Antonio Giannella-Neto

Positive end-expiratory pressure at minimal respiratory elastance represents the best compromise between mechanical stress and lung aeration in oleic acid induced lung injury.

IntroductionProtective ventilatory strategies have been applied to prevent ventilator-induced lung injury in patients with acute lung injury (ALI). However, adjustment of positive end-expiratory pressure (PEEP) to avoid alveolar de-recruitment and hyperinflation remains difficult. An alternative is to set the PEEP based on minimizing respiratory system elastance (Ers) by titrating PEEP. In the present study we evaluate the distribution of lung aeration (assessed using computed tomography scanning) and the behaviour of Ers in a porcine model of ALI, during a descending PEEP titration manoeuvre with a protective low tidal volume.MethodsPEEP titration (from 26 to 0 cmH2O, with a tidal volume of 6 to 7 ml/kg) was performed, following a recruitment manoeuvre. At each PEEP, helical computed tomography scans of juxta-diaphragmatic parts of the lower lobes were obtained during end-expiratory and end-inspiratory pauses in six piglets with ALI induced by oleic acid. The distribution of the lung compartments (hyperinflated, normally aerated, poorly aerated and non-aerated areas) was determined and the Ers was estimated on a breath-by-breath basis from the equation of motion of the respiratory system using the least-squares method.ResultsProgressive reduction in PEEP from 26 cmH2O to the PEEP at which the minimum Ers was observed improved poorly aerated areas, with a proportional reduction in hyperinflated areas. Also, the distribution of normally aerated areas remained steady over this interval, with no changes in non-aerated areas. The PEEP at which minimal Ers occurred corresponded to the greatest amount of normally aerated areas, with lesser hyperinflated, and poorly and non-aerated areas. Levels of PEEP below that at which minimal Ers was observed increased poorly and non-aerated areas, with concomitant reductions in normally inflated and hyperinflated areas.ConclusionThe PEEP at which minimal Ers occurred, obtained by descending PEEP titration with a protective low tidal volume, corresponded to the greatest amount of normally aerated areas, with lesser collapsed and hyperinflated areas. The institution of high levels of PEEP reduced poorly aerated areas but enlarged hyperinflated ones. Reduction in PEEP consistently enhanced poorly or non-aerated areas as well as tidal re-aeration. Hence, monitoring respiratory mechanics during a PEEP titration procedure may be a useful adjunct to optimize lung aeration.

Ability of dynamic airway pressure curve profile and elastance for positive end-expiratory pressure titration

ObjectiveTo evaluate the ability of three indices derived from the airway pressure curve for titrating positive end-expiratory pressure (PEEP) to minimize mechanical stress while improving lung aeration assessed by computed tomography (CT).DesignProspective, experimental study.SettingUniversity research facilities.SubjectsTwelve pigs.InterventionsAnimals were anesthetized and mechanically ventilated with tidal volume of 7xa0mlxa0kg−1. In non-injured lungs (nxa0=xa06), PEEP was set at 16xa0cmH2O and stepwise decreased until zero. Acute lung injury was then induced either with oleic acid (nxa0=xa06) or surfactant depletion (nxa0=xa06). A recruitment maneuver was performed, the PEEP set at 26xa0cmH2O and decreased stepwise until zero. CT scans were obtained at end-expiratory and end-inspiratory pauses. The elastance of the respiratory system (Ers), the stress index and the percentage of volume-dependent elastance (%E2) were estimated.Measurements and main resultsIn non-injured and injured lungs, the PEEP at which Ers was lowest (8–4 and 16–12xa0cmH2O, respectively) corresponded to the best compromise between recruitment/hyperinflation. In non-injured lungs, stress index and %E2 correlated with tidal recruitment and hyperinflation. In injured lungs, stress index and %E2 suggested overdistension at all PEEP levels, whereas the CT scans evidenced tidal recruitment and hyperinflation simultaneously.ConclusionDuring ventilation with low tidal volumes, Ers seems to be useful for guiding PEEP titration in non-injured and injured lungs, while stress index and %E2 are useful in non-injured lungs only. Our results suggest that Ers can be superior to the stress index and %E2 to guide PEEP titration in focal loss of lung aeration.

Comparison of computerized methods for detecting the ventilatory thresholds

The aim of this study was to compare computerized automatic methods to detect the ventilatory threshold (VT). Thirty apparently healthy and physically active volunteers [22.5 (6.5) years; 1.72 (0.08)xa0m; 71.9 (8.5)xa0kg] were submitted to a progressive and maximal cycle exercise. The gas exchange was monitored breath-by-breath with a fast gas analyser. The VT and respiratory compensation (RC) were automatically detected based on the respiratory exchange ratio, the ventilatory equivalent for O2 and the ventilatory equivalent for CO2, pulmonary ventilation, end-tidal PO2 and PCO2, and v-slope. In addition, VT and RC were also determined independently by visual inspection by two experienced investigators, and the results were compared with those of the automatic procedures. The automatic VT averaged 77% of the maximal V̇O2 and the RC 88%. The agreement between the experienced observers was very close [mean difference: 44.4 (16.1)xa0ml, r=0.94, not significant]. Data were expressed as the mean value together with the standard deviation in each case. The automatic and visual inspection procedures did not present significant differences, resulting in 29.6xa0(29.6)xa0ml with a reliability of r=0.86. All methods were significantly correlated for VT and RC (r=0.93 on average, P<0.01). ANOVA did not show differences between either the VT methods (P=0.131) or the RC methods (P=0.41). In conclusion, the present study has compared several simultaneous breath-by-breath ergospirometric methods that are used to describe the anaerobic threshold, showing high confidence when compared to visual inspection. No statistical differences were found between the VT and RC techniques for physically active subjects indicating that these methods may be equally effectively employed.

Effects of descending positive end-expiratory pressure on lung mechanics and aeration in healthy anaesthetized piglets

IntroductionAtelectasis and distal airway closure are common clinical entities of general anaesthesia. These two phenomena are expected to reduce the ventilation of dependent lung regions and represent major causes of arterial oxygenation impairment in anaesthetic conditions. The behaviour of the elastance of the respiratory system (Ers), as well as the lung aeration assessed by computed tomography (CT) scan, was evaluated during a descendent positive end-expiratory pressure (PEEP) titration. This work sought to evaluate the potential usefulness of Ers monitoring to set the PEEP in order to prevent tidal recruitment and hyperinflation of healthy lungs under general anaesthesia.MethodsPEEP titration (from 16 to 0 cmH2O, tidal volume of 8 ml/kg) was performed, and at each PEEP, CT scans were obtained during end-expiratory and end-inspiratory pauses in six healthy, anaesthetized and paralyzed piglets. The distribution of lung aeration was determined and the tidal re-aeration was calculated as the difference between end-expiratory and end-inspiratory poorly aerated and normally aerated areas. Similarly, tidal hyperinflation was obtained as the difference between end-inspiratory and end-expiratory hyperinflated areas. Ers was estimated from the equation of motion of the respiratory system during all PEEP titration with the least-squares method.ResultsHyperinflated areas decreased from PEEP 16 to 0 cmH2O (ranges decreased from 24–62% to 1–7% at end-expiratory pauses and from 44–73% to 4–17% at end-inspiratory pauses) whereas normally aerated areas increased (from 30–66% to 72–83% at end-expiratory pauses and from 19–48% to 73–77% at end-inspiratory pauses). From 16 to 8 cmH2O, Ers decreased with a corresponding reduction in tidal hyperinflation. A flat minimum of Ers was observed from 8 to 4 cmH2O. For PEEP below 4 cmH2O, Ers increased in association with a rise in tidal re-aeration and a flat maximum of the normally aerated areas.ConclusionIn healthy piglets under a descending PEEP protocol, the PEEP at minimum Ers presented a compromise between maximizing normally aerated areas and minimizing tidal re-aeration and hyperinflation. High levels of PEEP, greater than 8 cmH2O, reduced tidal re-aeration but increased hyperinflation with a concomitant decrease in normally aerated areas.

New impedance spectrometer for scientific and clinical studies of the respiratory system

The simple techniques generally used to investigate the biomechanical properties associated with the respiratory process are able to identify only basic characteristics, just supplying an initial representation of the complex behavior of the respiratory system. A much more detailed description can be obtained by the forced oscillation technique (FOT). Requiring very little cooperation of the subject, this technique allows a noninvasive evaluation of the mechanical impedance of the respiratory system. Although the FOT has a great scientific and clinical potential, this is attended by technical difficulties and has yet to be put to widespread use. In this context, this work describes a new flexible, open architecture, virtual-instrument-based impedance spectrometer for the FOT. After the design details are described, the system accuracy is evaluated investigating mechanical models simulating normal and individuals with restrictive disease. This analysis revealed errors smaller than 4% in modulus and 7° in p...

Comparative effects of vaporized perfluorohexane and partial liquid ventilation in oleic acid-induced lung injury

Background:It is currently not known whether vaporized perfluorohexane is superior to partial liquid ventilation (PLV) for therapy of acute lung injury. In this study, the authors compared the effects of both therapies in oleic acid–induced lung injury. Methods:Lung injury was induced in 30 anesthetized and mechanically ventilated pigs by means of central venous infusion of oleic acid. Animals were assigned to one of the following groups: (1) control or gas ventilation (GV), (2) 2.5% perfluorohexane vapor, (3) 5% perfluorohexane vapor, (4) 10% perfluorohexane vapor, or (5) PLV with perfluorooctane (30 ml/kg). Two hours after randomization, lungs were recruited and positive end-expiratory pressure was adjusted to obtain minimal elastance. Ventilation was continued during 4 additional hours, when animals were killed for lung histologic examination. Results:Gas exchange and elastance were comparable among vaporized perfluorohexane, PLV, and GV before the open lung approach was used and improved in a similar fashion in all groups after positive end-expiratory pressure was adjusted to optimal elastance (P < 0.05). A similar behavior was observed in functional residual capacity (FRC) in animals treated with vaporized perfluorohexane and GV. Lung resistance improved after recruitment (P < 0.05), but values were higher in the 10% perfluorohexane and PLV groups as compared with GV (P < 0.05). Interestingly, positive end-expiratory pressure values required to obtain minimal elastance were lower with 5% perfluorohexane than with PLV and GV (P < 0.05). In addition, diffuse alveolar damage was significantly lower in the 5% and 10% perfluorohexane vapor groups as compared with PLV and GV (P < 0.05). Conclusions:Although the use of 5% vaporized perfluorohexane permitted the authors to reduce pressures needed to stabilize the lungs and was associated with better histologic findings than were PLV and GV, none of these perfluorocarbon therapies improved gas exchange or lung mechanics as compared with GV.

A closed-loop mechanical ventilation controller with explicit objective functions

A closed-loop lung ventilation controller was designed, aiming to: 1) track a desired end-tidal CO/sub 2/ pressure (P/sub et/CO/sub 2/), 2) find the positive end-expiratory pressure (PEEP) of minimum estimated respiratory system elastance (E/sub rs,e/), and 3) follow objective functions conjectured to reduce lung injury. After numerical simulations, tests were performed in six paralyzed piglets. Respiratory mechanics parameters were estimated by the recursive least squares (RLS) method. The controller incorporated a modified PI controller for P/sub et/CO/sub 2/ and a gradient descent method for PEEP. In each animal, three automated PEEP control runs were performed, as well as a manual PEEP titration of E/sub rs,e/ and a multiple P/sub et/CO/sub 2/ step change trial. Overall performance indexes were obtained from PEEP control, such as minimum E/sub rs,e/ (37.0/spl plusmn/4.5cmH/sub 2/O.L/sup -1/), time to reach the minimum E/sub rs,e/ (235/spl plusmn/182 s) and associated PEEP (6.5/spl plusmn/1.0 cmH/sub 2/O), and from P/sub et/CO/sub 2/ control, such as rise time (53 /spl plusmn/ 22 s), absolute overshoot/undershoot of P/sub et/CO/sub 2/ (3/spl plusmn/1 mmHg), and settling time (145 /spl plusmn/ 72 s). The resulting CO/sub 2/ controller dynamics approximate physiological responses, and results from PEEP control were similar to those obtained by manual titration. Multiple dependencies linking the involved variables are discussed. The present controller can help to implement and evaluate objective functions that meet clinical goals.

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 byDaróczy andHantos (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≃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

OBJECTIVEnTo compare gas exchange at rest and during exercise in patients with chronic Chagas heart disease grouped according to the Los Andes clinical hemodynamic classification.nnnMETHODSnWe 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).nnnRESULTSnWhen 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.nnnCONCLUSIONnThe 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 patients 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.