Evaluation of an apparatus for continuous monitoring of gas exchange in mechanically ventilated patients

Indirect calorimetry has been integrated with a commercially available ventilator, permitting continuous monitoring of energy expenditure and respiratory quotient. This equipment, the ventilator Erica, the Engström metabolic computer EMC and the carbon dioxide analyzer Eliza (Gambro Engström AB, Sweden) was evaluated in laboratory tests and in critically ill patients. In the laboratory evaluation, the variability in FIO₂ was less than 0.05% at FIO₂<0.47. The linearity error of the oxygen sensor and the CO₂ analyzer was less than 0.06%. The variability of the oxygen sensor was 0.01% O₂ and of the CO₂ analyzer 0.04% CO₂. The CO₂ analyzer was sensitive to oxygen concentration and underestimated CO₂ concentration by 3.3% at FIO₂=0.5 if calibrated on room air. The pneumotachometer was oxygen dependent and overestimated inspired volume by 5.5% at 100% O₂, if calibrated on room air, and was negligibly affected by changes in minute volume or PEEP within the physiological range for adults. In the patient evaluation, 15 measurements of gas exchange were performed in 9 critically ill patients and the results were compared with data obtained simultaneously by the Scholander technique or mass spectrometry. The standard deviation of the mean difference in comparison with the reference methods was 9.4% for O₂ uptake, 6.6% for CO₂ elimination, 12.5% for respiratory quotient and 3.9% for minute volume. Paired t-test analysis showed no systematic difference between the various methods.It is concluded that the EMC has an accuracy sufficient for clinical use and represents a commercially available technique of potential clinical value.     

Energy expenditure in the acute renal failure patient mechanically ventilated

Twenty mechanically ventilated patients with acute renal failure were studied on 31 occasions to determine their energy expenditure (EE) during a 2 h period before a hemodialysis. Oxygen consumption and CO₂ elimination were measured continuously with a mass spectrometer system. EE (1660±48 kcal day^-1) was close to the total caloric intake (1682±83 kcal day-1) and represented 1.19±0.03 times the predicted resting energy expenditure (PREE) with large inter-individual variations (0.7-1.7 PREE). EE/PREE was higher when sepsis was present (1.31±0.03 versus 1.14±0.02; p<0.05). Glucose oxidation rate (4.35 mg kg^-1 min^-1) exceeded glucose intake (2.6 mg kg^-1 min^-1). Respiratory quotient was 1.02±0.01. Nitrogen loss was 17.3±1.7 g day^-1 and nitrogen balance-11.9±1.9 g day^-1. In conclusion, EE values were scattered but never exceeded 1.7 times the PREE. Sepsis increased EE. With a nutritional support covering EE, nitrogen balance remained markedly negative and a preferential utilisation of glucose and lipogenesis occurred.     

Comparison of measured and predicted energy expenditure in mechanically ventilated children

Objective: To determine the energy requirements in mechanically ventilated pediatric patients using indirect calorimetry and to compare the results with the predicted metabolic rate. Design: In 50 mechanically ventilated children with a moderate severity of illness, energy expenditure was measured by indirect calorimetry. Daily caloric intake was recorded for all patients. Total urinary nitrogen excretion was determined in 31 patients. Results: Although there was a close correlation between the measured total energy expenditure (mTEE) and the predicted basal metabolic rate (pBMR) (r = 0.93, p < 0.001), Bland–Altman analysis showed lack of agreement between individual mTEE and pBMR values. The ratio of caloric intake/mTEE was significantly higher in the patients with a positive nitrogen balance (1.4 ± 0.07) compared with those with a negative nitrogen balance (0.8 ± 0.1; p < 0.001). Conclusions: Standard prediction equations are not appropriate to calculate the energy needs of critically ill, mechanically ventilated children. Individual measurements of energy expenditure and respiratory quotient by means of indirect calorimetry in combination with nitrogen balance are necessary for matching adequate nutritional support. Received: 27 June 1997 Accepted: 12 February 1998     

Comparison between the Datex-Ohmeda M-COVX metabolic monitor and the Deltatrac II in mechanically ventilated patients

OBJECTIVE: To compare the M-COVX and the Deltatrac II metabolic monitors under clinical conditions. DESIGN: Prospective clinical comparison. SETTING: A general Intensive Care Unit of a university hospital. PATIENTS: Twenty mechanically ventilated critically ill patients. INTERVENTIONS: The monitors were compared at FiO(2) 0.3, 0.5, and 0.7 in each patient where possible. MEASUREMENTS AND RESULTS: Pulmonary gas exchange measurements were recorded using the two monitors sequentially (Deltatrac(before), M-COVX, Deltatrac(after)). Each measurement consisted of five consecutive 1-min readings of VO(2) and VCO(2). We compared the Deltatrac(before) with the Deltatrac(after) and the mean of the Deltatrac with the M-COVX. There was no clinically significant bias between the two monitors for VO(2) or VCO(2) but the limits of agreement (LOA) were wide (bias +/-95% LOA: VCO(2) -13 +/- 30 ml/min, -8 +/- 36 ml/min, 7 +/- 50 ml/min; VO(2) -7 +/- 50 ml/min, -5 +/- 56 ml/min, 6 +/- 64 ml/min, at FiO(2) 0.3, 0.5, and 0.7, respectively). The Deltatrac before and after measurements displayed good agreement for VCO(2) but poorer agreement for VO(2) (bias +/- 95% LOA: VCO(2) 0 +/- 18 ml/min, -6 +/- 16 ml/min, -1 +/- 12 ml/min; VO(2) 2 +/- 12 ml/min, 3 +/- 38 ml/min, 10 +/- 42 ml/min, at FiO(2) 0.3, 0.5, and 0.7, respectively). Using within-patient standard deviation as a measure of reproducibility suggested that for VO(2) the M-COVX performed better than the Deltatrac at high FiO(2), and for VCO(2) Deltatrac was better at lower FiO(2). CONCLUSIONS: The M-COVX is a suitable integrated device for measuring metabolic gas exchange in ventilated patients.     

Measuring the breathing workload in mechanically ventilated patients

Conclusion All the methods for the evaluation of the breathing workload in mechanically ventilated patients are subject to serious theoretical criticisms, due to the numerous assumptions and approximations on which they are based. Nevertheless they are of great interest in providing an objective evaluation of the ability of different forms of partial support ventilation to reduce the breathing workload. They have been applied successfully to assist-controlled ventilation [29, 30], SIMV [31], and pressure support ventilation [10, 11].Supported by a grant INSERM CNEP 89 CN 24     

The effect of neuromuscular blockade on oxygen consumption in sedated and mechanically ventilated pediatric patients after cardiac surgery

Objective  To measure the effect of intense neuromuscular blockade (NMB) on oxygen consumption (VO₂) in deeply sedated and mechanically ventilated children on the first day after complex congenital cardiac surgery. Design  Prospective clinical interventional study. Setting  Pediatric intensive care unit of an university medical centre. Measurements and results  Nine mechanically ventilated and sedated children (weight 2.8–8.7 kg) were included. All children were treated with vasoactive drugs. The level of sedation was quantified using the comfort score, Ramsay score and bispectral index (BIS). The intensity of NMB was quantified using acceleromyography and VO₂ was measured using indirect calorimetry. Analgo-sedation using various intravenous agents was targeted at a deep level (comfort score < 18, BIS < 60 and Ramsay score > 4). NMB was achieved by intravenous administration of rocuronium. All measurements were conducted before, during and after recovery from a period of intense NMB. Baseline values were VO₂ 6.1 ml/(kg min) (SD 1.3), comfort score 13 (SD 0.7), BIS 42.5 (SD 14.2), mean blood pressure 54.0 mmHg (SD 10.5), mean heart rate 129.9 bpm (SD 28.9) and mean core temperature 36.7°C (SD 0.5). There were no significant differences in VO₂ or other parameters between baseline, during NMB and the recovery phase. Conclusion  Neuromuscular blocking agents do not reduce oxygen consumption in deeply sedated and mechanically ventilated children after congenital cardiac surgery.     

机械通气患者贫血的临床观察研究

目的 观察贫血对机械通气患者预后的影响.方法 采用前瞻性研究方法,收集入住重症监护病房(ICU)预计机械通气时间≥72 h,血红蛋白(Hb)浓度≥100 g/L的患者.根据患者机械通气第3日的Hb浓度分为贫血组和非贫血组.比较两组患者1、3、7d血清促红细胞生成素(EPO)、Fe3+、转铁蛋白(TRF)水平,14 d 内人均输血量,3、7、14 d内人均日采血量以及机械通气时间、28 d脱机存活率、住院时间和28 d病死率.结果 共入选40例患者,贫血组18例,非贫血组22例.与非贫血组比较,贫血组患者血清Fe3+较低,血清EPO、TRF较高;贫血组患者14 d内人均输血量(U)较多[4.0(2.0,6.0)比2.0 (0.0,2.0),P＜0.01],ICU病死率较高(44.4％比13.6％,P＜0.05),住院时间(d)较长[35.0( 16.5,51.6)比24.5( 10.0,35.8),P＜ 0.05 ],28 d脱机存活率较低(44.4％比72.7％,P＜ 0.05).而贫血组和非贫血组机械通气时间(d:18.3±10.8比11.6±8.2,P＞0.05)、ICU住院时间[d:16.5(8.0,21.5)比11.0(5.8,18.3),P＞0.05]和住院病死率(61.1％比31.8％,P＞0.05)比较差异无统计学意义.结论 贫血患者机械通气时间及住院时间较长,ICU病死率较高,28 d脱机存活率较低.     

Sleep in mechanically ventilated patients

People dependent on mechanical ventilation in an intensive care unit sleep poorly. Less than half of their sleep occurs at night. Sleep architecture is abnormal, with occasional disappearance of some stages of sleep, and there are frequent arousals and awakenings. Because most arousals are associated with caregiving activities or noise, caregivers should consolidate care and decrease environmental noise. More research is needed on sleep and sleep promotion in mechanically ventilated patients.     

Capnography in mechanically ventilated patients

Capnography, the science of CO2 waveforms analysis, can play a role in the management of mechanically ventilated patients. Mass spectrometers are the devices most commonly used to collect sequentially and examine CO2 waveforms from multiple patients in the ICU or operating rooms. We present here a review of some clinical and technical problems, which may be resolved efficiently and expeditiously through the use of mass spectrometry and capnography. Mechanical failures, especially those that lead to rebreathing of exhaled gases, can be readily detected. The patient's progress during weaning and the consequences of changes in mechanical assistance can be virtually and noninvasively determined. An expanded role of capnography in mechanically ventilated patients can increase the use of mass spectrometers in the ICU.     

Clinical estimation of trunk position among mechanically ventilated patients

OBJECTIVES: Trunk position at 45 degrees from the horizontal is associated with a decreased risk of gastroesophageal aspiration. The objectives of this study were to determine the accuracy of trunk flexion estimates compared to a reference standard measurement, and to determine agreement about trunk flexion among ICU clinicians. DESIGN: Prospective observational study. SETTING: Two university-affiliated medical-surgical ICUs. PATIENTS AND PARTICIPANTS: Thirty-three mechanically ventilated ICU patients, seven residents, two fellows, three intensivists, and twenty-eight bedside nurses. INTERVENTIONS: Prospectively, concurrently, and independently during rounds, one bedside nurse, one resident, one fellow, and one intensivist clinically estimated the trunk flexion of mechanically ventilated patients. To record the reference standard, a trained investigator measured trunk position in the vertical plane using a goniometer. MEASUREMENTS AND RESULTS: We made 438 clinical assessments on 33 patients aged 57.2+/-19.4 (SD) years with an APACHE II score of 27.3+/-9.4. Mean trunk flexion estimates were: nurses 24.3+/-12.3 degrees from the horizontal, residents 20.2+/-13.7, fellows 20.3+/-10.8, and intensivists 21.1+/-13.1 compared to the reference standard measurement 16.2+/-9.0 degrees. The accuracy of trunk flexion estimates was fair to moderate [intraclass correlation for reference standard versus nurses (ICC 0.42), residents (ICC 0.52), fellows (ICC 0.36), and intensivists (ICC 0.55)]. The agreement among different groups of clinicians was moderate. CONCLUSIONS: In mechanically ventilated patients, we found that clinical estimates of trunk position were moderately good, agreement amongst caregivers was moderately good, but that all clinicians tended to overestimate the angle of semirecumbency.     

Evaluation of oxygen uptake and delivery in critically ill patients: A statistical reappraisal

Objective The evaluation of oxygen consumption (VO₂) and oxygen delivery (DO₂) has gained increasing importance in the monitoring of critically ill patients. They can be obtained from either direct measurements or by indirect calculations based on the Fick principle. However the choice between these two approaches remains controversial. The aim of the study was to investigate whether these 2 methods provide similar results, and if not, to define the best one in terms of reproducibility. Design Oxygen delivery and oxygen consumption were prospectively analyzed in 171 consecutive critically ill patients. Metabolic data were obtained simultaneously. Setting The study was completed in the intensive care unit as part of the management of the patients studied. Patients A first “group” of 279 evaluations was carried out in 73 consecutive critically ill patients. The results were subsequently validated by 423 observations performed in the 98 following patients. Interventions Before and during each evaluation, the patients were kept in stable hemodynamic and metabolic conditions. All were mechanically ventilated. Measurements and results VO₂ was evaluated by calculation (Fick principle) and direct measurement using indirect calorimetry. Cardiac output was both measured by the thermodilution technique and calculated (Fick principle) and the data were used for the evaluation of the directly measured and indirectly calculated DO₂. For both VO₂ and DO₂ the agreement between direct and indirect evaluations was not satisfactory. Differences as great as 55 ml/min·m² and 267 ml/min·m² between simultaneously measured and calculated VO₂ and DO₂ respectively may be expected. Finally, the indirect calculated methods were less reproducible than the measured ones. These observations resulted mainly from the cumulative effects of the random errors in the metabolic data entering into the calculation of VO₂ and DO₂. Conclusions Our data suggested that the indirect calculation (Fick equation) and the direct measurement (indirect calorimetry, thermodilution) of both VO₂ and DO₂ did not provide similar results. Direct measurements are more reproducible methods and must be preferred.     

Current oxygen management in mechanically ventilated patients: A prospective observational cohort study

Purpose

Oxygen (O₂) is the most common therapy in mechanically ventilated patients, but targets and dose are poorly understood. We aimed to describe current O₂ administration and titration in such patients in an academic intensive care unit.

Materials and Methods

In consecutive ventilated (> 48 hours) patients we prospectively obtained fraction of inspired O₂ (Fio₂), pulse oximetry O₂ saturation (Spo₂) and arterial O₂ tension (Pao₂) every 6 hours. We calculated the amount of excess O₂ delivery and the intensivists’ response to hyperoxemia (Spo₂ > 98%).

Results

During 358 mechanical ventilation days in 51 critically ill patients, median calculated excess O₂ delivery was 3472 L per patient. Patients spent most of their time with their Spo₂ > 98% (59% [29-83]) and Pao₂ between 80 and 120 mm Hg (59% [38–72]). In addition, 50% of all observations showed hyperoxemia and 4% severe hyperoxemia (Pao₂ > 202.5 mm Hg). Moreover, 71% of the calculated total excess 263,841 L of O₂ was delivered when the Fio₂ was 0.3 to 0.5. When hyperoxemia occurred with an Fio₂ between 0.3 and 0.4, for 88% of episodes, no Fio₂ adjustments were made.

Conclusions

Excess O₂ delivery and liberal O₂ therapy were common in mechanically ventilated patients. Current O₂ therapy practice may be suboptimal and further investigations are warranted.     

吸气压力支持通气在机械通气中应用的临床观察

目的：探讨采用吸气压力支持（ＩＰＳ）通气时压力支持（ＰＳ）水平与潮气量（ＶＴ）、呼吸频率（ｆ）、闭合压（Ｐ０．１）和血气分析（ＢＧＡ）的关系,以及如何选择适宜的ＰＳ水平成功地预测脱机的方法。方法：采用对照研究的方法将３４例机械通气患者按最终是否经ＩＰＳ脱机成功分为２组：脱机成功组２５例,ＡＰＡＣＨＥⅡ记分平均（２５．４±５．４）分;脱机失败组９例,ＡＰＡＣＨＥⅡ记分平均（３０．２±５．２）分。每８小时或改变ＰＳ水平后记录ＶＴ、ｆ、Ｐ０．１、经皮氧饱和度（ＳｐＯ２）,每日查ＢＧＡ。结果：ＩＰＳ脱机成功率７４％（２５／３４）。脱机时ＰＳ＜０．６９ｋＰａ（１ｋＰａ＝１０．２０ｃｍＨ２Ｏ）,Ｐ０．１＜０．３９ｋＰａ。机械通气＜１４日者,用ＩＰＳ０．５～２日后结合常规参数指标很容易脱机;超过１４日带机者,用ＩＰＳ３～７日可脱机,脱机指标除常规参数外还应结合Ｐ０．１的动态观察,ＰＳ常需降至０．３９～０．５９ｋＰａ,结合ｆ、Ｐ０．１及ＶＴ调整ＰＳ水平。结论：ＩＰＳ作为患者触发呼吸机的一种辅助通气方式,结合Ｐ０．１监测能提供最佳ＰＳ水平,并能较早地成功预测脱机。     

Artifacts in the assessment of metabolic gas exchange

In mechanically ventilated patients metabolic gas exchange recordings are frequently influenced by routine patient therapy. In this study the influence of such artifacts is investigated and a method for automatic detection and suppression proposed. This method reduced the influence of artifacts on diurnal oxygen and carbon dioxide exchange from up to 10% to a maximum of 1%.     

Aerosolized antibiotics in mechanically ventilated patients

Aerosolized antibiotics are potentially useful in intensive care. At State University of New York at Stony Brook we developed a human model of tracheobronchitis in intubated patients. The model provides daily specimens of airway secretions, allowing serial studies of airway inflammation and testing of therapy modes. The presence of local infection is defined by a unique method of quantified sputum collection. Bench models have been developed that illustrate the factors that limit aerosol delivery to intubated patients. With those models clinical trials have defined possible indications for targeted aerosol therapy to patients at risk for deep lung infection. An efficient aerosolized-antibiotics method that delivers the aerosol past the endotracheal tube has been established, and with that method the drug levels in pulmonary secretions exceed by several orders of magnitude the levels expected with intravenous therapy. Potential end points of therapy are being evaluated, including the rate of bacterial resistance and the incidence and definition of deep lung infection.     

Design and validation of an automatic metabolic monitor

A self-calibrating fully automatic instrument for the measurement of oxygen consumption, carbon dioxide production and the respiratory quotient of mechanically ventilated patients has been developed. The instrument is based on commercially available conventional oxygen and carbon dioxide gas analysers and a domestic natural gas volumetric flow meter. The distribution of the different gas flows, i.e. calibration gases, the inspiratory mixture sample and the expiratory mixture sample, are controlled by in inexpensive microprocessor, which also performs the necessary calculations. The accuracy of the instrument has been validated by bench tests. The present prototype has been in use for over 3000 h without major failures.     

Sizing the lung of mechanically ventilated patients

Introduction  

This small observational study was motivated by our belief that scaling the tidal volume in mechanically ventilated patients to the size of the injured lung is safer and more 'physiologic' than scaling it to predicted body weight, i.e. its size before it was injured. We defined Total Lung Capacity (TLC) as the thoracic gas volume at an airway pressure of 40 cm H2O and tested if TLC could be inferred from the volume of gas that enters the lungs during a brief 'recruitment' maneuver.