Ferenc Peták

Changes in respiratory mechanics during cardiac surgery

We investigated the role of cardiopulmonary bypass (CPB) in compromised lung function associated with cardiac surgery. Low-frequency respiratory impedance (Zrs) was measured in patients undergoing cardiac surgery with (n = 30; CPB group) or without (n = 29; off-pump coronary artery bypass [OPCAB] group) CPB. Another group of CPB patients received dopamine (DA) (n = 12; CPB-DA group). Extravascular lung water was determined in five CPB subjects. Zrs was measured before skin incision and after chest closure. Airway resistance and inertance and tissue damping and elastance were determined from Zrs data. Airway resistance increased in the CPB group (74.9% ± 20.8%;P < 0.05), whereas it did not change in the OPCAB group (11.8% ± 7.9%; not significant) and even decreased in the CPB-DA patients (−40.6% ± 9.2%;P < 0.05). Tissue damping increased in the CPB and OPCAB groups, whereas it remained constant in the CPB-DA patients. Significant increases in elastance were observed in all groups. There was no difference in extravascular lung water before and after CPB, suggesting that edema did not develop. These results indicate a significant and heterogeneous airway narrowing during CPB, which was counteracted by the administration of DA. The mild deterioration in tissue mechanics, reflecting partial closure of the airways, may be a consequence of the anesthesia itself.

Methacholine and Ovalbumin Challenges Assessed by Forced Oscillations and Synchrotron Lung Imaging

RATIONALE Methacholine (Mch) is routinely used to assess bronchial hyperreactivity; however, little is known about the differences in the lung response pattern between this provocation and that observed with ovalbumin (Ova) after allergic sensitization. OBJECTIVES To compare (1) the central versus peripheral effects of Mch and Ova within the lung by combining measurements of airway and tissue mechanics with synchrotron radiation (SR) imaging, and (2) to assess the extent to which mechanical and imaging parameters are correlated. METHODS We used the low-frequency forced oscillation technique and SR imaging in control (n = 12) and ovalbumin-sensitized (n = 13) rabbits, at baseline, during intravenous Mch infusion (2.5 microg/kg/min, 5.0 microg/kg/min, or 10.0 microg/kg/min), after recovery from Mch, and after intravenous Ova injection (2.0 mg). We compared intravenous Mch challenge with inhaled Mch (125 mg/ml, 90 s) in a separate group of control animals (n = 5). MEASUREMENTS AND MAIN RESULTS Airway conductance and tissue elastance were measured by low-frequency forced oscillation technique. The central airway cross-sectional area, the ventilated alveolar area, and the heterogeneity of specific ventilation were quantified by SR imaging. Mch infusion induced constriction predominantly in the central airways, whereas Ova provocation affected mainly the peripheral airways, leading to severe ventilation heterogeneities in sensitized animals. Mch inhalation affected both conducting and peripheral airways. The correlations between airway conductance and central airway cross-sectional area (R = 0.71) and between tissue elastance and ventilated alveolar area (R = -0.72) were strong. CONCLUSIONS The pattern of lung response caused by intravenous Mch and Ova are fundamentally different. Although inhaled Mch induces a heterogeneous lung response similar to that observed with intravenous allergen, these similar patterns are due to different mechanisms.

Protective Effects of Volatile Agents against Methacholine-induced Bronchoconstriction in Rats

BackgroundThe protective properties of common volatile agents against generalized lung constriction have previously been addressed only via estimations of parameters that combine airway and tissue mechanics. Their effectiveness in preventing airway constriction have not been compared systematically. Therefore, the authors investigated the abilities of halothane, isoflurane, sevoflurane, and desflurane to provide protection against airway constriction induced by methacholine. MethodsLow-frequency pulmonary impedance data were collected in open-chest rats under baseline conditions and during three consecutive intravenous infusions of methacholine (32 &mgr;g · kg−1 · min−1) while the animals were anesthetized with intravenous pentobarbital (control group). Methacholine challenges were performed in four other groups of rats, first during intravenous anesthesia and then repeated during the inhalation of halothane, isoflurane, sevoflurane, or desflurane at concentrations of 1 and 2 minimum alveolar concentration (MAC). Airway resistance and inertance, parenchymal damping, and elastance were estimated from the impedance data by model fitting. ResultsThe methacholine-induced increases in airway resistance during intravenous pentobarbital anesthesia (204 ± 53%) were markedly and significantly (P < 0.005) reduced by 1-MAC doses of halothane (80 ± 48%), isoflurane (112 ± 59%), sevoflurane (68 ± 34%), and desflurane (96 ± 34%), with no significant difference between the gases applied. Increasing the concentration to 2 MAC did not lead to any significant further protection against the increase in airway resistance. ConclusionsThese data demonstrate that isoflurane, sevoflurane, and desflurane are as effective as the widely accepted halothane in protecting against methacholine-induced airway constriction.

Methacholine responsiveness in infants assessed with low frequency forced oscillation and forced expiration techniques

BACKGROUND The contribution of the pulmonary tissues to the mechanical behaviour of the respiratory system is well recognised. This study was undertaken to detect airway and lung tissue responses to inhaled methacholine (Mch) using the low frequency forced oscillation technique (LFOT). METHODS The respiratory system impedance (Zrs, 0.5–20 Hz) was determined in 17 asymptomatic infants. A model containing airway resistance (Raw) and inertance (Iaw) and a constant phase tissue damping (G) and elastance (H) was fitted to Zrs data. Tissue hysteresivity (η) was calculated as η=G/H. The raised volume rapid thoracic compression technique (RVRTC) was used to generate forced expiratory volume in 0.5 seconds (FEV0.5). Lung function was determined at baseline and following inhaled Mch in doubling doses (0.25–16 mg/ml) until the maximal dose was reached or a fall of 15% in FEV0.5 was achieved (PC15FEV0.5). The response to Mch was defined in terms of the concentration of Mch provoking a change in lung function parameters of more than two standard deviation units (threshold concentration). RESULTS At PC15FEV0.5 a response in Raw, Iaw, G, and η, but not H, was detected (mean (SE) 61.28 (12.22)%, 95.43 (34.31)%, 46.28 (22.36)%, 44.26 (25.83)%, and –6.48 (4.94)%, respectively). No significant differences were found between threshold concentrations of LFOT parameters and FEV0.5. CONCLUSIONS Inhaled Mch alters both airway and respiratory tissue mechanics in infants.

Perioperative use of oxygen: variabilities across age

Enormous interest has emerged in the perioperative use of high concentrations of inspired oxygen in an attempt to increase tissue oxygenation and thereby improve postoperative outcome. An extensive debate has arisen regarding the risk/benefit ratio of oxygen therapy, with some researchers advocating the benefits of perioperative hyperoxia, particularly with regard to surgical site infection, whereas others emphasize its detrimental consequences on multiple organs, particularly the lungs and the brain. As one aspect of this debate, there is increased awareness of effects of reactive oxygen metabolites, a feature that contributes to the complexity of achieving consensus regarding optimum oxygen concentration in the perioperative period. Many reviews have discussed the pros and cons in the use of perioperative oxygen supplementation, but the potential importance of age-related factors in hyperoxia has not been addressed. The present narrative review provides a comprehensive overview of the physiological mechanisms and clinical outcomes across the age range from neonates to the elderly. Risks greatly outweigh the benefits of hyperoxia both in the very young, where growth and development are the hallmarks, and in the elderly, where ageing increases sensitivity to oxidative stress. Conversely, in middle age, benefits of short-term administration of perioperative oxygen therapy exceed potential adverse change effects, and thus, oxygen supplementation can be considered an important therapy to improve anaesthesia management.

Influence of inertance on respiratory mechanics measurements in mechanically ventilated puppies

The complete equation of motion for a single compartment model (SCM) includes an inertance term to describe pressure changes in phase with acceleration, as well as terms for resistance and elastance. Inertance has traditionally been excluded from the model when measuring respiratory mechanics at conventional ventilatory frequencies in mature respiratory systems. However, this omission has been questioned recently for measurements of respiratory mechanics in intubated infants where higher ventilation frequencies and smaller tracheal tubes are the norm. We investigated 1) the significance of inertance in an immature respiratory system during mechanical ventilation, and 2) the effect of omitting it from the model on estimates of respiratory mechanics. Six anesthetised, paralysed and mechanically ventilated puppies (2.6–3.9 kg) were studied. A SCM, including an inertance term was fitted to measurements of flow and airway opening (PAO) or transpulmonary (PTP) pressure using multiple linear regression to estimate respiratory system and lung resistance (RRS, RL), elastance (ERS, EL) and inertance (IRS, IL) respectively, at various ventilation frequencies (0.2–2 Hz). Data obtained at each ventilation frequency were also fitted with a similar model without the inertance term.

Effect of positive end-expiratory pressure on regional ventilation distribution during mechanical ventilation after surfactant depletion.

Background:Ventilator-induced lung injury occurs due to exaggerated local stresses, repeated collapse, and opening of terminal air spaces in poorly aerated dependent lung, and increased stretch in nondependent lung. The aim of this study was to quantify the functional behavior of peripheral lung units in whole-lung lavage-induced surfactant depletion, and to assess the effect of positive end-expiratory pressure. Methods:The authors used synchrotron imaging to measure lung aeration and regional specific ventilation at positive end-expiratory pressure of 3 and 9 cm H2O, before and after whole-lung lavage in rabbits. Respiratory mechanical parameters were measured, and helium-washout was used to assess end-expiratory lung volume. Results:Atelectatic, poorly, normally aerated, hyperinflated, and trapped regions could be identified using the imaging technique used in this study. Surfactant depletion significantly increased atelectasis (6.3 ± 3.3 [mean ± SEM]% total lung area; P = 0.04 vs. control) and poor aeration in dependent lung. Regional ventilation was distributed to poorly aerated regions with high (16.4 ± 4.4%; P < 0.001), normal (20.7 ± 5.9%; P < 0.001 vs. control), and low (5.7 ± 1.2%; P < 0.05 vs. control) specific ventilation. Significant redistribution of ventilation to normally aerated nondependent lung regions occurred (41.0 ± 9.6%; P = 0.03 vs. control). Increasing positive end-expiratory pressure level to 9 cm H2O significantly reduced poor aeration and recruited atelectasis, but ventilation redistribution persisted (39.2 ± 9.5%; P < 0.001 vs. control). Conclusions:Ventilation of poorly aerated dependent lung regions, which can promote the local concentration of mechanical stresses, was the predominant functional behavior in surfactant-depleted lung. Potential tidal recruitment of atelectatic lung regions involved a smaller fraction of the imaged lung. Significant ventilation redistribution to aerated lung regions places these at risk of increased stretch injury.

Changes in functional residual capacity and lung mechanics during surgical repair of congenital heart diseases: effects of preoperative pulmonary hemodynamics

Background:To characterize the impact of lung volume changes in the lung function impairment after the surgical repair of congenital heart diseases, combined measurements of functional residual capacity, lung clearance index, and respiratory mechanics were performed in children with hypoperfused lungs (tetralogy of Fallot [TOF]) or with pulmonary hyperperfusion (ventricular septal defect [VSD]). Methods:Lung volume and clearance were assessed by using a sulfur hexafluoride washout technique, and the mechanical properties of the respiratory system were assessed using a low-frequency oscillation technique. Lung volume and oscillatory measurements were made preoperatively, before and after cardiopulmonary bypass and aortic clamping (AC), and after chest closure. Results:Impairments in airway (36 ± 2%) and tissue mechanics (22 ± 3%) were observed in the children with TOF after bypass; AC and chest closure were associated with marked decreases in functional residual capacity (−24 ± 3% and −13 ± 2% for TOF and VSD after AC, respectively) and increases in lung clearance index (−60 ± 6% and −24 ± 3% for TOF and VSD after AC, respectively). Smaller impairments in lung mechanics were observed after bypass and AC in children with VSD. Conclusions:These findings suggest that the lung volume loss and lung mechanical deteriorations are probably caused by a diminished tethering effect of the lung periphery through a reduced filling of the pulmonary capillaries. This effect seems to be more pronounced in children with hypoperfused lungs (TOF) than in those with pulmonary hyperperfusion (VSD). The beneficial postoperative changes in children with VSD are consequences of the reversal of the pulmonary vascular engorgement after surgical repair.

Crackle-sound recording to monitor airway closure and recruitment in ventilated pigs.

It was hypothesised that the recruitment of atelectatic lung areas is signified by changes in the airway and tissue mechanics, and by the appearance of crackle activity attributed to the sudden reopening of collapsed airways. The authors also assumed that the acoustic activity is an earlier indicator of lung recruitment than the change in the overall mechanical state of the lungs. Six thoracotomised and mechanically ventilated mini-pigs were studied. Low-frequency pulmonary impedance was measured at end-expiratory pauses at transpulmonary pressures of 4 and 1 hPa to estimate airway resistance (Raw) and the coefficient of lung tissue elastance (H), and tracheal sounds were recorded during subsequent slow inflations to 30 hPa, in the control state and following increasing doses of i.v. methacholine (Mch). Raw and H were higher at baseline and increased more in response to Mch at 1 hPa than at 4 hPa. The crackles detected during the subsequent inflations were concentrated around and associated with the development of the lower knee of the pressure–volume curve. The number of crackles increased faster following the Mch doses and reached statistical significance earlier than Raw and H. Crackle recording during mechanical ventilation can be employed as a simple method with which to monitor lung recruitment–derecruitment.

Prevention of bronchoconstriction in sensitized guinea pigs: efficacy of common prophylactic drugs

Efficacy of beta2-agonists (B2), anticholinergics, corticosteroids, anti-inflammatory drugs or antihistamines against methacholine, histamine, or allergen-induced lung constriction was tested in ovalbumin sensitized guinea pigs. Airway resistance (Raw) and parenchymal damping (G) and elastance (H) were determined from low-frequency forced oscillatory input impedance (ZL). ZL was measured under control conditions, during iv infusions of methacholine, and following iv boluses of histamine or ovalbumin. In decreasing potency, ipratropium, salmeterol, fluticasone or cromoglycate reduced the methacholine-induced increases in Raw and G. Only antihistamines had any effects on the histamine-induced increased in Raw, G, and H. The ovalbumin-induced increases in Raw, G and H in the control animals (120 +/- 90%, 201 +/- 126%, 86 +/- 71%) were markedly reduced by pretreatments with antihistamines (18 +/- 11%, 13 +/- 9%, 3+/- 3%) and cromoglycate (29 +/- 13%, 35 +/- 22%, 18 +/- 10%). Bronchoconstriction induced by muscarinic receptor stimulation is inhibited by anticholinergic, anti-inflammatory or beta2-agonist pretreatment; antihistaminic or anti-inflammatory premedication is beneficial if the release of histamine via an exposure to allergen is responsible for the fall in lung function.