Changes in airway and respiratory tissue mechanics after cardiac surgery

BACKGROUND

Because of the critical importance of the first postoperative week in the development of respiratory complications after cardiac surgery, the mechanical properties of the respiratory system in this period were followed up systematically.

METHODS

The input impedance of the respiratory system (Zrs) was measured during spontaneous breathing in patients (n=35) undergoing cardiac surgery on the day before surgery to establish the baseline, and for six days thereafter. The airway resistance was inferred from the average of the resistive component of Zrs, while the changes in respiratory elastance were assessed from the imaginary part of Zrs by model fitting. An assessment was made of the impact on the postoperative changes of factors characteristic of the patients (gender, age, smoking, and obesity) or the surgery duration and the need or not for a cardiopulmonary bypass.

RESULTS

Airway resistance increased immediately after extubation (peak rise on day 1, evening: 48+/-10%) and subsequently gradually decreased to the initial level, the recovery proving prolonged in obese patients. Postoperative elevation in elastance peaked later (day 2, evening: 83+/-14%), lasted longer, and was affected by both cardiopulmonary bypass (p<0.05) and obesity (p<0.005).

CONCLUSIONS

These findings demonstrate the need for particular attention in the postoperative management of patients after cardiac surgery in order to reduce the immediate airway symptoms, and to take steps to maintain the lungs open during the critical postoperative days 2 and 3, especially in obese patients and (or) if the surgery involves the use of cardiopulmonary bypass.

Mechanisms for lung function impairment and airway hyperresponsiveness following chronic hypoxia in rats

Although chronic normobaric hypoxia (CH) alters lung function, its potential to induce bronchial hyperreactivity (BHR) is still controversial. Thus the effects of CH on airway and tissue mechanics separately and changes in lung responsiveness to methacholine (MCh) were investigated. To clarify the mechanisms, mechanical changes were related to end-expiratory lung volume (EELV), in vivo results were compared with those in vitro, and lung histology was assessed. EELV was measured plethysmographically in two groups of rats exposed to 21 days of CH (11% O(2)) or to normoxia. Total respiratory impedance was measured under baseline conditions and following intravenous MCh challenges (2-18 microg x kg(-1) x min(-1)). The lungs were then excised and perfused, and the pulmonary input impedance was measured, while MCh provocations were repeated under a pulmonary capillary pressure of 5, 10, and 15 mmHg. Airway resistance, tissue damping, and elastance were extracted from the respiratory impedance and pulmonary input impedance spectra. The increases in EELV following CH were associated with decreases in airway resistance, whereas tissue damping and elastance remained unaffected. CH led to the development of severe BHR to MCh (206 +/- 30 vs. 95 +/- 24%, P < 0.001), which was not detectable when the same lungs were studied in vitro at any pulmonary capillary pressure levels maintained. Histology revealed pulmonary arterial vascular remodeling with overexpression of alpha-smooth muscle actin antibody in the bronchial wall. These findings suggest that, despite the counterbalancing effect of the increased EELV, BHR develops following CH, only in the presence of intact autonomous nervous system. Thus neural control plays a major role in the changes in the basal lung mechanics and responsiveness following CH.

Impact of elevated pulmonary blood flow and capillary pressure on lung responsiveness

Since alterations in pulmonary hemodynamics may lead to airway hyperreactivity, the consequences of individual changes in pulmonary blood flow (Qp) and capillary pressure (Pc) on lung responsiveness were investigated. During maintenance of a steady-state Pc of 5, 10, or 15 mmHg (groups 1-3), acute increases of Qp were generated in isolated, perfused rat lungs by simultaneous pulmonary arterial pressure elevation and venous pressure lowering. Conversely, at constant low (groups 4 and 5) or high Qp (groups 6 and 7), Pc was lowered or elevated by changing, in parallel, the pulmonary arterial and venous pressures. Pulmonary input impedance was measured under baseline conditions and during methacholine provocation (2-18 microg*kg(-1)*min(-1)), whereas the pulmonary hemodynamics were altered in accordance with the group allocation. The airway resistance and constant-phase parenchymal model parameters were identified from the pulmonary input impedance spectra. Increases of Qp at constant Pc had no effect on the basal lung mechanics, whereas they enhanced the lung reactivity to methacholine, particularly when high Pc was maintained [peak airway resistance increases of 299 +/- 99% (SE) vs. 609 +/- 217% at Qp levels of 5 and 10 ml/min, respectively, P < 0.05]. In contrast, the change of Pc at constant Qp slightly deteriorated the basal parenchymal mechanics without affecting the lung responsiveness. These findings suggest that increases in Qp per se may lead to the development of airway hyperreactivity. This phenomenon may contribute to the airway susceptibility under conditions associated with simultaneous elevations in pulmonary vascular pressures and Qp, such as exercise-induced asthma and the situation in children with congenital heart diseases.

Lung mechanical and vascular changes during positive- and negative-pressure lung inflations: importance of reference pressures in the pulmonary vasculature

The continuous changes in lung mechanics were related to those in pulmonary vascular resistance (Rv) during lung inflations to clarify the mechanical changes in the bronchoalveolar system and the pulmonary vasculature. Rv and low-frequency lung impedance data (Zl) were measured continuously in isolated, perfused rat lungs during 2-min inflation-deflation maneuvers between transpulmonary pressures of 2.5 and 22 cmH(2)O, both by applying positive pressure at the trachea and by generating negative pressure around the lungs in a closed box. ZL was averaged and evaluated for 2-s time windows; airway resistance (Raw), parenchymal damping and elastance (H) were determined in each window. Lung inflation with positive and negative pressures led to very similar changes in lung mechanics, with maximum decreases in Raw [-68 +/- 4 (SE) vs. -64 +/- 18%] and maximum increases in H (379 +/- 36 vs. 348 +/- 37%). Rv, however, increased with positive inflation pressure (15 +/- 1%), whereas it exhibited mild decreases during negative-pressure expansions (-3 +/- 0.3%). These results demonstrate that pulmonary mechanical changes are not affected by the opposing modes of lung inflations and confirm the importance of relating the pulmonary vascular pressures in interpreting changes in Rv.

Differential roles of endothelin-1 ETA and ETB receptors and vasoactive intestinal polypeptide in regulation of the airways and the pulmonary vasculature in isolated rat lung

The available treatment strategies against pulmonary hypertension include the administration of endothelin-1 (ET-1) receptor subtype blockers (ET(A) and ET(B) antagonists); vasoactive intestinal polypeptide (VIP) has recently been suggested as a potential new therapeutic agent. We set out to investigate the ability of these agents to protect against the vasoconstriction and impairment of lung function commonly observed in patients with pulmonary hypertension. An ET(A) blocker (BQ123), ET(B) blocker (BQ788), a combination of these selective blockers (ET(A) + ET(B) blockers) or VIP (V6130) was administered into the pulmonary circulation in four groups of perfused normal rat lungs. Pulmonary vascular resistance (PVR) and forced oscillatory lung input impedance (Z(L)) were measured in all groups under baseline conditions and at 1 min intervals following ET-1 administrations. The airway resistance, inertance, tissue damping and elastance were extracted from the Z(L) spectra. While VIP, ET(A) blocker and combined ET(A) and ET(B) blockers significantly prevented the pulmonary vasoconstriction induced by ET-1, ET(B) blockade enhanced the ET-1-induced increases in PVR. In contrast, the ET(A) and ET(B) blockers markedly elevated the ET-1-induced increases in airway resistance, while VIP blunted this constrictor response. Our results suggest that VIP potently acts against the airway and pulmonary vascular constriction mediated by endothelin-1, while the ET(A) and ET(B) blockers exert a differential effect between airway resistance and PVR.

Mechanisms of airway hyper-responsiveness after coronary ischemia

We explored the consequences of myocardial ischemia (MI) on the lung responsiveness and identified the pathophysiological mechanisms involved. Airway resistance (R(aw)) was identified from the respiratory system input impedance (Z(rs)) in rats. Z(rs) was determined under baseline conditions, and following iv boluses of 20 and 30 microg/kg serotonin. MI was then induced in the animals in Group I by ligating the left-interventricular coronary artery, while rats in Group C underwent sham surgery. Four weeks later, baseline Z(rs) and its changes following serotonin administration were reassessed. Lung morphological changes were assessed by histology, and alpha smooth muscle actin cells (alpha-SMA) were identified. MI induced no changes in baseline R(aw) but led to bronchial hyper-reactivity (BHR) with 2.7+/-0.5-times (p<0.05) greater responses in R(aw) to 30 microg/kg serotonin. Perivascular edema and alpha-SMA cell proliferation were observed after MI. The development of BHR following MI is a consequence of the expression of alpha-SMA, while the geometrical alterations caused by the pulmonary vascular engorgement have smaller impact.

An improved technique for repeated bronchoalveolar lavage and lung mechanics measurements in individual rats

Lung function and bronchoalveolar lavage (BAL) fluid are commonly analyzed to assess the severity of lung disease in sacrificed animals. The input impedance of the respiratory system (Z(rs)) was measured and BAL fluid was collected in intubated, anesthetized, mechanically ventilated rats on three occasions 1 week apart. Measurements were performed in control animals (group C), while lung injury was induced in the other group (group LPS) by i.p. injection of lipopolysaccharide (LPS) before the second measurement. The airway resistance (R(aw)), tissue damping (G) and elastance (H) were determined from the Z(rs) spectra. The total cell counts (TC) from 0.3- to 0.4-ml BAL fluid were also determined. R(aw) exhibited no significant change in either group C (-6.7+/-3.6[S.E.]%) or LPS (-0.9+/-3.7%). Reproducible G and H values were obtained in group C (2.5+/-5.3%, -7.0+/-4.4%), while G and H increased in group LPS (18.4+/-6.5%, 14.9+/-13.8%, p<0.05). The changes in TC followed a similar pattern to those observed in G, with no change in group C (-7.9+/-30%), but with a marked increase in group LPS (580+/-456%, p<0.05). The method devised for repeated BAL measurements in another group of rats without intubation and muscle relaxant resulted in similar results in BAL profile. We conclude that longitudinal follow-up of the airway and tissue mechanics and inflammatory cells in the BAL fluid are feasible in rats. The current method allows an early detection of lung injury, even in a relatively mild form.

Development of Bronchoconstriction After Administration of Muscle Relaxants in Rabbits with Normal or Hyperreactive Airways

Neuromuscular blocking drugs can induce intraoperative bronchospasm. We characterized the magnitude and the temporal profile of the constriction in normal or in hyperresponsive airways after injections of neuromuscular blocking drugs. Respiratory system impedance (Zrs) was measured continuously over a 90-s apneic period in naïve and rabbits sensitized to allergens by ovalbumin. Fifteen s after the start of Zrs recordings, succinylcholine, mivacurium, or pipecuronium was administered in random order. Zrs was then also recorded during the administration of increasing doses of exogenous histamine. To monitor the changes in the airway mechanics during these maneuvers, Zrs was averaged for 2-s time windows, and the airway resistance (Raw) was determined by model fitting. The increases in Raw were significantly larger in the sensitized rabbits than in the naïve animals. The largest increases in Raw and the maximum rate of change in Raw were obtained for succinylcholine (146% +/- 29% and 0.80 +/- 0.12 cm H2O/L, respectively) and mivacurium (80% +/- 25% and 0.71 +/- 0.13 cm H2O/L) and the smallest were obtained for pipecuronium (40% +/- 12% and 0.41 +/- 0.04 cm H2O/L). Allergic sensitization leads to severe and rapidly developing bronchospasm after administrations of mivacurium or succinylcholine. These deleterious side effects should be considered when succinylcholine or mivacurium is administered in the presence of bronchial hyperreactivity.

Measuring end-expiratory lung volume and pulmonary mechanics to detect early lung function impairment in rabbits

We investigated whether end-expiratory lung volume (EELV) or lung mechanical parameters are more sensitive for the detection of a compromised gas exchange during bronchoconstriction and after surfactant depletion. EELV was determined via SF(6) multiple breath wash-outs in mechanically ventilated rabbits while a positive end-expiratory pressure (PEEP) of 1, 3 or 7 cm H(2)O was maintained. Airway resistance (R(aw)) and parenchymal elastance (H) were estimated from the pulmonary input impedance measured at each PEEP level by means of forced oscillations. Measurements were repeated during i.v. methacholine (MCh) infusions and following lung injury induced by saline lavage. MCh induced marked elevations in R(aw), with no significant change in EELV or H at any PEEP. After lavage, the severity of hypoxia was reflected systematically in significant decreases in EELV at all PEEP levels (-42+/-13%, -26+/-4%, and -18+/-5% at 1, 3 and 7 cm H(2)O, respectively), whereas compromised gas exchange was not associated with consistent changes in the mechanical parameters at a PEEP of 7 cm H(2)O (20+/-9% and 14+/-9% in R(aw) and H, respectively; p=0.2). We conclude that R(aw) is the only sensitive indicator for the detection of a compromised lung function during MCh infusions, whereas the estimation of EELV is necessary to follow the progression of a lung injury when a high PEEP level is applied.

The role of endothelin-1 in hyperoxia-induced lung injury in mice

Background

As prolonged hyperoxia induces extensive lung tissue damage, we set out to investigate the involvement of endothelin-1 (ET-1) receptors in these adverse changes.

Methods

Experiments were performed on four groups of mice: control animals kept in room air and a group of mice exposed to hyperoxia for 60 h were not subjected to ET-1 receptor blockade, whereas the dual ETA/ETB-receptor blocker tezosantan (TEZ) was administered via an intraperitoneal pump (10 mg/kg/day for 6 days) to other groups of normal and hyperoxic mice. The respiratory system impedance (Zrs) was measured by means of forced oscillations in the anesthetized, paralyzed and mechanically ventilated mice before and after the iv injection of ET-1 (2 μg). Changes in the airway resistance (Raw) and in the tissue damping (G) and elastance (H) of a constant-phase tissue compartment were identified from Zrs by model fitting.

Results

The plasma ET-1 level increased in the mice exposed to hyperoxia (3.3 ± 1.6 pg/ml) relative to those exposed to room air (1.6 ± 0.3 pg/ml, p < 0.05). TEZ administration prevented the hyperoxia-induced increases in G (13.1 ± 1.7 vs. 9.6 ± 0.3 cmH₂O/l, p < 0.05) and H (59 ± 9 vs. 41 ± 5 cmH₂O/l, p < 0.05) and inhibited the lung responses to ET-1. Hyperoxia decreased the reactivity of the airways to ET-1, whereas it elevated the reactivity of the tissues.

Conclusion

These findings substantiate the involvement of the ET-1 receptors in the physiopathogenesis of hyperoxia-induced lung damage. Dual ET-1 receptor antagonism may well be of value in the prevention of hyperoxia-induced parenchymal damage.

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.

Investigation of penetratin peptides. Part 2.In vitro uptake of penetratin and two of its derivatives

As endocytic uptake of the Antennapedia homeodomain-derived penetratin peptide (RQIKIWFQNRRMKWKK) is finally being revealed, some of the early views about penetratin need to be reconsidered. Endocytic uptake seems to contradict the indispensability of tryptophans and also the minimum length of 16 amino acid residues for efficient internalization. To revise the membrane translocation of penetratin, two penetratin analogs were designed and synthesized: a peptide in which tryptophans were replaced by phenylalanines (Phe(6,14)-penetratin, RQIKIFFQNRRMKFKK) and a shortened analog (dodeca-penetratin, RQIKIWF-R-KWKK) made up of only 12 residues. The peptides were fluorescently labeled and applied to live, unfixed cells from various lines. Cellular uptake was analysed by confocal microscopy and flow cytometry. Low temperature or ATP-depletion blocked the intracellular entry of all three penetratin peptides. A decrease in membrane fluidity or cholesterol depletion with methyl-beta-cyclodextrin greatly inhibited peptide uptake, showing the involvement of cholesterol-rich lipid rafts in internalization. Exogenous heparan sulfate also diminished the internalization of penetratin and its derivatives, reflecting the paramount importance of electrostatic interactions with polyanionic cell-surface proteoglycans. The beneficial presence of tryptophans is supported by observations on the decreased cellular uptake of Phe(6, 14)-penetratin. The maintained translocational efficiency of dodeca-penetratin demonstrates that a thorough understanding of penetratin internalization can yield new penetratin analogs with unaltered translocational abilities. This study provides evidence on the energy-dependent and lipid raft-mediated endocytic uptake of penetratin and highlights the necessity of revealing those pathways that cationic cell-penetrating peptides employ to enter live cells.

Preoperative pulmonary hemodynamics determines changes in airway and tissue mechanics following surgical repair of congenital heart diseases

To characterize the effect of changes in pulmonary hemodynamics on airway and tissue mechanics, forced oscillatory input impedance of the respiratory system (Zrs) was measured between 0.4-12 Hz in two groups of children undergoing surgical repair of congenital heart disease (CHD) immediately before sternotomy and after chest closure during short apneic intervals. Children with lesions associated with high pulmonary blood flow and/or pressure (septal defects; HP group, n = 12) and children with hypoperfused lungs (tetralogy of Fallot; LP group, n = 12) were included in the study. Airway resistance (Raw), and coefficients of respiratory tissue damping (G) and elastance (H), were estimated from Zrs by model-fitting. A postoperative reduction in pulmonary blood flow and/or pressure in the HP group resulted in an immediate decrease in Raw of 29 +/- 9 (SE)% (P < 0.05), whereas children in the LP group had increases in Raw (24 +/- 17%, no significance) after surgery. No significant change was observed in G in either the HP (6.4 +/- 13%) or LP (27 +/- 23%) group, while H increased in children of both the HP (23 +/- 8%, P < 0.05) and LP (36 +/- 7%, P < 0.01) groups. These results suggest that the preoperative pulmonary hemodynamic condition determines changes in airway mechanics: surgical repair of CHD leads to an improvement in airway function only in children with congested lungs. The adverse effects of surgery, mechanical ventilation, and/or cardiopulmonary bypass may be responsible for the increased stiffness of the respiratory system observed in both groups of children.

Impact of microvascular circulation on peripheral lung stability

The involvement of pulmonary circulation in the mechanical properties was studied in isolated rat lungs. Pulmonary input impedance (ZL) was measured at a mean transpulmonary pressure (Ptpmean) of 2 cmH2O before and after physiological perfusion with either blood or albumin. In these lungs and in a group of unperfused lungs, ZL was also measured at Ptpmean values between 1 and 8 cmH2O. Airway resistance ( Raw) and parenchymal damping (G) and elastance (H) were estimated from ZL. End-expiratory lung volume (EELV) was measured by immersion before and after blood perfusion. The orientation of the elastin fibers relative to the basal membrane was assessed in additional unperfused and blood-perfused lungs. Pressurization of the pulmonary capillaries significantly decreased H by 31.5 ± 3.7% and 18.7 ± 2.7% for blood and albumin, respectively. Perfusion had no effect on Raw but markedly altered the Ptpmean dependences of G and H <4 cmH2O, with significantly lower values than in the unperfused lungs. At a Ptpmean of 2 cmH2O, EELV increased by 31 ± 11% ( P = 0.01) following pressurization of the capillaries, and the elastin fibers became more parallel to the basal membrane. Because the organization of elastin fibers results in smaller H values of the individual alveolus, the higher H in the unperfused lungs is probably due to a partial alveolar collapse leading to a loss in lung volume. We conclude that the physiological pressure in the pulmonary capillaries is an important mechanical factor in the maintenance of the stability of the alveolar architecture.

Acoustic evidence of airway opening during recruitment in excised dog lungs

The aim of this study was to test the hypothesis that the mechanism of recruitment and the lower knee of the pressure-volume curve in the normal lung are primarily determined by airway reopenings via avalanches rather than simple alveolar recruitments. In isolated dog lung lobes, the pressure-volume loops were measured, and crackle sounds were recorded intrabronchially during both the first inflation from the collapsed state to total lobe capacity and a second inflation without prior degassing. The inflation flow contained transients that were accompanied by a series of crackles. Discrete volume increments were estimated from the flow transients, and the energy levels of the corresponding crackles were calculated from the sound recordings. Crackles were concentrated in the early phase of inflation, with the cumulative energy exceeding 90% of its final value by the lower knee of the pressure-volume curve. The values of volume increments were correlated with crackle energy during the flow transient for both the first and the second inflations ( r2 = 0.29–0.73 and 0.68–0.82, respectively). Because the distribution of volume increments followed a power law, the correlation between crackle energy and discrete volume increments suggests that an avalanche-like airway opening process governs the recruitment of collapsed normal lungs.

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.

Effects of volatile anaesthetic agents on enhanced airway tone in sensitized guinea pigs

BACKGROUND

Although volatile anaesthetics afford protection against bronchospasm, their potential to reverse a sustained constriction of hyperreactive airways has not been characterized. Accordingly, we investigated the ability of halothane, isoflurane, sevoflurane and desflurane to reverse lung constriction induced by prolonged stimulation of the muscarinic receptors in guinea pigs sensitized to ovalbumin.

METHODS

Pulmonary input impedance (ZL) was measured using forced oscillations in five groups of ovalbumin-sensitized, mechanically ventilated guinea pigs. ZL was measured under baseline conditions, during steady-state bronchoconstriction induced by an i.v. infusion of methacholine (MCh), and after administration of one of the volatile agents at 1 MAC after the induction of a steady-state bronchoconstriction. Airway resistance (Raw), and parenchymal tissue resistive and elastic coefficients were extracted from ZL by model fitting.

RESULTS

All four volatile agents exhibited an initial relaxation of the MCh-induced airway constriction followed by gradual increases in Raw. The bronchodilatory effect of isoflurane was the most potent (-28.9 (SE 5.5)% at 2 min, P<0.05) and lasted longest (7 min); sevoflurane and halothane had shorter and more moderate effects (-21.1 (3.9)%, P<0.05, and -6.1 (1.7)%, P<0.05, respectively, at 1 min). Desflurane caused highly variable changes in Raw, with a tendency to enhance airway tone.

CONCLUSIONS

Volatile agents can reverse sustained MCh-induced airway constriction only transiently in sensitized guinea pigs. Isoflurane proved most beneficial in temporally improving lung function in the presence of a severe constriction of allergic inflamed airways. Desflurane displayed potential to induce further airway constriction.

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.

IMPLICATIONS

We observed that cardiopulmonary bypass deteriorates lung function by inducing a heterogeneous airway constriction, whereas no such effects were observed in patients undergoing cardiac surgery without bypass. The impairment in parenchymal mechanics, which was obtained in both groups, may result from peripheral airway closure and/or be a consequence of mediator release.

Airway and tissue mechanics in anesthetized paralyzed children

To estimate the mechanical properties of the airways and respiratory tissues, respiratory system impedance (Zrs) was measured with low-frequency forced oscillations in 26 anesthetized, paralyzed children (aged 3 months-10 years) undergoing surgical correction of congenital heart diseases. Zrs was determined from the signals of tracheal flow and pressure between 0.4-12 Hz before surgery at zero mean transrespiratory pressure. The pulmonary (Z(L)) and chest wall (Z(W)) components of Zrs were also determined in 5 children by measuring esophageal pressure. A model containing frequency-independent resistance (R) and inertance (I), and coefficients of tissue-damping (G) and elastance (H), was fitted to the Zrs, Z(L), and Z(W) spectra. The total respiratory parameters normalized to body weights were 82.2 +/- 8.5 (SE) hPa x sec x l(-1) x kg, 0.152 +/- 0.05 hPa x sec(2) x l(-1) x kg, 293.8 +/- 20.0 hPa. l(-1) x kg, and 1,583 +/- 65.5 hPa x l(-1) x kg, for R, I, G, and H, respectively. The measurements of Z(L) and Z(W) revealed the dominance of the lungs in R (91 +/- 4.3%) and I (109 +/- 16%), and the major contribution of the lung parenchyma to G (61 +/- 7.3%) and H (66 +/- 7.4%) of the total respiratory system. It is concluded that anesthesia-paralysis provides an ideal condition for the measurement of low-frequency forced oscillatory impedance and its partitioning into airway and tissue components in mechanically ventilated children. The separation of pulmonary and chest wall mechanics demonstrates that airway properties can be estimated appropriately from Zrs data, while the chest wall may damp the changes in parenchymal properties.

Components of respiratory resistance monitored in mechanically ventilated patients

The interrupter technique is commonly adopted to monitor respiratory resistance (Rrs,int) during mechanical ventilation; however, Rrs,int is often interpreted as an index of airway resistance (Raw). This study compared the values of Rrs,int provided by a Siemens 940 Lung Mechanics Monitor with total respiratory impedance (Zrs) parameters in 39 patients with normal spirometric parameters, who were undergoing elective coronary bypass surgery. Zrs was determined at the airway opening with pseudorandom oscillations of 0.2-6 Hz at end inspiration. Raw and tissue resistance (Rti) were derived from the Zrs data by model fitting; Rti and total resistance (Rrs,osc=Raw+Rti) were calculated at the actual respirator frequencies. Lower airway resistance (Rawl) was estimated by measuring tracheal pressure. Although good agreement was obtained between Rrs,osc and Rrs,int, with a ratio of 1.07+/-0.19 (mean+/-SD), they correlated poorly (r2=0.36). Rti and the equipment component of Raw accounted for most of Rrs,osc (39.8+/-11.9 and 43.0+/-6.9%, respectively), whereas only a small portion belonged to Rawl (17.2+/-6.3%). It is concluded that respiratory resistance may become very insensitive to changes in lower airway resistance and therefore, inappropriate for following alterations in airway tone during mechanical ventilation, especially in patients with relatively normal respiratory mechanics, where the tissue and equipment resistances represent the vast majority of the total resistance.

Role of endogenous histamine in altered lung mechanics in rabbits

BACKGROUND

Unlike the effects of exogenous histamine, those of endogenous histamine on the lung mechanics have not yet been characterized. The site of endogenous histamine liberation by mivacurium was determined, as were the effects of this histamine on the airway and parenchymal mechanics in control rabbits (group C) and rabbits pretreated with H1 and H2 receptor blockers (group AH). The effectiveness of the receptor blockade was ensured by challenges with exogenous histamine.

METHODS

Pulmonary input impedance at low frequencies (ZL) was measured in anesthetized mechanically ventilated open-chest rabbits under control conditions and every minute after administration of an intravenous bolus of mivacurium (2 mg/kg) and exogenous histamine (10 microg/kg). Histamine levels were determined in serum samples taken from the carotid artery and jugular vein before and 1, 3, and 6 min after mivacurium injection. Parameters of airway resistance (Raw) and inertance and parenchymal damping (G) and elastance (H) were extracted from ZL spectra.

RESULTS

Mivacurium induced significant increases in plasma histamine levels, with the venous concentrations being significantly higher than those in the artery. The mivacurium-induced increase in Raw (28.7 +/- 2.3%; mean +/- SD) in group C was significantly higher than that in group AH (6.6 +/- 3.4%), whereas the responses in G were not inhibited significantly (23.9 +/- 6.9% vs. 15.5 +/- 3.0%). The significant increases in Raw (70.6 +/- 12.6%) and G (21.0 +/- 4.9%) after exogenous histamine administration were virtually completely abolished by antihistamine pretreatment (3.6 +/- 3.7% and 0.3 +/- 2.6%).

CONCLUSIONS

After mivacurium administration, endogenous histamine is liberated at least partly in the systemic circulation, and it induces primarily a heterogeneous airway constriction with minor changes in the parenchymal properties. This response was considerably reduced but not abolished by antihistamine pretreatment, a circumstance suggesting that mivacurium may liberate other constrictor mediators that might also contribute to the airway and parenchymal constriction.

Effects of pulmonary vascular pressures and flow on airway and parenchymal mechanics in isolated rat lungs

Changes in pulmonary hemodynamics have been shown to alter the mechanical properties of the lungs, but the exact mechanisms are not clear. We therefore investigated the effects of alterations in pulmonary vascular pressure and flow (Q(p)) on the mechanical properties of the airways and the parenchyma by varying these parameters independently in three groups of isolated perfused normal rat lungs. The pulmonary capillary pressure (Pc(est)), estimated from the pulmonary arterial (Ppa) and left atrial pressure (Pla), was increased at constant Q(p) (n = 7), or Q(p) was changed at Pc(est) = 10 mmHg (n = 7) and at Pc(est) = 20 mmHg (n = 6). In each condition, the airway resistance (Raw) and parenchymal damping (G) and elastance (H) were identified from the low-frequency pulmonary input impedance spectra. The results of measurements made under isogravimetric conditions were analyzed. The changes observed in the mechanical parameters were consistent with an altered Pla: monotonous increases in Raw were observed with increasing Pla, whereas G and H were minimal at Pla of approximately 7-10 mmHg and increased at lower and higher Pla. The results indicate that Pla, and not Ppa or Q(p), is the primary determinant of the mechanical condition of the lungs after acute changes in pulmonary hemodynamics: the parenchymal mechanics are impaired if Pla is lower or higher than physiological, whereas airway narrowing occurs at high Pla.

Hyperoxia-induced changes in mouse lung mechanics: forced oscillations vs. barometric plethysmography

Hyperoxia-induced lung damage was investigated via airway and respiratory tissue mechanics measurements with low-frequency forced oscillations (LFOT) and analysis of spontaneous breathing indexes by barometric whole body plethysmography (WBP). WBP was performed in the unrestrained awake mice kept in room air ( n = 12) or in 100% oxygen for 24 ( n = 9), 48 ( n = 8), or 60 ( n = 9) h, and the indexes, including enhanced pause (Penh) and peak inspiratory and expiratory flows, were determined. The mice were then anesthetized, paralyzed, and mechanically ventilated. Airway resistance, respiratory system resistance at breathing frequency, and tissue damping and elastance were identified from the LFOT impedance data by model fitting. The monotonous decrease in airway resistance during hyperoxia correlated best with the increasing peak expiratory flow. Respiratory system resistance and tissue damping and elastance were unchanged up to 48 h of exposure but were markedly elevated at 60 h, with associated decreases in peak inspiratory flow. Penh was increased at 24 h and sharply elevated at 60 h. These results indicate no adverse effect of hyperoxia on the airway mechanics in mice, whereas marked parenchymal damage develops by 60 h. The inconsistent relationships between LFOT parameters and WBP indexes suggest that the changes in the latter reflect alterations in the breathing pattern rather than in the mechanical properties. It is concluded that, in the presence of diffuse lung disease, Penh is inadequate for characterization of the mechanical status of the respiratory system.

Endothelin-1-induced airway and parenchymal mechanical responses in guinea-pigs: the roles of ETA and ETB receptors

Endothelin-1 (ET-1) has been shown to have a constrictor effect on the airways and parenchyma; however, the roles of the ETA and ETB receptors in the ET-1-induced changes in the airway and tissue compartments have not been fully explored. Low-frequency pulmonary impedance (ZL) was measured in anaesthetized, paralysed, open-chest guinea-pigs. ZL spectra were fitted by a model to estimate airway resistance (Raw) and inertance (Iaw), and coefficients of tissue damping (G) and elastance (H), and hysteresivity (eta = G/H). Two successive doses of ET-1 (0.05 and 0.2 nmol x kg(-1)) each evoked significant dose-related increases in Raw, G, H and eta. Pretreatment with 20 nmol x kg(-1) BQ-610 (a highly selective ETA receptor antagonist) resulted in a significantly decreased elevation only in H after the lower dose of ET-1. However, all parameters changed significantly less on the administration of ET-1 after pretreatment with 80 nmol-kg(-1) BQ-610, with 20 nmol x kg(-1) ETR-P1/fl (a novel ETA receptor antagonist) or with 20 nmol x kg(-1) IRL 1038 (an ETB receptor antagonist). The results of the separate assessments of the airway and tissue mechanics demonstrate that endothelin-1 induces airway and parenchymal constriction via stimulation of both receptor types in both compartments.