Individualized positive end-expiratory pressure in obese patients during general anaesthesia: a randomized controlled clinical trial using electrical impedance tomography.

Background: General anaesthesia leads to atelectasis, reduced end-expiratory lung volume (EELV), and diminished arterial oxygenation in obese patients. We hypothesized that a combination of a recruitment manoeuvre (RM) and individualized positive end-expiratory pressure (PEEP) can avoid these effects. Methods: Patients with a BMI ≥35 kg m -2 undergoing elective laparoscopic surgery were randomly allocated to mechanical ventilation with a tidal volume of 8 ml kg -1 predicted body weight and (i) an RM followed by individualized PEEP titrated using electrical impedance tomography (PEEP IND ) or (ii) no RM and PEEP of 5 cm H 2 O (PEEP 5 ). Gas exchange, regional ventilation distribution, and EELV (multiple breath nitrogen washout method) were determined before, during, and after anaesthesia. The primary end point was the ratio of arterial partial pressure of oxygen to inspiratory oxygen fraction ( P aO 2 / F iO 2 ). Results: For PEEP IND ( n =25) and PEEP 5 ( n =25) arms together, P aO 2 / F iO 2 and EELV decreased by 15 kPa [95% confidence interval (CI) 11-20 kPa, P <0.001] and 1.2 litres (95% CI 0.9-1.6 litres, P <0.001), respectively, after intubation. Mean ( sd ) PEEP IND was 18.5 (5.6) cm H 2 O. In the PEEP IND arm, P aO 2 / F iO 2 before extubation was 23 kPa higher (95% CI 16-29 kPa; P <0.001), EELV was 1.8 litres larger (95% CI 1.5-2.2 litres; P <0.001), driving pressure was 6.7 cm H 2 O lower (95% CI 5.4-7.9 cm H 2 O; P <0.001), and regional ventilation was more equally distributed than for PEEP 5 . After extubation, however, these differences between the arms vanished. Conclusions: In obese patients, an RM and higher PEEP IND restored EELV, regional ventilation distribution, and oxygenation during anaesthesia, but these differences did not persist after extubation. Therefore, lung protection strategies should include the postoperative period. Clinical trial registration: German clinical trials register DRKS00004199, www.who.int/ictrp/network/drks2/en/ .

Cardio-respiratory interactions and relocation of heartbeats within the respiratory cycle during spontaneous and paced breathing.

The capability of respiratory sinus arrhythmia (RSA) to generate privileged locations for the occurrence of R-peaks within the respiratory cycle has been questioned in recent works, challenging the hypothesis that RSA might play a role in improving pulmonary gas exchange. We assessed such a capability submitting healthy humans to spontaneous and paced breathing (SB and PB) protocols, estimating the fraction of beats occurring during inspiration, at low, medium, and high respiratory volumes, and during the first and second half of inspiration and expiration. Then, the same fractions were computed assuming a random uniform distribution of heartbeats, and the differences were compared. The results found are as follows: (1) during PB at 6 rpm, heartbeats redistribute toward inspiration; (2) during SB and PB at 12 rpm, heartbeats tend to cluster when respiratory volume is high; (3) since such redistributions are limited in magnitude, it is possible that its physiological relevance is marginal, for instance, in terms of within-cycle variations in lung perfusion; (4) two groups of subjects with considerably different levels of RSA showed similar redistribution of heartbeats, suggesting that this phenomenon might be an underlying effect of the overall cardio-respiratory interactions, and not directly of RSA.

Low sampling rates bias outcomes from the Wingate test.

The purpose of this work was to apply a simple method for acquisition of power output (PO) during the Wingate Anaerobic Test (WAnT) at a high sampling rate ( S(R)) and to compare the effect of lower S(R) on the measurements extracted from the PO. 26 male subjects underwent 2 WAnTs on a cycle ergometer. The reference PO was calculated at 30 Hz as a function of the linear velocity, the moment of inertia and the frictional load. The PO was sampled at 0.2, 0.5, 1, 2 and 5 Hz. Both the peak (16.03±2.22 W·kg (-1)) and mean PO (10.34±1.01 W·kg (-1)) presented lower relative values when the S(R) was lower. Peak PO was attenuated by 0.29-42.07% for decreasing sampling rates, resulting in different values for 0.2 and 1 Hz ( P<0.001). When the S(R) was 0.2 Hz, the time to peak was delayed by 53.81% ( P<0.001) and the fatigue index was attenuated by 22.12% ( P<0.001). In conclusion, due to the differences achieved here and the fact that the peak flywheel frequency is around 2.3 Hz, we strongly recommend that the PO be sampled at 5 Hz instead of 0.2 Hz in order to avoid biased errors and misunderstandings of the WAnT results.

Comparison of objective methods to classify the pattern of respiratory sinus arrhythmia during mechanical ventilation and paced spontaneous breathing.

Respiratory sinus arrhythmia (RSA) is a fluctuation of heart period that occurs during a respiratory cycle. It has been suggested that inspiratory heart period acceleration and expiratory deceleration during spontaneous ventilation (henceforth named positive RSA) improve the efficiency of gas exchange compared to the absence or the inversion of such a pattern (negative RSA). During mechanical ventilation (MV), for which maximizing the efficiency of gas exchange is of critical importance, the pattern of RSA is still the object of debate. In order to gain a better insight into this matter, we compared five different methods of RSA classification using the data of five mechanically ventilated piglets. The comparison was repeated using the data of 15 volunteers undergoing a protocol of paced spontaneous breathing, which is expected to result in a positive RSA pattern. The results showed that the agreement between the employed methods is limited, suggesting that the lack of a consensus about the RSA pattern during MV is, at least in part, of methodological origin. However, independently of the method used, the pattern of RSA within the respiratory cycle was not consistent among the subjects and conditions of MV considered. Also, the outcomes showed that even during paced spontaneous breathing a negative RSA pattern might be present, when a low respiratory frequency is imposed.

Pulmonary capillary blood flow by partial CO2 rebreathing: importance of the regularity of the respiratory pattern.

The partial CO2 rebreathing technique has been shown to be a reliable non-invasive method for measurement of pulmonary capillary blood flow (Qc), but experience with this technique has been limited to controlled mechanical ventilation. In this study, we evaluated this technique during spontaneous and oriented ventilation in nine subjects without known cardiopulmonary disease. Subjects underwent 10 consecutive (Qc) measurements with both spontaneous and oriented ventilation. Breath-by-breath gas exchange was measured and (Qc) was calculated from changes in CO2 elimination and PETCO2, which were achieved by sudden increase of the apparatus deadspace (rebreathing period). An exponential curve was fitted to the PETCO2 values in the rebreathing period in order to estimate PETCO2 at equilibrium. We found that mean (Qc) values were not influenced by the ventilation pattern (P=0.51), but that the intra-individual variability with oriented ventilation (median=16.0%) was significantly lower than with spontaneous ventilation (median=31.8%, P=0.039). Accordingly, the curve fitting for rebreathing PETCO2 rise failed in 4.4% of measurements with oriented ventilation vs. 18.9% of measurements with spontaneous ventilation (P=0.039). Our results suggest that the performance of the partial CO2 rebreathing technique is adversely affected by spontaneous ventilation and, consequently, that this method should be reserved for patients with regular respiratory patterns.

Effect of generator nonlinearities on the accuracy of respiratory impedance measurements by forced oscillation.

Measurements of respiratory impedance by means of the forced oscillation technique (FOT) are usually made using a loudspeaker as the excitation device. Its nonlinear nature can introduce artifacts that coincide with the frequencies applied to excite the respiratory system, limiting the accuracy of the impedance estimation. In this paper, this hypothesis is evaluated in the case of both a traditional estimator and the unbiased estimator proposed by Daróczy and Hantos (1982). A simulated study under apnoea conditions in the pressure range 0.5-3.0 cmH2O peak-to-peak reveals that loudspeaker nonlinearities introduce a characteristic pattern of dispersion in both the resistance and reactance curves that can be significantly decreased (p approximately equal to 0.03, signtest) by reducing the nonlinearities. A simulation of spontaneous breathing shows the same pattern, and is observed in the case of traditional as well as unbiased estimators. The dispersion is quantified by the mean absolute distance between the theoretical and simulated data and decreases with the reduction of nonlinearities when impedance is estimated with a traditional estimator (from 6.63 to 4.72% in real estimates and from 6.78 to 3.47% in imaginary estimates) as well as with an unbiased estimator (real estimates from 4.84 to 1.57% and 5.61 to 2.06% in imaginary estimates). Studies with normal subjects show the same dispersion pattern, which decreases if the generator nonlinearities are reduced. These results supply substantial evidence that reducing generator nonlinearities can contribute to the production of more reliable mechanical impedance FOT measurements.

Gas exchange during exercise in different evolutional stages of chronic Chagas' heart disease.

OBJECTIVE: To compare gas exchange at rest and during exercise in patients with chronic Chagas' heart disease grouped according to the Los Andes clinical hemodynamic classification. METHODS: We studied 15 healthy volunteers and 52 patients grouped according to the Los Andes clinical and hemodynamic classification as follows: 17 patients in group IA (normal electrocardiogram and echocardiogram), 9 patients in group IB (normal electrocardiogram and abnormal echocardiogram), 14 patients in group II (abnormal electrocardiogram and echocardiogram, without congestive heart failure), and 12 patients in group III (abnormal electrocardiogram and echocardiogram with congestive heart failure). The following variables were analyzed: oxygen consumption (V O2), carbon dioxide production (V CO2), gas exchange rate (R), inspiratory current volume (V IC), expiratory current volume (V EC), respiratory frequency, minute volume (V E), heart rate (HR), maximum load, O2 pulse, and ventilatory anaerobic threshold (AT). RESULTS: When compared with the healthy group, patients in groups II and III showed significant changes in the following variables: V O2 peak, V CO2 peak, V IC peak, V EC peak, E, HR, and maximum load. Group IA showed significantly better results for these same variables as compared with group III. CONCLUSION: The functional capacity of patients in the initial phase of chronic Chagas' heart disease is higher than that of patients in an advanced phase and shows a decrease that follows the loss in cardiac-hemodynamic performance.

Linear servo-controlled pressure generator for forced oscillation measurements.

In respiratory input impedance measurements, the low-frequency range contains important clinical and physiological information. However, the patient's spontaneous ventilation can contaminate the data in this range, leading to unreliable results. Unbiased estimators are a good alternative to overcome this problem, provided that the generator is considered linear. This condition is not fulfilled by most existing generators as they are based on loudspeakers, which have strong nonlinearities. The present work aims to contribute to the solution of this problem, and describes a pressure generator that minimises the nonlinearities by an optical sensor placed in a position feedback loop. The static evaluation shows a high linearity for the optical system. The well known frequency response of pressure transducers is used in the dynamic evaluation of the instrument. The analysis of the generator shows that the use of position feedback improved the frequency response. The total harmonic distortion (THD) measurement shows that closed loop resulted in an effective decrease in the nonlinearities. The reduction of THD achieved by the servo-controlled generator can contribute to the practical implementation of the unbiased estimators, increasing the reliability of the impedance data, especially in the low-frequency range. This system is compared with conventional generators and with another servo-controlled system.

Design and calibration of unicapillary pneumotachographs.

This study presents a method for design and calibration of unicapillary pneumotachographs for small-animal experiments. The design, based on Poiseuille's law, defines a set of internal radius and length values that allows for laminar flow, measurable pressure differences, and minimal interference with animal's respiratory mechanics and gas exchange. A third-order polynomial calibration (Pol) of the pressure-flow relationship was employed and compared with linear calibration (Lin). Tests were done for conditions of ambient pressure (Pam) and positive pressure (Ppos) ventilation at different flow ranges. A physical model designed to match normal and low compliance in rats was used. At normal compliance, Pol provided lower errors than Lin for mixed (1-12 ml/s), mean (4-10 ml/s), and high (8-12 ml/s) flow rate calibrations for both Pam and Ppos inspiratory tests (P < 0.001 for all conditions) and expiratory tests (P < 0.001 for all conditions). At low compliance, they differed significantly with 8.6 +/- 4.1% underestimation when Lin at Pam was used in Ppos tests. Ppos calibration, preferably in combination with Pol, should be used in this case to minimize errors (Pol = 0.8 +/- 0.5%, Lin = 6.5 +/- 4.0%, P < 0.0005). Nonlinear calibration may be useful for improvement of flow and volume measurements in small animals during both Pam and Ppos ventilation.

Decision support system to assist mechanical ventilation in the adult respiratory distress syndrome.

This paper presents a knowledge-based decision support system to assist mechanical ventilation in patients with the Adult Respiratory Distress Syndrome (DSSARDS). The knowledge base uses clinical algorithms developed from interviews and seminars with experts. The system contains 140 rules, applies backward chaining and was built on an IBM-PC compatible microcomputer. Clinical and physiological data and ventilator settings were used for suggestions of ventilatory support mode (VSMODE) and settings (MVSET) and for hemodynamic evaluation and therapy (HEMO). Success rates (s) and kappa coefficient (k) were used to measure agreement between DSSARDS and physicians at 4 decision steps related to: beginning of mechanical ventilation (FIRSTSET), VSMODE, MVSET and HEMO, DSSARDS prototype was evaluated in a development phase with 6 patients aged 48.6 +/- 15.9 years. Agreement results for 142 decision steps were: FIRSTSET k = 0.90, s = 0.93; VSMODE k = 0.76, s = 0.92; HEMO k = 0.58, s = 0.70, MVSET k = 0.86, s = 0.92 (p < 0.05 for all k). Improvements in the knowledge base were performed mainly in HEMO and VSMODE modules. The subsequent test phase studied 5 patients aged 54.8 +/- 11.0 years in a total of 900 decision steps. Results were: FIRSTSET k = 0.93, s = 0.95; VSMODE k = 0.93, s = 0.96; HEMO k = 0.97, s = 0.99, MVSET k = 0.96, s = 0.97 (p < 0.05 for all k). The results indicate significant agreement between DSSARDS and physicians for all decision steps. This suggests that DSSARDS may be used as a support for decision making and a training tool for mechanical ventilation in patients with the adult respiratory distress syndrome.

Sistema computadorizado para monitorizaçäo cardio-respiratória / Computerized system for cardiorespiratory monitoring

Um sistema baseado em computador pessoal é usado para a monitorização cardio-respiratória de pacientes, em tempo-real, integrando sinais obtidos de diferentes equipamentos: monitor fisiológico (ECG), espectrômetro de massa respiratória (EMR: frações gasosas) e pneumotacógrafo (fluxo ventilatório). O sistema detecta complexos QRS e ciclos respiratórios, e calcula freqüência cardíaca e vários parâmetros respiratórios. O desempenho dos algoritmos é avaliado com sinais simulados, o MIT-BIH Arrhytmia Database e experimentos in-vivo. O sistema é empregado em pesquisa da fisiologia do exercício

Abstract- A modular system based on a personal computer is used to perform real-time cardio-respiratory monitoring of patients. integrating data from different equipments: a bed side monitor (ECG ), a respiratory mass spectrometer (RMS: gas fractions) and a pneumothacograph (respiratory tlow). The system includes an automatic detection of the QRS complexes and the respiratory cycles and it calculates heart rate and severa! respiratory parameters. The performance of the algorithms is estimated with simulated data. the MIT-BIH Arrhythrnia Database and in rivo experirnents. This system is being used for research in exercise physiology

Mechanisms of hypoxemia and hypocapnia in pulmonary embolism.

Mechanisms of hypoxemia and hypocapnia in pulmonary embolism (PE) are incompletely understood. We studied 10 patients at diagnosis (D) and five of these again after 10 to 14 d of heparin treatment (T). Patients had right heart catheterization, assessment of ventilation-perfusion ratio (VA/Q) distribution by inert gas, radioisotopic perfusion and ventilation scans, and angiography. At D, two-thirds of the pulmonary circulation was obstructed, patients were hypoxemic (PaO2 = 63.0 +/- 11.7 mm Hg) and hypocapnic (PaCO2 = 30.0 +/- 4.1 mm Hg), mixed venous oxygen pressure (PvO2) was reduced (30.9 +/- 3.9 mm Hg), minute ventilation (VE) markedly increased (14.1 +/- 5.1 L/min), and cardiac output measured by applying the Fick principle to arteriovenous oxygen content difference (QT) slightly low (4.7 +/- 1.7 L/min). Hypoxemia was mainly explained by VA/Q inequality, reduced PvO2 also contributed. Hypocapnia was the result of hyperventilation. VA/Q inequality was characterized by shift of VA and Q distribution mean to regions with higher VA/Q ratio through a fraction of blood flow (19.0 +/- 24.3% of cardiac output) went to lung units with low VA/Q ratio. Log SDQ and log SDvA were increased. Shunt, diffusion limitation, or true alveolar dead space occurred in occasional patients but were generally insignificant. Regional ventilation and perfusion maps indicated that in the unperfused lung segments, ventilation was reduced. Furthermore, they disclosed overperfused lung segments. At T, hypoxemia and hypocapnia improved considerably. However, temporal imbalances in recovery between regional ventilation and perfusion occurred with the former normalizing sooner. However, perfusion recovered sooner than ventilation in some regions.

Natural course of treated pulmonary embolism. Evaluation by perfusion lung scintigraphy, gas exchange, and chest roentgenogram.

Perfusion lung scintigrams, pulmonary gas exchange data, and chest roentgenograms were obtained in 33 patients during acute embolism and over the following six months in order to assess their clinical usefulness in monitoring the effect of therapy. To this purpose, the measurement of pulmonary gas exchange and the presence of chest x-ray findings were compared with perfusion lung scintigraphic abnormalities both at diagnosis and after 7, 30, and 180 days during treatment. More than 50 percent of the pulmonary arterial tree was obstructed at diagnosis, and a large part of perfusion recovery was complete within the first month. All of the gas exchange parameters were abnormal at diagnosis, and the rate of their improvement was related to that of perfusion recovery. Interestingly, PaO2st (ie, PaO2 corrected for hyperventilation) and VE tended to return to normal during the first month as a consequence of the progressive recovery of perfusion, whereas oxygen and carbon dioxide gradients and physiologic dead space showed the persistence of some abnormalities six months after diagnosis. Significant correlations were observed between the number of ULSs evaluated on the perfusion lung scintigram (and considered an index of the severity of pulmonary embolization) and all of the gas exchange parameters at diagnosis (correlation coefficients averaged from 0.41 to 0.73) and after 7 and 30 days. The enlargement of the right descending pulmonary artery and particularly the "sausage" sign and the Westermark sign were significantly associated with a higher degree of gas exchange impairment and with a more severe embolization. In conclusion, this study demonstrates that perfusion lung scintigraphy has a primary role in monitoring the recovery of patients with pulmonary embolism under treatment. Moreover, the chest roentgenogram may help in this purpose. A second major result is that the simple measurement of some gas exchange parameters may allow the assessment of functional recovery of these patients, thus giving additional information about the effect of therapy.

Estimates of mean alveolar gas in patients with chronic airways obstruction.

Alveolar to arterial differences (AaD) may be computed from representative alveolar gas pressures (PA). These are particularly difficult to obtain in patients with chronic airways obstruction (CAO) and severe inhomogeneity of the ventilation/perfusion (VA/Q) ratio. In 25 such patients, using a computerized mass spectrometer, representative values of alveolar gas were obtained: 1. as end-tidal concentrations (ET); 2. using the alveolar air equation with PACO2 derived from Bohr's equation (BE); 3. putting PACO2 = PACO2 into the alveolar air equation to compute ideal O2 alveolar pressure (ID); 4. applying Rahn's definition of "mean" alveolar gas (RD), i.e. taking within each jth breath, the alveolar pressures corresponding to the moment when the instantaneous gas exchange ratio R(t)j equals the overall exchange ratio for that breath Rj. During quiet breathing, the point where R(t)j equals Rj occurred at 71% of tidal volume (VT). Alveolar pressures by BE and RD resulted in similar values (115.5 +/- 6.9 and 115.5 +/- 6.6 mmHg for O2 and 30.3 +/- 5.7 and 30.8 +/- 5.7 mmHg for CO2, respectively), but they were significantly different from those by ET and ID (106.2 +/- 9.5 and 95.6 +/- 13.5 mmHg for O2 and 36.3 +/- 7.7 and 48.2 +/- 11.2 mmHg for CO2, respectively). These findings may be explained considering that PA(BE) and PA(RD) represent both high and low VA/Q units, whereas PA(ET) and PA(ID) represent mainly low VA/Q units. AaDCO2 by RD and BE appeared proportional to the severity of CAO as estimated from VT. Automated techniques make it possible to substitute end-tidal determinations with more accurate estimates either by measuring anatomical dead-space and using Bohr's equation or by applying Rahn's definition.

Automatisation of the single-breath nitrogen washout test.

The paper presents a computer-based system for performance and analysis of the single-breath nitrogen washout test (Sb-N2 test). A pneumotachometer and a rapid gas analyser are used for volume flow rate and nitrogen fraction measurements. The data are acquired and analysed by a PDP-11/34 computer. Several sources of measurement errors are considered. An algorithm is proposed to reduce errors in flow measurements due to viscosity variation in respired air. The algorithm for closing volume (CV) detection and calculation of other Sb-N2 parameters is developed from 50 tracings of 18 subjects. The CV is obtained by an iterative linear fitting of the later half of the Sb-N2 curve. Comparisons between computer and hand measurements of three trained readers show good correlations (r = 0.87, 0.93, 0.91). The variability of computer CV measurements was comparable with that of the hand reader with the lowest variance. The coefficient of variation of residual volume and slope of alveolar plateau measurements are apparently reduced. The results demonstrate the adequacy and convenience of using the computer to analyse the Sb-N2 test and indicate that the automation allows for reduction of experimental errors and of test parameter variabilities.

The assessment of gas exchange by automated analysis of O2 and CO2 alveolar to arterial differences.

A computer program to measure breath by breath alveolar pressure (PA) and alveolar to arterial difference (AaD) for O2 and CO2, by a mass-spectrometer has been implemented. The program allows the determination of alveolar gas by different methods: 1. Bohr's equation (BE); 2. ideal alveolar air equation for O2 (IDO2); 3. end-tidal (ET); 4. by the Rahn's definition of 'mean alveolar gas', i.e., alveolar pressures are defined when instantaneous respiratory exchange ratio (IRQ) equals mean respiratory exchange ratio (MRQ). This automated technique has been used in 16 patients with chronic obstructive lung disease (COLD) and 15 patients with pulmonary embolism (APE). In both groups of patients it was always possible to find in each breath the point where IRQ = MRQ and therefore to measure AaD by RD. IDO2 was significantly lower than PAO2 by the other methods. Also ET values of O2 and CO2 were significantly different from RD and BE in both groups of patients, however the difference was consistently higher in COLD patients. The different shape of the expirograms (steeper expirograms in COLD) is responsible for this different result. RD and BE AaD characterize gas exchange more precisely than ET, because the contribution of high VA/Q units is also evaluated. This is particularly important in COLD patients. Consideration on dead space measurements are also reported both for COLD and APE patients. In conclusion this automated technique provides the assessment of gas exchange for the use in clinical respiratory physiology and for the monitoring of gas-exchange in critically ill patients.