Effects of mean airway pressure and tidal volume on lung and chest wall mechanics in the dog

In vivo imaging of canine lung deformation: effects of posture, pneumonectomy, and inhaled erythropoietin

Mechanical stresses on the lung impose the major stimuli for developmental and compensatory lung growth and remodeling. We used computed tomography (CT) to noninvasively characterize the factors influencing lobar mechanical deformation in relation to posture, pneumonectomy (PNX), and exogenous proangiogenic factor supplementation. Post-PNX adult canines received weekly inhalations of nebulized nanoparticles loaded with recombinant human erythropoietin (EPO) or control (empty nanoparticles) for 16 wk. Supine and prone CT were performed at two transpulmonary pressures pre- and post-PNX following treatment. Lobar air and tissue volumes, fractional tissue volume (FTV), specific compliance (Cs), mechanical strains, and shear distortion were quantified. From supine to prone, lobar volume and Cs increased while strain and shear magnitudes generally decreased. From pre- to post-PNX, air volume increased less and FTV and Cs increased more in the left caudal (LCa) than in other lobes. FTV increased most in the dependent subpleural regions, and the portion of LCa lobe that expanded laterally wrapping around the mediastinum. Supine deformation was nonuniform pre- and post-PNX; strains and shear were most pronounced in LCa lobe and declined when prone. Despite nonuniform regional expansion and deformation, post-PNX lobar mechanics were well preserved compared with pre-PNX because of robust lung growth and remodeling establishing a new mechanical equilibrium. EPO treatment eliminated posture-dependent changes in FTV, accentuated the post-PNX increase in FTV, and reduced FTV heterogeneity without altering absolute air or tissue volumes, consistent with improved microvascular blood volume distribution and modestly enhanced post-PNX alveolar microvascular reserves.NEW & NOTEWORTHY Mechanical stresses on the lung impose the major stimuli for lung growth. We used computed tomography to image deformation of the lung in relation to posture, loss of lung units, and inhalational delivery of the growth promoter erythropoietin. Following loss of one lung in adult large animals, the remaining lung expanded and grew while retaining near-normal mechanical properties. Inhalation of erythropoietin promoted more uniform distribution of blood volume within the remaining lung.

Elevated Mean Airway Pressure and Central Venous Pressure in the First Day of Mechanical Ventilation Indicated Poor Outcome

OBJECTIVES

The relationship between respiratory mechanical parameters and hemodynamic variables remains unclear. This study was performed to determine whether mean airway pressure and central venous pressure in the first day of mechanical ventilation are associated with patient outcomes.

DESIGN

Retrospective first 24-hour comparison during ICU stay.

SETTING

The Department of Critical Care Medicine of Peking Union Medical College Hospital.

PATIENTS

Patients with mechanical ventilation.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The clinical data of patients who received mechanical ventilation, especially respiratory and hemodynamic data, were collected and analyzed. In terms of the hemodynamic and perfusion data, the nonsurvivors group (177/2,208) had higher heart rate, respiratory rate, central venous pressure, and lactates and a lower perfusion index and P(v-a)CO2 (p < 0.05). In terms of respiratory condition, mean airway pressure, peak airway pressure, positive end-expiratory pressure, driving pressure, and inspiratory time/total respiration time of nonsurvivors were significantly higher, and arterial oxygen pressure and dynamic compliance worsened and were lower than the survivors (p < 0.05). Increased central venous pressure (odds ratio, 1.125; 95% CI, 1.069-1.184; p < 0.001) and elevated mean airway pressure (odds ratio, 1.125; 95% CI, 1.069-1.184; p < 0.001) were independently associated with 28-day mortality. The area under receiver operating characteristic demonstrated that central venous pressure and mean airway pressure were measured at 0.795 (95% CI, 0.654-0.757) and 0.833 (95% CI, 0.608-0.699), respectively. Based on the cutoff of central venous pressure and mean airway pressure, all of the participants were divided into the following groups: low central venous pressure and mean airway pressure, only high central venous pressure or mean airway pressure, or high central venous pressure and mean airway pressure. Post hoc tests showed significant differences among these three groups based on 28-day survival (log rank [Mantel-Cox], 131.931; p < 0.001).

CONCLUSIONS

During the first 24 hours of mechanical ventilation, patients with elevated mean airway pressure and elevated central venous pressure had worse outcomes.

Respiratory Effects of Sarafotoxins from the Venom of Different Atractaspis Genus Snake Species

Sarafotoxins (SRTX) are endothelin-like peptides extracted from the venom of snakes belonging to the Atractaspididae family. A recent in vivo study on anesthetized and ventilated animals showed that sarafotoxin-b (SRTX-b), extracted from the venom of Atractaspis engaddensis, decreases cardiac output by inducing left ventricular dysfunction while sarafotoxin-m (SRTX-m), extracted from the venom of Atractaspis microlepidota microlepidota, induces right ventricular dysfunction with increased airway pressure. The aim of the present experimental study was to compare the respiratory effects of SRTX-m and SRTX-b. Male Wistar rats were anesthetized, tracheotomized and mechanically ventilated. They received either a 1 LD₅₀ IV bolus of SRTX-b (n = 5) or 1 LD₅₀ of SRTX-m (n = 5). The low-frequency forced oscillation technique was used to measure respiratory impedance. Airway resistance (Raw), parenchymal damping (G) and elastance (H) were determined from impedance data, before and 5 min after SRTX injection. SRTX-m and SRTX-b injections induced acute hypoxia and metabolic acidosis with an increased anion gap. Both toxins markedly increased Raw, G and H, but with a much greater effect of SRTX-b on H, which may have been due to pulmonary edema in addition to bronchoconstriction. Therefore, despite their structural analogy, these two toxins exert different effects on respiratory function. These results emphasize the role of the C-terminal extension in the in vivo effect of these toxins.

Correlated variability in the breathing pattern and end-expiratory lung volumes in conscious humans

In order to characterize the variability and correlation properties of spontaneous breathing in humans, the breathing pattern of 16 seated healthy subjects was studied during 40 min of quiet breathing using opto-electronic plethysmography, a contactless technology that measures total and compartmental chest wall volumes without interfering with the subjects breathing. From these signals, tidal volume (V_T), respiratory time (T_TOT) and the other breathing pattern parameters were computed breath-by-breath together with the end-expiratory total and compartmental (pulmonary rib cage and abdomen) chest wall volume changes. The correlation properties of these variables were quantified by detrended fluctuation analysis, computing the scaling exponentα. V_T, T_TOT and the other breathing pattern variables showed α values between 0.60 (for minute ventilation) to 0.71 (for respiratory rate), all significantly lower than the ones obtained for end-expiratory volumes, that ranged between 1.05 (for rib cage) and 1.13 (for abdomen) with no significant differences between compartments. The much stronger long-range correlations of the end expiratory volumes were interpreted by a neuromechanical network model consisting of five neuron groups in the brain respiratory center coupled with the mechanical properties of the respiratory system modeled as a simple Kelvin body. The model-based α for V_T is 0.57, similar to the experimental data. While the α for T_TOT was slightly lower than the experimental values, the model correctly predicted α for end-expiratory lung volumes (1.045). In conclusion, we propose that the correlations in the timing and amplitude of the physiological variables originate from the brain with the exception of end-expiratory lung volume, which shows the strongest correlations largely due to the contribution of the viscoelastic properties of the tissues. This cycle-by-cycle variability may have a significant impact on the functioning of adherent cells in the respiratory system.

Oscillation mechanics of the respiratory system

The mechanical impedance of the respiratory system defines the pressure profile required to drive a unit of oscillatory flow into the lungs. Impedance is a function of oscillation frequency, and is measured using the forced oscillation technique. Digital signal processing methods, most notably the Fourier transform, are used to calculate impedance from measured oscillatory pressures and flows. Impedance is a complex function of frequency, having both real and imaginary parts that vary with frequency in ways that can be used empirically to distinguish normal lung function from a variety of different pathologies. The most useful diagnostic information is gained when anatomically based mathematical models are fit to measurements of impedance. The simplest such model consists of a single flow-resistive conduit connecting to a single elastic compartment. Models of greater complexity may have two or more compartments, and provide more accurate fits to impedance measurements over a variety of different frequency ranges. The model that currently enjoys the widest application in studies of animal models of lung disease consists of a single airway serving an alveolar compartment comprising tissue with a constant-phase impedance. This model has been shown to fit very accurately to a wide range of impedance data, yet contains only four free parameters, and as such is highly parsimonious. The measurement of impedance in human patients is also now rapidly gaining acceptance, and promises to provide a more comprehensible assessment of lung function than parameters derived from conventional spirometry.

Constant-phase descriptions of canine lung, chest wall, and total respiratory system viscoelasticity: effects of distending pressure

The dynamic mechanical properties of the respiratory system reflect the ensemble behavior of its constituent structural elements. This study assessed the appropriateness of constant-phase descriptions of respiratory tissue viscoelasticity at various distending pressures. We measured the mechanical input impedance (Z) of the lungs, chest wall and total respiratory system in 12 dogs at mean airway pressures from 5 to 30 cm H(2)O. Each Z was fitted with a constant-phase model which provided estimates tissue damping (G), elastance (H), and hysteresivity (η=G/H). Both G and H sharply increased with increasing distending pressure for the lungs and chest wall, while η attained a minimum near 15-20 cm H(2)O. Model fitting errors for the lungs and total respiratory system increased for distending pressures greater than 20 cm H(2)O, indicating that constant-phase descriptions of parenchymal and respiratory system viscoelasticty may be inappropriate at volumes closer to total lung capacity. Such behavior may reflect alterations in load distribution across various parenchymal stress-bearing elements.

Effects of components derived from diesel exhaust particles on lung physiology related to antigen

Our previous study has shown that diesel exhaust particles (DEP), main constituents in ambient particulate matters (PM), enhance airway hyperresponsivness in a murine model of allergic asthma (Takano et al., 1998). However, it remains unknown which components in DEP are responsible for the enhancement. The present study investigated the effects of repeated pulmonary exposure to DEP components (extracted organic chemicals in DEP; DEP-OC, carbonaceous nuclei of DEP after extraction; washed DEP) on lung physiology in the presence or absence of antigen. ICR mice were divided into six experimental groups. Vehicle, DEP components, ovalbumin (OVA), or DEP components plus OVA was administered intratrachally for 6 weeks. Twenty-four hr after the last instillation, cholinergic lung reactivity was examined. DEP components alone did not induce any facilitation of lung function as compared to vehicle alone. The values of total respiratory system resistance (R), elastance (E), Newtonian resistance (R(n)), tissue damping (G), and tissue elastance (H) were higher and the value of compliance (C) was lower in the OVA or the DEP component + OVA groups than in the vehicle group. In particular, the hyperreactivity was most prominent in the washed DEP + OVA group. The values in the DEP-OC + OVA group were not significantly different from those in the OVA group. These data suggest that carboneous component in DEP, rather than organic chemical one, can be attributable to the enhancement of lung hyperresponsiveness in allergic asthma.

Hyperoxia confers myocardial protection in mechanically ventilated rats through the generation of free radicals and opening of mitochondrial ATP-sensitive potassium channels

1. One hour exposure to hyperoxia has been shown previously to limit a subsequent ischaemia-reperfusion injury in spontaneously breathing rats. We tested the cardioprotective effect of a shorter period of hyperoxia during mechanical ventilation and the possible contribution of reactive oxygen species (ROS) and mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels. 2. Mechanically ventilated rats were exposed to normoxia (Fi O2 = 0.3) or hyperoxia (Fi O2 = 1.0) for 30 min and pH, P CO2, PO2, heart rate, airway and blood pressure were measured at baseline and after 30 min mechanical ventilation. Isolated hearts were subsequently subjected to 30 min ischaemia and 120 min reperfusion. Infarct size and left ventricular end-diastolic pressure (LVEDP), developed pressure (LVDP) and coronary flow (CF) were measured. In order to investigate the role of ROS and KATP channels within the mechanism leading to cardioprotection, the free radical scavenger N-acetylcysteine (NAC; 150 mg/kg) was infused in mechanically ventilated rats and the KATP channel blockers glibenclamide (200 mmol/L) or 5-hydroxydecanoate (10 mmol/L) were infused in isolated hearts immediately before ischaemia. 3. No differences were detected in P CO2, pH, heart rate, airway and blood pressure between the groups. However, the PO2 in hyperoxic groups was significantly higher compared with that in normoxic groups (P < 0.01). After 30 min ischaemia, we found that hyperoxic preconditioning significantly improved CF (P < 0.01), LVDP (P < 0.01) and LVEDP (P < 0.01) and reduced the extent of infarct size in the reperfused heart compared with the normoxic group (P < 0.01). When rats were pretreated either with NAC before hyperoxic ventilation or with K(ATP) channel blockers before ischaemia, myocardial protection was abolished. 4. Hyperoxic mechanical ventilation, prior to ischaemia, reduces myocardial reperfusion injury. This is likely to occur through the induction of oxidative stress, which leads to myocyte mitoKATP channel opening.

Effects of heterogeneities on the partitioning of airway and tissue properties in normal mice

We measured the mechanical properties of the respiratory system of C57BL/6 mice using the optimal ventilation waveform method in closed- and open-chest conditions at different positive end-expiratory pressures. The tissue damping (G), tissue elastance (H), airway resistance (Raw), and hysteresivity were obtained by fitting the impedance data to three different models: a constant-phase model by Hantos et al. (Hantos Z, Daroczy B, Suki B, Nagy S, Fredberg JJ. J Appl Physiol 72: 168-178, 1992), a heterogeneous Raw model by Suki et al. (Suki B, Yuan H, Zhang Q, Lutchen KR. J Appl Physiol 82: 1349-1359, 1997), and a heterogeneous H model by Ito et al. (Ito S, Ingenito EP, Arold SP, Parameswaran H, Tgavalekos NT, Lutchen KR, Suki B. J Appl Physiol 97: 204-212, 2004). Both in the closed- and open-chest conditions, G and hysteresivity were the lowest and Raw the highest in the heterogeneous Raw model, and G and H were the largest in the heterogeneous H model. Values of G, Raw, and hysteresivity were significantly higher in the closed-chest than in the open-chest condition. However, H was not affected by the conditions. When the tidal volume of the optimal ventilation waveform was decreased from 8 to 4 ml/kg in the closed-chest condition, G and hysteresivity significantly increased, but there were smaller changes in H or Raw. In summary, values of the obtained mechanical properties varied among these models, primarily due to heterogeneity. Moreover, the mechanical parameters were significantly affected by the chest wall and tidal volume in mice. Contribution of the chest wall and heterogeneity to the mechanical properties should be carefully considered in physiological studies in which partitioning of airway and tissue properties are attempted.

Vitamin A deficiency alters pulmonary parenchymal collagen and tissue mechanics

The mechanical properties of the pulmonary parenchyma are strongly influenced by the collagen and elastic fibers that course through the alveolar interstitium and interconnect the bronchovascular bundles. Vitamin A deficiency (VAD) produces effacement and remodeling of the alveolar architecture, resulting in alternating areas of alveolar dilatation and collapse. To better understand the mechanical consequences and reversibility of this remodeling process, we have examined how the remodeling of collagen and elastic fibers correlates with the mechanical properties of the lung parenchyma in VAD rats. An oscillatory impulse was applied at different levels of stress on the fiber network and the tissue damping (G), elastance (H), hysteresivity (G/H, eta) were analyzed. At a supra-physiological functional residual capacity, the lung parenchyma of VAD rats exhibited a lower G and H than Vitamin A sufficient (VAS) rats, which was accompanied by a significant decrease in the quantity of parenchymal collagen and collagen fibers. Retinoic acid (RA) administration restored the parenchymal collagen and mechanical properties.

Canine awake head-out plethysmography (HOP): Characterization of external resistive loading and spontaneous laryngeal paralysis

We applied a novel head-out plethysmographic (HOP) method to study awake canine responses to external resistive loading and natural laryngeal paralysis. Measurements of inspiratory and expiratory specific airway resistance (sRaw(insp), sRaw(exp)) were obtained before and after uni- and bidirectional loading (R(add) = 5 cmH(2)O/L/s) in large-breed dogs (n = 9). Mean sRaw(insp) after inspiratory, and sRaw(exp) after expiratory loading were 31.4 and 33.3 cmH(2)Os, respectively. Bidirectional loads induced a significantly greater rise in both sRaw(insp) and sRaw(exp) (55.1 and 61.3 cmH(2)Os) compared to unidirectional loading (P < 0.001). Yet, type of loading did not affect flow-volume indices. The mean R(aw) of dogs was 4.81 cmH(2)O/L/s. Expiratory loading resulted in a significant 8.8% increase in functional-residual-capacity (FRC), compared to FRC(baseline) (76.7 ml/kg). Dogs (n = 5) with laryngeal paralysis demonstrated a significant increase in sR(aw) and R(aw) compared to controls without changes in FRC. In conclusion, HOP precisely characterized sR(aw) in response to external resistive loading. Hence, we could accurately quantify airway obstruction in awake dogs with laryngeal paralysis.

Influence of breathing pattern and lung inflation on impulse oscillometry measurements in horses

The objective of this paper was to determine if changes in ventilation patterns could influence the outcome of respiratory function measurements performed with our impulse oscillometry system (IOS) in horses. In a first study, IOS tests were performed in vitro on six isolated equine lungs. Lung inflation levels were controlled by modifying depressurisation inside an artificial thorax and different ventilation patterns were imposed. In a second in vivo study, transient variations in breathing pattern were evaluated both with the IOS and a current reference technique (CRT) in five healthy mature horses after an intravenous (i.v.) injection of lobeline hydrochloride. In both studies, respiratory rate (RR, range: 7-42 breaths/min.) and tidal volume (V(T), range: 0.4-25 L) had minor or no influence on IOS parameters. The influence of lung inflation, most marked for resistance at 5 Hz (R(5 Hz)), was limited for the considered physiological range. In vivo, statistical models indicated that maximal changes in pleural pressure (Max Delta Ppl) and peak flows were the main determinants of the variability of the resistance (R(rs)) and the reactance (X(rs)) of the respiratory system. The fourfold increase in baseline Max Delta Ppl and peak flows obtained during hyperpnoea caused a significant increase in R(rs) at 5 and 10 Hz and a decrease in X(rs) at all frequencies. We conclude that IOS parameters are not influenced by tachypnoea, but will reflect alterations in respiratory mechanics caused by hyperpnoeic breathing.

Tracking of airway and tissue mechanics during TLC maneuvers in mice

A tracking impedance estimation technique was developed to follow the changes in total respiratory impedance (Zrs) during slow total lung capacity maneuvers in six anesthetized and mechanically ventilated BALB/c mice. Zrs was measured with the wave-tube technique and pseudorandom forced oscillations at nine frequencies between 4 and 38 Hz during inflation from a transrespiratory pressure of 0-20 cmH2O and subsequent deflation, each lasting for approximately 20 s. Zrs was averaged for 0.125 s and fitted by a model featuring airway resistance (Raw) and inertance, and tissue damping and elastance (H). Lower airway conductance (Glaw) was linearly related to volume above functional residual capacity (V) between 0 and 75-95% maximum V, with a mean slope of dGlaw/dV = 13.6 +/- 4.6 cmH2O-1. s-1. The interdependence of Raw and H was characterized by two distinct and closely linear relationships for the low- and high-volume regions, separated at approximately 40% maximum V. Comparison of Raw with the highest-frequency resistance of the total respiratory system revealed a marked volume-dependent contribution of tissue resistance to total respiratory system resistance, resulting in the overestimation of Raw by 19 +/- 8 and 163 +/- 40% at functional residual capacity and total lung capacity, respectively, whereas the lowest frequency reactance was proportional to H; these findings indicate that single-frequency resistance values may become inappropriate as surrogates of Raw when tissue impedance is changing.

Comparative respiratory system mechanics in rodents

Because of the wide utilization of rodents as animal models in respiratory research and the limited data on measurements of respiratory input impedance (Zrs) in small animals, we measured Zrs between 0.25 and 9.125 Hz at different levels (0-7 hPa) of positive end-expiratory pressure (PEEP) in mice, rats, guinea pigs, and rabbits using a computer-controlled small-animal ventilator (Schuessler TF and Bates JHT, IEEE Trans Biomed Eng 42: 860-866, 1995). Zrs was fitted with a model, including a Newtonian resistance (R) and inertance in series with a constant-phase tissue compartment characterized by tissue damping (Gti) and elastance (Hti) parameters. Inertance was negligible in all cases. R, Gti, and Hti were normalized to body weight, yielding normalized R, Gti, and Hti (NHti), respectively. Normalized R tended to decrease slightly with PEEP and increased with animal size. Normalized Gti had a minimal dependence on PEEP. NHti decreased with increasing PEEP, reaching a minimum at approximately 5 hPa in all species except mice. NHti was also higher in mice and rabbits compared with guinea pigs and rats at low PEEPs, which we conclude is probably due to a relatively smaller air space volume in mice and rabbits. Our data also suggest that smaller rodents have proportionately wider airways than do larger animals. We conclude that a detailed, comparative study of respiratory system mechanics shows some evidence of structural differences among the lungs of various species but that, in general, rodent lungs obey scaling laws similar to those described in other species.

Effects of lung volume on lung and chest wall mechanics in rats

To investigate the effect of lung volume on chest wall and lung mechanics in the rats, we measured the impedance (Z) under closed- and open-chest conditions at various positive end-expiratory pressures (0-0.9 kPa) by using a computer-controlled small-animal ventilator (T. F. Schuessler and J. H. T. Bates. IEEE Trans. Biomed. Eng. 42: 860-866, 1995) that we have developed for determining accurately the respiratory Z in small animals. The Z of total respiratory system and lungs was measured with small-volume oscillations between 0.25 and 9.125 Hz. The measured Z was fitted to a model that featured a constant-phase tissue compartment (with dissipation and elastance characterized by constants G and H, respectively) and a constant airway resistance (Z. Hantos, B. Daroczy, B. Suki, S. Nagy, and J. J. Fredberg. J. Appl. Physiol. 72: 168-178, 1992). We matched the lung volume between the closed- and open-chest conditions by using the quasi-static pressure-volume relationship of the lungs to calculate Z as a function of lung volume. Resistance decreased with lung volume and was not significantly different between total respiratory system and lungs. However, G and H of the respiratory system were significantly higher than those of the lungs. We conclude that chest wall in rats has a significant influence on tissue mechanics of the total respiratory system.

Volume dependence of respiratory impedance in infants

We previously studied low-frequency respiratory impedance (Zrs) data at an elevated lung volume to separate airway and tissue mechanical properties in normal infants (Am. I. Respir. Crit. Care Med. 1996; 154:161-166). The aim of the present study was to determine the volume dependence of the airway and tissue mechanics by extending Zrs measurements to lower lung volumes. Zrs spectra between 0.5 and 21 Hz were measured in supine sleeping infants (n = 8; 7 to 26 mo of age) at mean transrespiratory pressures (Ptr[mean]) of 20, 10, and 0 cm H2O, during periods of apnea induced by inflating the infants' lungs to a pressure of 20 cm H2O through a face mask. At each inflation pressure, a model containing airway resistance (Raw) and inertance (law) and tissue damping (G) and elastance (H) was fitted to Zrs data. At FRC, the values of Raw, law, G, and H were 20.6+/-4.9 (SD) cm H2O x s/L, 0.037+/-0.014 cm H2O x s2/L, 39.6+/-10.3 cm H2O/L, and 147+/-35 cm H2O/L, respectively. Increase of Ptr(mean) caused a monotonous decrease in Raw (42+/-7% of the value at FRC), while law remained constant. The tissue parameters were minimal at a Ptr(mean) of 10 cm H2O (68+/-10% and 78+/-6% in G and H, respectively) and significantly higher at both 0 and 20 cm H2O. Although Zrs measurements can be made in most infants at lung volumes as low as FRC, an inflation pressure of 20 cm H2O provides a higher success rate and is therefore a more suitable condition for general use.

Chest wall and lung mechanics during acute hemorrhage in anesthetized dogs

OBJECTIVES

In trauma and in surgical patients, respiratory mechanics may change because of many factors, including the hypotension induced by hemorrhage. The effects of acute hemorrhage on elastic and resistive characteristics of the respiratory system were studied.

DESIGN

Prospective study.

SETTING

Anesthesia research laboratory.

INTERVENTIONS

Acute hemorrhagic shock was induced in 24 supine anesthetized/paralyzed, mechanically ventilated dogs by blood withdrawal over a 12-minute period to decrease systolic arterial pressure to 50 mmHg; additional blood was subsequently withdrawn to maintain this pressure for 2 hours. Total respiratory system dynamic compliance and resistance and lung and chest wall compliances and resistances were measured.

MEASUREMENTS AND MAIN RESULTS

Total respiratory system dynamic compliance decreased from control (0.03 +/- 0.002 L/cmH2O) by the first 10 minutes of shock (p < 0.05) and was 9.8 +/- 2% lower than control 2 hours after the induction of shock because of decreases in both lung (9.6 +/- 3%) and chest wall (7.7 +/- 3%) compliances. Total respiratory resistance increased 12.8 +/- 3% from control (3.08 +/- 0.19 cmH2O/L/s) after 2 hours of shock (p < 0.05) because of an increase in chest wall resistance (21.6 +/- 8%, p < 0.05). Pulmonary resistance was not significantly increased (p > 0.05). In six control dogs, prepared similarly but not hemorrhaged, chest wall compliance and resistance did not change, but lung compliance gradually decreased by 17.8% during 150 minutes of anesthesia/paralysis. Lung resistance increased only after 100 minutes (p < 0.05).

CONCLUSIONS

(1) Hemorrhagic shock caused slight changes in the chest wall, but effects on lung mechanics were a consequence of prolonged mechanical ventilation during anesthesia/paralysis, and (2) changes in respiratory mechanics caused by hemorrhagic shock are small and, unless other deleterious factors are present, would probably have little clinical significance.

Measurements of PEEP-induced changes in lung volume: evaluation of a laser monitor

STUDY OBJECTIVE

To evaluate a newly developed laser monitor in the measurement of positive end-expiratory pressure (PEEP)-induced changes in end-expiratory lung volume (EELV) in spontaneously breathing subjects.

DESIGN

An open comparison between two different methods to assess breathing parameters.

SETTING

A respiratory research unit.

PARTICIPANTS

Six spontaneously breathing, healthy volunteers.

INTERVENTIONS

Stepwise increases of PEEP from 0 to 0.8 to 1.6 kPa during spontaneous breathing; repeated validation of the laser monitor in each subject.

MEASUREMENTS AND RESULTS

Pressure and flow were recorded at the airway opening. Abdominal wall displacement (AWD) measured by the laser sensor was recorded simultaneously. The time lag between the volume and the laser signals during baseline was 0.068+/-0.052 s and during maximal PEEP, 0.108+/-0.093 s. There was no baseline drift in either the PEEP or the AWD signals. Mean EELV decreased by 290 mL to 1,157 mL after the PEEP valve was removed. Within each individual, the ratio between EELV and AWD showed only small variations. Measurements of tidal volume (VT) and AWD showed good agreement at all PEEP levels. Mean VT decreased in all but one subject during PEEP. With the increase in PEEP, the end-expiratory abdominal baseline increased linearly over a large range of end-expiratory pressures with a flattening of the curve at high PEEP levels in all subjects.

CONCLUSIONS

The laser monitor is sufficiently accurate for measuring PEEP-induced changes in EELV during spontaneous breathing in healthy adult subjects. Monitors incorporating multiple laser sensors may have considerable clinical promise.

Respiratory impedances and acinar gas transfer in a canine model for emphysema

We examined how the changes in the acini caused by emphysema affected gas transfer out of the acinus (Taci) and lung and chest wall mechanical properties. Measurements were taken from five dogs before and 3 mo after induction of severe bilateral emphysema by exposure to papain aerosol (170-350 mg/dose) for 4 consecutive wk. With the dogs anesthetized, paralyzed, and mechanically ventilated at 0.2 Hz and 20 ml/kg, we measured Taci by the rate of washout of 133Xe from an area of the lung with occluded blood flow. Measurements were repeated at positive end-expiratory pressures (PEEP) of 10, 5, 15, 0, and 20 cmH2O. We also measured dynamic elastances and resistances of the lungs (EL and RL, respectively) and chest wall at the different PEEP and during sinusoidal forcing in the normal range of breathing frequency and tidal volume. After final measurements, tissue sections from five randomly selected areas of the lung each showed indications of emphysema. Taci during emphysema was similar to that in control dogs. EL decreased by approximately 50% during emphysema (P < 0.05) but did not change its dependence on frequency or tidal volume. RL did not change (P > 0.05) at the lowest frequency studied (0.2 Hz), but in some dogs it increased compared with control at the higher frequencies. Chest wall properties were not changed by emphysema (P > 0.05). We suggest that although large changes in acinar structure and EL occur during uncomplicated bilateral emphysema, secondary complications must be present to cause several of the characteristic dysfunctions seen in patients with emphysema.

Effects of PEEP on respiratory mechanics are tidal volume and frequency dependent

How the effects of frequency, tidal volume (VT) and PEEP interact to determine the mechanical properties of the respiratory system is unclear. Airway flow and airway and esophageal pressures were measured in ten intubated, anesthetized/paralyzed patients during mechanical ventilation at 10-30 breaths/min and VT of 250-800 ml. From these measurements, Fourier transformation was used to calculate elastance (E) and resistance (R) of the total respiratory system (subscript rs), lungs (subscript L) and chest wall (subscript cw) at 5, 10 and 0 cm PEEP. As PEEP increased from 0-5 cmH2O, all elastances and resistances decreased (P < 0.05). Increasing PEEP to 10 cmH2O decreased EL, Rrs, and RL further (P < 0.05). The changes in Ers, EL, Rrs and RL caused by PEEP were less (P < 0.05) as VT increased, while changes in Rrs, RL and Ers were less (P < 0.05) as frequency increased. VT dependences in Ers and Rrs were enhanced (P < 0.05) at 0 cmH2O PEEP. The ratio of EL to chest wall elastance was not affected by PEEP (P > 0.05), but increased (P < 0.05) with increasing VT at 5 and 10 cmH2O PEEP. We conclude that it is critical to standardize ventilatory parameters when comparing groups of patients or testing clinical intervention efficacy and that the differential effects on the lungs and chest wall must be considered in optimizing the application of PEEP.

Airway and tissue constrictions are greater in closed than in open-chest conditions

We measured lung impedance (ZL) before and after four doses of methacholine (Mch) infusion in five intact chest (with esophageal balloon) and six open-chest dogs from 0.2 to 8 Hz with an optimal ventilator waveform. From ZL, we estimated airway resistance (R(aw)) and inertance (Iaw) and tissue viscance (GL) and elastance (HL). Two-way analysis of variance revealed that: (1) Mch had a strong influence on all parameters (p < 0.001), but small effect on hysteresivity, nL = GL/HL; (2) closed-chest GL and HL were significantly higher and Iaw lower than their open-chest values (p < 0.002, p < 0.05 and p < 0.0001); and (3) at the highest Mch dose, the relative increase in R(aw) was six times higher in the closed-chest condition. The reduced impact of Mch on open-chest mechanics may be due to constrictions superimposed on grossly different lung configurations and/or some humoral effects initiated by the thoracotomy. We conclude that Mch doses that elicit mild constriction in open-chest condition can cause a severe constriction in intact animals.

Methacholine-induced bronchoconstriction in rats: effects of intravenous vs. aerosol delivery

To determine the predominant site of action of methacholine (MCh) on lung mechanics, two groups of open-chest Sprague-Dawley rats were studied. Five rats were measured during intravenous infusion of MCh (i.v. group), with doubling of concentrations from 1 to 16 micrograms.kg-1.min-1. Seven rats were measured after aerosol administration of MCh with doses doubled from 1 to 16 mg/ml (ae group). Pulmonary input impedance (ZL) between 0.5 and 21 Hz was determined by using a wave-tube technique. A model containing airway resistance (Raw) and inertance (Iaw) and parenchymal damping (G) and elastance (H) was fitted to the ZL spectra. In the iv group, MCh induced dose-dependent increases in Raw [peak response 270 +/- 9 (SE) % of the control level; P < 0.05] and in G (340 +/- 150%; P < 0.05), with no increase in Iaw (30 +/- 59%) or H (111 +/- 9%). In the ae group, the dose-dependent increases in Raw (191 +/- 14%; P < 0.05) and G (385 +/- 35%; P < 0.05) were associated with a significant increase in H (202 +/- 8%; P < 0.05). Measurements with different resident gases [air vs. neon-oxygen mixture, as suggested (K.R. Lutchen, Z. Hantos, F. Peták, A. Adamicza, and B. Suki J. Appl. Physiol. 80: 1841-1849, 1996)] in the control and constricted states in another group of rats suggested that the entire increase seen in G during the i.v. challenge was due to ventilation inhomogeneity, whereas the ae challenge might also have involved real tissue contractions via selective stimulation of the muscarinic receptors.

Effects of Trendelenburg and reverse Trendelenburg postures on lung and chest wall mechanics

STUDY OBJECTIVE

To test whether the Trendelenburg ("head-down") or reverse Trendelenburg ("head-up") postures change lung and chest wall mechanical properties in a clinical condition.

DESIGN

Unblinded study, each patient serving as own control.

SETTING

University of Maryland at Baltimore Hospital, Baltimore, Maryland.

PATIENTS

15 patients scheduled for laparoscopic surgery.

INTERVENTIONS

Patients were anesthetized and paralyzed, tracheally intubated and mechanically ventilated at 10 to 30 per minute and at a tidal volume of 250 to 800 ml. Measurements were made before surgery in supine, head-up (10 degrees from horizontal) and head-down (15 degrees from horizontal) postures.

MEASUREMENTS AND MAIN RESULTS

Airway flow and airway and esophageal pressures were measured. From these measurements, discrete Fourier transformation was used to calculate elastances and resistances of the total respiratory system, lungs, and chest wall. Total respiratory elastance and resistance increased in the head-down posture compared with supine due to increases in lung elastance and resistance (p < 0.05); but chest wall elastance and resistance did not change (p > 0.05). Lung elastance also exhibited a negative dependence on tidal volume while head-down that was not observed in the supine posture. The change in lung elastance compared with supine was positively correlated to body mass index (weight/height2) and negatively correlated to tidal volume. Lung and chest wall elastance and resistance were not affected by shifting from supine to head-up (p > 0.05).

CONCLUSIONS

The Trendelenburg posture increases the mechanical impedance of the lung to inflation, probably due to decreases in lung volume. This effect may become clinically relevant in patients predisposed with lung disease and in obese patients.

Changes in lung and chest wall properties with abdominal insufflation of carbon dioxide are immediately reversible

Previously we have reported that large increases in lung and chest wall elastances as well as lung resistance occur with abdominal insufflation of carbon dioxide during laparoscopic surgery. To examine whether these effects were reversible with abdominal deflation, we calculated lung and chest wall elastances and resistances from measurement of airway flow and pressure and esophageal pressure in 17 anesthetized/paralyzed patients undergoing laparoscopic surgery. Measurements were made immediately prior to abdominal insufflation and after deflation. Lung and chest wall elastances and resistances were not changed from baseline (P > 0.05), although total respiratory elastance remained slightly increased compared to baseline (P < 0.05). The change in total respiratory elastance did not correlate with abdominal insufflation time, surgical site, smoking history, or physical characteristics of the patients. There were no differences in frequency and tidal volume dependences of the elastances and resistances before and after abdominal insufflation (P > 0.5). We conclude that residual changes in respiratory mechanics caused by carbon dioxide insufflation during laparoscopic surgery are minor, and that the reported compromise of respiratory function indicated by pulmonary function tests after laparoscopy does not appear to be due to changes in passive mechanical properties of the lungs or chest wall.

The effects of increased abdominal pressure on lung and chest wall mechanics during laparoscopic surgery

We tested the hypothesis that increases in pressure in the abdomen (Pab) exerted by CO2 insufflation during laparoscopy would increase elastance (E) and resistance (R) of both the lungs and chest wall. We measured airway flow and airway and esophageal pressures of 12 anesthetized/paralyzed tracheally intubated patients during mechanical ventilation at 10-30/min and tidal volume of 250-800 mL. From these measurements, we used discrete Fourier transformation to calculate E and R of the lungs and chest wall. Measurements were made at 0, 15, and 25 mm Hg Pab in the 15 degrees head-down (Trendelenburg) posture and at 0 and 15 mm Hg Pab in the 10 degrees head-up (reverse Trendelenburg) posture. Lung and chest wall Es and Rs while head-down increased at Pab = 15 mm Hg, and both Es increased further at Pab = 25 mm Hg (P < 0.05). Both Es and Rs also increased while head-up at Pab = 15 mm Hg (P < 0.05), but increases in lung E and R were less than while head-down (P < 0.05). The increase in lung E and R at Pab = 15 mm Hg in either posture were positively correlated to body weight or body mass index, whereas the increases in chest wall E and R were negatively correlated to the same factors (P < 0.05). Lung and chest wall mechanical impedances increase with increasing Pab; the increases depend on body configuration and are greater while head-down.(ABSTRACT TRUNCATED AT 250 WORDS)