh gas flow inlet did not change the circuit work vs. fresh gas flow rate relationship. Addition of a face mask resulted in a smaller increase in circuit work for the modified Mapleson-D with increasing fresh gas flow rate. However, unlike the modified Mapleson-A circuit alone, the addition of a mask caused circuit work to increase with increasing fresh gas flow rate. CONCLUSIONS: The modified Mapleson-A circuit at a fresh gas flow rate equal to V minimizes circuit work, and hence represents an optimal CPAP circuit. The increases in circuit work at fresh gas flow rates above V that were found with the modified Mapleson-D circuit are not due to inertial differences, and are likely due to turbulent gas flow.     

A pilot study quantifying the shape of tidal breathing waveforms using centroids in health and COPD

During resting tidal breathing the shape of the expiratory airflow waveform differs with age and respiratory disease. While most studies quantifying these changes report time or volume specific metrics, few have concentrated on waveform shape or area parameters. The aim of this study was to derive and compare the centroid co-ordinates (the geometric centre) of inspiratory and expiratory flow–time and flow–volume waveforms collected from participants with or without COPD. The study does not aim to test the diagnostic potential of these metrics as an age matched control group would be required. Twenty-four participants with COPD and thirteen healthy participants who underwent spirometry had their resting tidal breathing recorded. The flow–time data was analysed using a Monte Carlo simulation to derive the inspiratory and expiratory flow–time and flow–volume centroid for each breath. A comparison of airflow waveforms show that in COPD, the breathing rate is faster (17 ± 4 vs 14 ± 3 min^?1) and the time to reach peak expiratory flow shorter (0.6 ± 0.2 and 1.0 ± 0.4 s). The expiratory flow–time and flow–volume centroid is left-shifted with the increasing asymmetry of the expired airflow pattern induced by airway obstruction. This study shows that the degree of skew in expiratory airflow waveforms can be quantified using centroids.     

A recruitment manoeuvre performed after endotracheal suction does not increase dynamic compliance in ventilated paediatric patients: a randomised controlled trial

QUESTION: Does a recruitment manoeuvre after suctioning have any immediate or short-term effect on ventilation and gas exchange in mechanically-ventilated paediatric patients? DESIGN: Randomised controlled trial with concealed allocation, assessor blinding, and intention-to-treat analysis. PARTICIPANTS: Forty-eight paediatric patients with heterogeneous lung pathology. Fourteen patients were subsequently excluded from analysis due to large leaks around the endotracheal tube. INTERVENTION: The experimental group received a single standardised suctioning procedure followed five minutes later by a standardised recruitment manoeuvre. The control group received only the single suctioning procedure. OUTCOME MEASURES: Measurements of ventilation (dynamic lung compliance, expiratory airway resistance, mechanical and spontaneous expired tidal volume, respiratory rate) and gas exchange (transcutaneous oxygen saturation) were recorded, on three occasions before and on two occasions after the recruitment manoeuvre, using a respiratory profile monitor. RESULTS: There was no difference between the experimental and the control group in dynamic compliance, expired airway resistance, or oxygen saturation either immediately after the recruitment manoeuvre, or after 25 minutes. The experimental group decreased mechanical expired tidal volume by 0.3 ml/kg (95% CI 0.1 to 0.6), increased spontaneous expired tidal volume by 0.3 ml/kg (95% CI 0.0 to 0.6), and increased total respiratory rate by 3 bpm (95% CI 1 to 4) immediately after the recruitment manoeuvre compared with the control group, but these differences disappeared after 25 minutes. CONCLUSION: There is insufficient evidence to support performing recruitment manoeuvres after suctioning infants and children.     

Optimal phasic tracheal gas insufflation timing: an experimental and mathematical analysis

OBJECTIVE: To investigate the modulation of CO2 clearance by changes in the duration of tracheal gas flow application during tracheal gas insufflation (TGI). DESIGN: Combination of bench studies using a commercial test lung and a commercially available intensive care ventilator and mathematical analysis using a clearance model derived from first principles. SETTING: University pulmonary research laboratory. PATIENTS: None. INTERVENTIONS: Experiments using TGI were performed on a test lung at two combinations of tidal volume and frequency. TGI was limited to part of the expiratory phase (the terminal 10-100% of expiration), and two different TGI catheter flow rates were studied. Permutations over a range of compliances, dead-space volumes, catheter flows, and TGI durations were collected. A mathematical model incorporating key ventilatory and TGI-related variables was developed to provide a first-principles theoretical foundation for interpreting the experimental results. MEASUREMENTS AND MAIN RESULTS: In the physical model, alveolar Pco2 attained a minimum value with TGI flow applied during the terminal 40-60% of the expiratory phase, a finding that was consistent over an almost eight-fold range of expiratory time constants. The mathematical model shows the same qualitative pattern as the experimental model, indicating that the observed behaviors are not an experimental artifact. CONCLUSION: The optimal duration of expiratory TGI flow application is stable over a wide range of impedance characteristics. Such stability suggests that near maximal effect of expiratory TGI could be obtained by applying TGI flow solely within the final 50% of the expiratory phase. Such uniform restriction of the application profile might both simplify technique implementation and decrease adverse consequences.     

Sampled gas need not be returned during low-flow anesthesia

Objective. The purpose of this investigation was to study the N₂ flux between the patient and the breathing circuit, and the excess gas during N₂O anesthesia with the low, fresh gas flow technique.Methods. Forty patients were studied. After a 6-minute high, fresh gas flow denitrogenation period, the O₂ fresh gas flow was set at about 4 ml/kg/min and the N₂O fresh gas flow was set to maintain an inspired O₂ fraction of 0.30. The excess gas flow and N₂ excretion were measured by a variant of the Douglas bag method.Results. The mean inspired N₂ concentration reached a peak of 5.9% at 40 minutes. The estimated mean N₂ excretion was 39 ml/min at 10 minutes, declining to 18 ml/min at 60 minutes. A calculation of N₂ homeostasis during closed-circuit anesthesia based on the results of the patient study indicated that sampling for gas analysis actually reduces the gas costs if the sampled gas is scavenged instead of returned to the circle system, since intermittent flushing with high, fresh gas flow for denitrogenation is unnecessary in the former situation.Conclusions. Regardless of the fresh gas flow used, sampled gas need not be returned during N₂O anesthesia.Financial support was provided by AGA AB Medical Research Fund (supplier of O₂ and N₂O).     

Performance of a hydrophobic heat and moisture exchanger at different ambient temperatures

Objective To evaluate the effect of different room temperatures on hydrophobic heat and moisture exchangers (HME) humidifying capability and efficiency.Methods Stock HMEs were tested in vitro using an already described test model, with separation of inspiratory and expiratory gas. Absolute humidity (AH) was measured by means of dry-wet dual thermocouple, and HME efficiency was computed as the ratio between expired to inspired. AH, at room termperature of 20 and 26°C.Results Inspired gas temperature and AH were significantly higher at 26 than at 20°C; since expired AH remained substantially unchanged, HME efficiency was also higher in warmer environment.Conclusions Hydrophobic HMEs appear to be affected by room temperature, increasing their humidifying ability and their efficiency with its rise.     

Accuracy of expiratory carbon dioxide measurements using the coaxial and circle breathing circuits in small subjects

Mass spcctrometry is widely used to measure the end-tidal concentrations of inhalation anesthetics and other gases during surgery in order to estimate their arterial concentrations. When certain breathing circuits are used in newborns, however, fresh gas or ambient air may contaminate the expired sample, introducing a systematic error in the measurement of any end-tidal gas concentration. We estimated this error in newborn piglets using carbon dioxide as an indicator substance of expired gas. The capnograms and the difference between arterial carbon dioxide tension (PaCO₂) and peakexpired carbon dioxide tension (PeCO₂) were compared when either a coaxial (Bain) or circle breathing circuit was used. Gas was sampled from the proximal airway and distal trachea. No combination of circuit and sampling site produced a flat alveolar phase until the circle circuit was modified with diversion valves to reduce gas mixing. The mean PaCO₂-PeCO₂ gradients using the coaxial/proximal sampling, coaxial/distal sampling, and modified circle/proximal sampling circuits were 12.4, 9.2, and 8.8 mm Hg, respectively. The mean PeCO₂ in each of these combinations was significantly different from the corresponding mean PaCO₂ (p<0.05). Using the modified circle circuit with distal sampling, mean PeCO₂ was not significantly different from mean PaCO₂: the mean PaCO₂-PcCO₂ gradient was 2.2 ± 0.2 mm Hg (SEM), range, 0 to 6 mm Hg, with 95% confidence limits ⩽ 8 mm Hg. When a coaxial breathing circuit is used in small subjects, PaCO₂ may be significantly underestimated regardless of sampling site, although the circle breathing circuit with distal tracheal sampling yields accurate results. Supported in part by BRS Grant SO RR05507-20 from the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health, and by the American Heart Association, Lancaster, PA Chapter. The authors thank Robert Hirsch, PhD, for his statistical advice, and Greg Harris and Perkin-Elmer, Inc for loaning the mass spectrometer.     

In vitro validation of a metabolic monitor for gas exchange measurements in ventilated neonates

OBJECTIVE: To evaluate the Datex Deltatrac II for measurements in neonates requiring mechanical ventilation. DESIGN: Prospective laboratory evaluation, using a ventilated lung model and gas injection. During simulation of 79 neonatal respiratory settings, assessment of oxygen consumption (VO2), carbon dioxide production (VCO2) and respiratory quotient (RQ) was compared to a reference method (mass spectrometry, wet gas spirometry) using the statistical method of Bland and Altman. INTERVENTIONS: Respiratory variables, which may influence the accuracy and precision of gas exchange measurements, were varied within the following ranges: inspired oxygen fraction (FIO2): 0.21-0.8, expired carbon dioxide fraction (FECO2) and inspiratory-expiratory oxygen fraction (DFO2): 0.0032-0.0256, expiratory flow rate: 1.0-2.5 l/min, inspiratory pressure: 10-55 mbar, respiratory rate 25-60/min, constant RQ of 1. This resulted in 79 tests with VCO2 and VO2 ranging from 8-64 ml/min. MEASUREMENTS AND RESULTS: The coefficient of repeatability for ten single subsequent Deltatrac measurements was 8.09 ml/min for VO2 and 9.17 ml/min for VCO2 compared to 2.02 ml/min and 0.90 ml/min for VO2 and VCO2 with repeated reference measurements. The coefficient of repeatability of the Deltatrac measurements improved considerably when means of subsequent 5 min intervals were compared: 0.68 ml/min for VO2 and 0.28 ml/ min for VCO2. The difference between the two methods (Deltatrac-reference) was -3.8 % (2 s: 11.4%) for VO2, 13.2% (2s: 7.9%) for VCO2 and 17.6% (2s: 16.7%) for RQ. The agreement between methods deteriorated with smaller (FECO2) or DFO2 and increasing FIO2. CONCLUSIONS: Considering limits of agreement of less than +/- 20% as clinically acceptable, results for VO2 assessment indicate acceptable accuracy and precision whereas VCO2 and RQ assessments exceed this limit. Limited accuracy and precision result from detection of CO2 following dilution of expiratory gases and increased sensitivity to error propagation by Haldane equations due to the small differences between inspiratory and expiratory gas fractions.     

Upper Airway Obstruction--A Report on Sixteen Patients

In sixteen patients with upper airway obstruction, breathlessnesswas a symptom in all with maximum mid vital capacity flow ratesin inspiration or expiration of 1·7 litres per secondor less. With one exception, all these patients had stridor.The stridor was inspiratory in nine, expiratory in one and bothinspiratory and expiratory in two. There was no diagnostic difficulty in the twelve patients withextrathoracic airway obstruction and in this group tests ofinspiratory flow (forced inspired volume in one second, peakinspiratory flow or maximum mid inspiratory flow) were of mostvalue in following the progression of the disease and the responseto treatment. Flow volume loops were particularly useful whereextrathoracic obstruction and diffuse intrapulmonary airwayobstruction co-existed. The two patients with intrathoracic upper airway obstructioncaused considerable difficulty with diagnosis and both wereinitially thought to have, and treated unsuccessfully for, asthma.In each patient flow volume loops showed a low flow expiratoryplateau, diagnostic of severe intrathoracic airway obstructionbut recorded in the absence of any clinical or radiographicfeatures of emphysema. An obstructing lesion of the intrathoracictrachea was therefore suspected and this was confirmed by trachealtomography. In one patient serial expiratory flow volume curvesdemonstrated the combination of intrathoracic upper and lowerairway obstruction. Two patients had tracheal stenosis in the region of the suprasternalnotch. Each showed a characteristic twin humped expiratory flowvolume curve and in one patient the stenosis was demonstratedboth physiologically and radiologically to move in and out ofthe thorax. The importance of a standard posture during serialmeasurements is emphasized. The ratio of forced expired volume in one second measured inmillilitres, to the peak expiratory flow measured in litresper minute, was of limited value in differentiating upper fromlower airway obstruction in these patients. It is concluded that upper airway obstruction is likely to becomemore common and that respiratory function tests, in particularthe flow volume loop, play an essential part in the recognitionand management of this problem.     

Animal and lung model studies of tracheal gas insufflation

Tracheal gas insufflation (TGI) is the continuous or phasic insufflation of fresh gas into the central airways for the purpose of improving the efficiency of alveolar ventilation and/or minimizing the ventilatory pressure requirements. Fresh gas is insufflated near the main carina, usually at flow rates of 2-15 L/min. During expiration, TGI clears the anatomic and apparatus dead space proximal to the catheter tip, thus improving carbon dioxide (CO2) clearance. Moreover, at high catheter flow rates turbulence generated at the tip of the catheter may enhance distal gas mixing. CO2 elimination during TGI depends on catheter flow rate, as at higher flow rates a greater portion of the proximal dead space is flushed clear of CO2. Consequently, as TGI flow is increased, arterial carbon dioxide tension (PaCO2) decreases. Eventually, with increasing catheter flow rate, fresh gas completely flushes the available dead space during expiration and the PaCO2 reaches a plateau. At that point, increasing catheter flow rate decreases PaCO2 much less, probably because of turbulent mixing in the airways distal to the catheter tip. In clinical practice, TGI can be applied either to decrease PaCO2 while maintaining tidal volume constant or to decrease tidal volume while keeping PaCO2 constant. In the former strategy, TGI is used to protect pH, whereas in the latter it is used to minimize the stretch forces acting on the lung parenchyma, to minimize ventilator-associated lung injury.     

Expiratory function in complete tetraplegics: study of spirometry, maximal expiratory pressure, and muscle activity of pectoralis major and latissimus dorsi muscles.

Respiratory complications, such as pneumonia and atelectasis, are major causes of mortality and inhibit rehabilitation programs in spinal cord injury. Tetraplegic patients cannot cough enough to clear their sputum because of expiratory muscle weakness, mainly of the abdominal muscles. However, tetraplegics are still able to activate some muscles during coughing. Some tetraplegics, even though they cannot contract the abdominal muscles, can cough effectively. It was supposed that some accessory expiratory muscles were activated during coughing in tetraplegics. We, therefore, studied the peak expiratory flow rate, expiratory muscle strength, and the activities of the pectoralis major and latissimus dorsi muscles in 11 complete tetraplegics. Peak expiratory flow rate was measured by spirometry. Expiratory muscles strength was assessed by maximal expiratory mouth pressure; muscle activity was assessed by means of the root mean square voltage obtained by surface electromyography. The results showed that peak expiratory flow rate, maximal expiratory mouth pressure, and root mean square of these two muscles were correlated with neurological level. Peak expiratory flow rate was correlated with peak expiratory flow rate. Peak expiratory flow rate was correlated with the root mean square voltage of the pectoralis major and latissimus dorsi muscles. It was supposed that these two muscles were activated as accessory expiratory muscles and play an important role in expiratory function in tetraplegic patients.     

Continuous indirect calorimetry--a laboratory simulation of a new method for non-invasive metabolic monitoring of patients under anaesthesia or in critical care

A method was tested which permits continuous real time monitoring of O(2) uptake in patients attached to a breathing system. The method is an indirect calorimetry technique which uses fresh gas rotameters for control, regulation and measurement of the gas flows into the system, with continuous sampling of mixed exhaust gas. Testing of this approach was conducted using a lung gas exchange simulator, in order to determine its accuracy and precision under controlled conditions, when compared to a range of simulated O(2) uptake values. The overall mean bias (standard error) was -1.3 mL min(-1) (0.3) and the standard deviation was 6.5. The performance of the method was found to be consistent across a wide range of fresh gas flow rates and O(2) concentrations from 30 to 80%. The method warrants in vivo testing under clinical conditions.     

Volume dependence of respiratory system resistance during artificial ventilation in rabbits

The volume dependence of respiratory resistance (Rrs), usually observed during normal breathing, is expected to be accentuated during expiratory flow limitation (EFL). In order to quantify this dependence we studied the pressure, flow, and volume data obtained from eight New Zealand rabbits, artificially ventilated at different levels of applied expiratory pressure (0-10 hPa), before and during histamine i. v. infusion. EFL was provoked by lowering the expiratory pressure and was detected by the application of an additional negative expiratory pressure and by forced oscillations. The analysis of respiratory system mechanics was performed by multiple regression, using the classical linear first-order model and also a nonlinear model, accounting for volume dependence of Rrs. Both models satisfactorily fitted the data in the absence of EFL. The nonlinear model proved to be more appropriate in the presence of EFL. The coefficient expressing the volume dependence of Rrs (Rvd) was significantly more negative during EFL. Rvd values were highly correlated with the fraction of the tidal volume left to be expired at the onset of EFL. A threshold Rvd value of -1,000 (hPa x s x l(-2)) detected EFL with high sensitivity and specificity. We conclude that a strongly negative volume dependence of Rrs is a reliable and noninvasive index of EFL during artificial ventilation.     

Assessment of errors when expiratory condensate PCO2 is used as a proxy for mixed expired PCO2 during mechanical ventilation

Objectives. We designed a series of experiments to determine whetherexpiratory water condensate (PconCO2) can be used as a proxyfor mixed expired gas collection. Methods. In 18 adult mechanically ventilatedpatients with ARDS (40 samples), simultaneous collections of arterial blood,expiratory water trap condensate, mixed expired gas, and minute ventilationwere used to calculate VCO2 and VD/VT. To assess theeffect of temperature, a constant gas flow (PCO2 10-30mm Hg) was bubbled through water at temperatures of 19.5-37 °C. Gasand water samples were collected, immediately analyzed forPCO2, and a temperature correction factor was calculated. Alung model was constructed using a 5 L anesthesia bag connected to amechanical ventilator with a heated humidifier. Temperature at the Y-piece wasset to 37 °C and CO2 was injectedinto the bag to establish an end-tidal PCO2 of 20-70 mmHg. After equilibration, condensate was collected, PCO2 wasmeasured, and the temperature-corrected PCO2 was compared toPCO2. The capnogram at points along the expiratory limbcircuit was used to evaluate gas mixing. Results. There was anover-estimation of PCO2 by PconCO2 (p <0.001) for the patient data, resulting in an underestimation of VD/VT (p <0.001) and an overestimation of VCO2 (p < 0.001). Thetemperature correction factor for PCO2 in water was–0.010 (about half of the factor used for whole blood). The bias betweentemperature-corrected PconCO2 and PCO2 was0.3 ± 3.2 mm Hg in the lung model. Mixing in the expiratory limb waspoor, as evaluated by the capnogram. Conclusions. Even with temperaturecorrection, we failed to precisely predict PCO2 fromPconCO2. For measurement of VD/VT andVCO2, we do not recommend methods that usePconCO2.     

Expiratory flow limitation and the response to breathing a helium-oxygen gas mixture in a canine model of pulmonary emphysema

The pathophysiology of reduced maximum expiratory flow in a canine model of pulmonary emphysema was studied, and the results interpreted in terms of the wave-speed theory of flow limitation. According to this theory, maximum expiratory flow is related both to the cross-sectional area and compliance at an airway site where a critical gas velocity is first reached ("choke-point") and to gas density. Pulmonary emphysema was produced by the repeated instillations of the enzyme papain into the airways of six dogs. In five control dogs, a saline solution was instilled. During forced vital capacity deflation, in an open-chest preparation, maximum expiratory flow, choke-point locations, and the response to breathing an 80:20 helium/oxygen gas mixture were determined at multiple lung volumes. To locate choke-points, a pressure measuring device was positioned in the airway to measure lateral and end-on intrabronchial pressures, from which the relevant wave-speed parameters were obtained. In general, the reduced maximum expiratory flow in emphysema can be explained by diminished lung elastic recoil pressure and by altered bronchial pressure-area behavior, which results in a more peripheral location of choke-points that have smaller cross-sectional areas than controls. With respect to the density dependence of maximum expiratory flow, this response did not differ from control values in four dogs with emphysema in which frictional pressure losses upstream from choke-points did not differ on the two gas mixtures. In two dogs with emphysema, however, upstream frictional pressure losses were greater on helium/oxygen than on air, which resulted in a smaller cross-sectional area on helium/oxygen; hence density dependence decreased.     

Peak expiratory flow rate: relationship to risk variables and mortality: the Wisconsin Epidemiologic Study of diabetic retinopathy.

OBJECTIVE: To examine correlates of peak expiratory flow rate in people with type 1 diabetes and to evaluate the relationship of peak expiratory flow rate to mortality. RESEARCH DESIGN AND METHODS: A cohort study that was originally designed to determine the prevalence, incidence, and severity of diabetic retinopathy also provided the opportunity to measure peak expiratory flow rate. This was first measured at a 10-year follow-up and was evaluated in regard to risk factors for microvascular complications of diabetes. Mortality during 6 years of follow-up after the measurement was also ascertained. RESULTS: In multivariable analysis, peak expiratory flow rate was associated with sex, age, height, BMI, history of cardiovascular disease, pulse rate, duration of diabetes, glycosylated hemoglobin, and end-stage renal disease. Peak expiratory flow rate was significantly associated with survival in categorical analyses. Even after considering age, sex, renal disease, history of cardiovascular disease, respiratory symptoms, duration of diabetes, cigarette smoking, and hypertension, peak expiratory flow rate was still significantly related to survival (hazard ratio 0.61 [95% CI 0.46-0.82]). CONCLUSIONS: These data indicate that peak expiratory flow rate is associated with risk factors for other complications of diabetes. In addition, peak expiratory flow rate is a significant predictor of survival over even a relatively short period of time (6 years) in patients with younger-onset diabetes.     

Monitoring pulmonary function with superimposed pulmonary gas exchange curves from standard analyzers

Objective.A repetitive graphic display of the single breath pulmonary function can indicate changes in cardiac and pulmonary physiology brought on by clinical events. Parallel advances in computer technology and monitoring make real-time, single breath pulmonary function clinically practicable. We describe a system built from a commercially available airway gas monitor and off the shelf computer and data-acquisition hardware. Methods.Analog data for gas flow rate, O₂, and CO₂ concentrations are introduced into a computer through an analog-to-digital conversion board. Oxygen uptake (VO₂) and carbon dioxide output (VCO₂) are calculated for each breath. Inspired minus expired concentrations for O₂ and CO₂ are displayed simultaneously with the expired gas flow rate curve for each breath. Dead-space and alveolar ventilation are calculated for each breath and readily appreciated from the display. Results.Graphs illustrating the function of the system are presented for the following clinical scenarios; upper airway obstruction, bronchospasm, bronchopleural fistula, pulmonary perfusion changes and inadequate oxygen delivery. Conclusions.This paper describes a real-time, single breath pulmonary monitoring system that displays three parameters graphed against time: expired flow rate, oxygen uptake and carbon dioxide production. This system allows for early and rapid recognition of treatable conditions that may lead to adverse events without any additional patient measurements or invasive procedures. Monitoring systems similar to the one described in this paper may lead to a higher level of patient safety without any additional patient risk.