Genetic fate-mapping reveals surface accumulation but not deep organ invasion of pleural and peritoneal cavity macrophages following injury

During injury, monocytes are recruited from the circulation to inflamed tissues and differentiate locally into mature macrophages, with prior reports showing that cavity macrophages of the peritoneum and pericardium invade deeply into the respective organs to promote repair. Here we report a dual recombinase-mediated genetic system designed to trace cavity macrophages in vivo by intersectional detection of two characteristic markers. Lineage tracing with this method shows accumulation of cavity macrophages during lung and liver injury on the surface of visceral organs without penetration into the parenchyma. Additional data suggest that these peritoneal or pleural cavity macrophages do not contribute to tissue repair and regeneration. Our in vivo genetic targeting approach thus provides a reliable method to identify and characterize cavity macrophages during their development and in tissue repair and regeneration, and distinguishes these cells from other lineages.

Estimation of change in pleural pressure in assisted and unassisted spontaneous breathing pediatric patients using fluctuation of central venous pressure: A preliminary study

Background

It is important to evaluate the size of respiratory effort to prevent patient self-inflicted lung injury and ventilator-induced diaphragmatic dysfunction. Esophageal pressure (Pes) measurement is the gold standard for estimating respiratory effort, but it is complicated by technical issues. We previously reported that a change in pleural pressure (ΔPpl) could be estimated without measuring Pes using change in CVP (ΔCVP) that has been adjusted with a simple correction among mechanically ventilated, paralyzed pediatric patients. This study aimed to determine whether our method can be used to estimate ΔPpl in assisted and unassisted spontaneous breathing patients during mechanical ventilation.

Methods

The study included hemodynamically stable children (aged <18 years) who were mechanically ventilated, had spontaneous breathing, and had a central venous catheter and esophageal balloon catheter in place. We measured the change in Pes (ΔPes), ΔCVP, and ΔPpl that was calculated using a corrected ΔCVP (cΔCVP-derived ΔPpl) under three pressure support levels (10, 5, and 0 cmH₂O). The cΔCVP-derived ΔPpl value was calculated as follows: cΔCVP-derived ΔPpl = k × ΔCVP, where k was the ratio of the change in airway pressure (ΔPaw) to the ΔCVP during airway occlusion test.

Results

Of the 14 patients enrolled in the study, 6 were excluded because correct positioning of the esophageal balloon could not be confirmed, leaving eight patients for analysis (mean age, 4.8 months). Three variables that reflected ΔPpl (ΔPes, ΔCVP, and cΔCVP-derived ΔPpl) were measured and yielded the following results: -6.7 ± 4.8, − -2.6 ± 1.4, and − -7.3 ± 4.5 cmH2O, respectively. The repeated measures correlation between cΔCVP-derived ΔPpl and ΔPes showed that cΔCVP-derived ΔPpl had good correlation with ΔPes (r = 0.84, p< 0.0001).

Conclusions

ΔPpl can be estimated reasonably accurately by ΔCVP using our method in assisted and unassisted spontaneous breathing children during mechanical ventilation.

Differential mechanisms are required for phrenic long-term facilitation over the course of motor neuron loss following CTB-SAP intrapleural injections

Selective elimination of respiratory motor neurons using intrapleural injections of cholera toxin B fragment conjugated to saporin (CTB-SAP) mimics motor neuron death and respiratory deficits observed in rat models of neuromuscular diseases. This CTB-SAP model allows us to study the impact of motor neuron death on the output of surviving phrenic motor neurons. After 7(d) days of CTB-SAP, phrenic long-term facilitation (pLTF, a form of respiratory plasticity) is enhanced, but returns towards control levels at 28d. However, the mechanism responsible for this difference in magnitude of pLTF is unknown. In naïve rats, pLTF predominately requires 5-HT2 receptors, the new synthesis of BDNF, and MEK/ERK signaling; however, pLTF can alternatively be induced via A2A receptors, the new synthesis of TrkB, and PI3K/Akt signaling. Since A2A receptor-dependent pLTF is enhanced in naïve rats, we suggest that 7d CTB-SAP treated rats utilize the alternative mechanism for pLTF. Here, we tested the hypothesis that pLTF following CTB-SAP is: 1) TrkB and PI3K/Akt, not BDNF and MEK/ERK, dependent at 7d; and 2) BDNF and MEK/ERK, not TrkB and PI3K/Akt, dependent at 28d. Adult Sprague Dawley male rats were anesthetized, paralyzed, ventilated, and were exposed to acute intermittent hypoxia (AIH; 3, 5 min bouts of 10.5% O2) following bilateral, intrapleural injections at 7d and 28d of: 1) CTB-SAP (25 μg), or 2) un-conjugated CTB and SAP (control). Intrathecal C4 delivery included either: 1) small interfering RNA that targeted BDNF or TrkB mRNA; 2) UO126 (MEK/ERK inhibitor); or 3) PI828 (PI3K/Akt inhibitor). Our data suggest that pLTF in 7d CTB-SAP treated rats is elicited primarily through TrkB and PI3K/Akt-dependent mechanisms, whereas BDNF and MEK/ERK-dependent mechanisms induce pLTF in 28d CTB-SAP treated rats. This project increases our understanding of respiratory plasticity and its implications for breathing following motor neuron death.

Asymmetrical intrapleural pressure distribution: a cause for scoliosis? A computational analysis

PURPOSE

The mechanical link between the pleural physiology and the development of scoliosis is still unresolved. The intrapleural pressure (IPP) which is distributed across the inner chest wall has yet been widely neglected in etiology debates. With this study, we attempted to investigate the mechanical influence of the IPP distribution on the shape of the spinal curvature.

METHODS

A finite element model of pleura, chest and spine was created based on CT data of a patient with no visual deformities. Different IPP distributions at a static end of expiration condition were investigated, such as the influence of an asymmetry in the IPP distribution between the left and right hemithorax. The results were then compared to clinical data.

RESULTS

The application of the IPP resulted in a compressive force of 22.3 N and a flexion moment of 2.8 N m at S1. An asymmetrical pressure between the left and right hemithorax resulted in lateral deviation of the spine towards the side of the reduced negative pressure. In particular, the pressure within the dorsal section of the rib cage had a strong influence on the vertebral rotation, while the pressure in medial and ventral region affected the lateral displacement.

CONCLUSIONS

An asymmetrical IPP caused spinal deformation patterns which were comparable to deformation patterns seen in scoliotic spines. The calculated reaction forces suggest that the IPP contributes in counterbalancing the weight of the intrathoracic organs. The study confirms the potential relevance of the IPP for spinal biomechanics and pathologies, such as adolescent idiopathic scoliosis.

Esophageal pressure: benefit and limitations

The recording of esophageal pressure (Pes) in supine position as a substitute for pleural pressure is difficult and fraught with potential errors. Pes is affected by the: 1) elastance and weight of the lung; 2) elastance and weight of the rib cage; 3) weight of the mediastinal organs; 4) elastance and weight of the diaphragm and abdomen; 5) elastance of the esophageal wall; and 6) elastance of the esophageal balloon (if filled with too much air). If the purpose is to measure lung compliance in the intensive care patient, reasonably useful information might be obtained by measuring airway pressure alone, considering chest wall compliance to be a weight that is forced away by the ventilation. Such weight requires a constant pressure for displacement. The transpulmonary pressure, whether calculated with Pes or by another measure of abdominal pressure, may guide in PEEP titration. It may also enable calculation of stresses applied to the lung and these may be more important in guiding an optimal ventilator setting than an optimum compliance or oxygenation of blood. Diaphragm function can be estimated by esophageal minus gastric pressure and with even more precision, when combined with diaphragm electromyography.

[Production and law of variation of the pleural cavity intrinsic pressure and the pressure of alveolar wall during respiratory process]

All physiologic textbooks deal with pleural cavity pressure, alveolar wall pressure and pressure inside the lung, but they have not stated these ideas clearly. The present study reveals production and Law of variation of the intrinsic pressure of pleural cavity, the pressure of alveolar wall and the intrinsic pressure in the alveoli. Pleural cavity intrinsic pressure is produced by the pressure from pleura expanding or compressing force of the lungs. When the lungs calmly inhale, the thorax expands, pleural cavity negative pressure increase. When the lungs calmly exhale, thorax reduces, but thorax and lungs are still in the extended state, pleural cavity is still in negative pressure. With thorax reducing, negative pressure decreases. When the lungs are at the forced expiration, the lung pleura and wall pleura extrude pleural cavity, only to produce positive pressure. The pressure of alveolar wall is the algebraic sum of the intrinsic pressure of pleural cavity, the intrinsic pressure of pulmonary tissue and the additional pressure of alveolar wall. We did the calculation of additional pressure on the alveolar wall by using Laplace formula of spherical elastic membrane. The intrinsic pressure of alveoli depends on the moving speed or slowness of expansion or compression of alveolar wall and the size of trachea resistance.

Role of pleural pressure in the coupling between the intercostal muscles and the ribs

The inspiratory intercostal muscles elevate the ribs and thereby elicit a fall in pleural pressure (ΔPpl) when they contract. In the present study, we initially tested the hypothesis that this ΔPpl does, in turn, oppose the rib elevation. The cranial rib displacement (Xr) produced by selective activation of the parasternal intercostal muscle in the fourth interspace was measured in dogs, first with the rib cage intact and then after ΔPpl was eliminated by bilateral pneumothorax. For a given parasternal contraction, Xr was greater after pneumothorax; the increase in Xr per unit decrease in ΔPpl was 0.98 ± 0.11 mm/cmH2O. Because this relation was similar to that obtained during isolated diaphragmatic contraction, we subsequently tested the hypothesis that the increase in Xr observed during breathing after diaphragmatic paralysis was, in part, the result of the decrease in ΔPpl, and the contribution of the difference in ΔPpl to the difference in Xr was determined by using the relation between Xr and ΔPpl during passive inflation. With diaphragmatic paralysis, Xr during inspiration increased approximately threefold, and 47 ± 8% of this increase was accounted for by the decrease in ΔPpl. These observations indicate that 1) ΔPpl is a primary determinant of rib motion during intercostal muscle contraction and 2) the decrease in ΔPpl and the increase in intercostal muscle activity contribute equally to the increase in inspiratory cranial displacement of the ribs after diaphragm paralysis.

Regional recruitment of rat diaphragmatic lymphatics in response to increased pleural or peritoneal fluid load

The specific role of the diaphragmatic tendinous and muscular tissues in sustaining lymph formation and propulsion in the diaphragm was studied in 24 anaesthetized spontaneously breathing supine rats. Three experimental protocols were used: (a) control; (b) peritoneal ascitis, induced through an intraperitoneal injection of 100 ml kg(-1) of iso-oncotic saline; and (c) pleural effusion, induced through an intrapleural injection of 6.6 ml kg(-1) saline solution. A group of animals (n = 12) was instrumented to measure the hydraulic transdiaphragmatic pressure gradient between the pleural and peritoneal cavities in the three protocols. In the other group (n = 12), the injected iso-oncotic saline was enriched with 2% fluorescent dextrans (molecular mass = 70 kDa); at 30 min from the injections these animals were suppressed and their diaphragm excised and processed for confocal microscopy analysis. In control conditions, in spite of a favourable peritoneal-to-pleural pressure gradient, the majority of the tracer absorbed into the diaphragmatic lymphatic system converges towards the deeper collecting lymphatic ducts. This suggests that diaphragmatic lymph formation mostly depends upon pressure gradients developing between the serosal cavities and the lymphatic vessel lumen. In addition, the tracer distributes to lymph vessels located in the muscular diaphragmatic tissue, suggesting that active muscle contraction, rather than passive tendon stretch, more efficiently enhances local diaphragmatic lymph flow. Vice versa, a prevailing recruitment of the lymphatics of the tendinous diaphragmatic regions was observed in peritoneal ascitis and pleural effusion, suggesting a functional adaptation of the diaphragmatic network to increased draining requirements.

Telemetric recording of intrapleural pressure

BACKGROUND

Monitoring of intrapleural pressure (IPP) is used for evaluation of lung function in a number of pathophysiological conditions. We describe a telemetric method of non-invasive monitoring of the IPP in conscious animals intermittently or continuously for a prolonged period of time.

MATERIALS AND METHODS

After IACUC approval, six mongrel dogs were used for the study. After sedation, each dog was intubated and anesthetized using 0.5% Isoflurane. A telemetric implant model TL11M2-D70-PCT from Data Science International was secured subcutaneously. The pressure sensor tip of the catheter from the implant was inserted into the pleural space, and the catheter was secured with sutures. The IPP signals were recorded at a sampling rate of 100 points/second for 30 to 60 min daily for 4 days. From these recordings, the total mean negative IPP (mmHg), and the total mean negative IPP for a standard time of 30 min were calculated. In addition, the actual inspiratory and expiratory pressures were also measured from stable recording of the IPP waveforms.

RESULTS

In six dogs, the total mean +/- SD negative IPP was -10.8 +/- 10.6 mmHg. After normalizing with respect to acquisition time it was -13.2 +/- 11.2 mmHg/min. The actual inspiratory pressure was -19.7 +/- 15.3, and the expiratory pressure was -11.0 +/- 12.9.

CONCLUSIONS

Our study demonstrates that telemetric monitoring of IPP can be performed reliably and non-invasively in conscious experimental animals. The values for IPP in our study are compatible with the results of other investigators who used different methods of IPP measurement. Further work may show this method to be helpful in understanding the pathophysiology of various breathing disorders.

Effect of diaphragmatic contraction on the action of the canine parasternal intercostals

The inspiratory intercostal muscles enhance the force generated by the diaphragm during lung expansion. However, whether the diaphragm also alters the force developed by the inspiratory intercostals is unknown. Two experiments were performed in dogs to answer the question. In the first experiment, external, cranially oriented forces were applied to the different rib pairs to assess the effect of diaphragmatic contraction on the coupling between the ribs and the lung. The fall in airway opening pressure (ΔPao) produced by a given force on the ribs was invariably greater during phrenic nerve stimulation than with the diaphragm relaxed. The cranial rib displacement (Xr), however, was 40–50% smaller, thus indicating that the increase in ΔPao was exclusively the result of the increase in diaphragmatic elastance. In the second experiment, the parasternal intercostal muscle in the fourth interspace was selectively activated, and the effects of diaphragmatic contraction on the ΔPao and Xr caused by parasternal activation were compared with those observed during the application of external loads on the ribs. Stimulating the phrenic nerves increased the ΔPao and reduced the Xr produced by the parasternal intercostal, and the magnitudes of the changes were identical to those observed during external rib loading. It is concluded, therefore, that the diaphragm has no significant synergistic or antagonistic effect on the force developed by the parasternal intercostals during breathing. This lack of effect is probably related to the constraint imposed on intercostal muscle length by the ribs and sternum.

Respiratory and circulatory effects of parietal pleural afferent stimulation in rabbits

Respiratory symptoms accompanying pleural diseases combine dyspnea, tachypnea, rapid shallow breathing, and sometimes hypotension. There are no experimental data on the changes in respiratory and circulatory functions elicited by the activation of pleural afferents. After removal of all muscles covering the 5th to 10th intercostal spaces, we investigated in paralyzed, vagotomized rabbits the changes in phrenic discharge, transpulmonary pressure, and systemic arterial pressure in response to an outwardly directed force exerted on the parietal pleura or the local application of solutions containing lactic acid or inflammatory mediators. Mechanical stimulation of the pleura induced an immediate decrease in both integrated phrenic discharge and arterial blood pressure, the responses being positively correlated with the magnitude of force applied on the pleura. No accompanying changes in ventilatory timing, transpulmonary pressure, or heart rate were measured. Lactic acid solution also elicited an inhibition of phrenic activity and a fall in blood pressure. Section of the internal intercostal nerves supplying the stimulated intercostal spaces totally abolished the responses to mechanical stimulation or lactic acid. An inflammatory mixture elicited only modest respiratory and circulatory effects. We concluded that an acute mechanical distension of the parietal pleura as well as its chemical stimulation by lactic acid elicit a marked inhibition of phrenic motoneurons combined to a reduction of the sympathetic outflow to the circulatory system.

[Structure and physiology of the pleura and the pleural space]

The pleural space, derived from the intraembryonic coelom, is limited by a serous membrane including the mesothelium formed by cells possessing not only the characteristic features of epithelial cells but also the potential of secretory cells (cytokines and growth factor). Blood supply to visceral pleurae differs depending on the species while the lymphatic circulation is directly connected to the pleural space via pores in the parietal pleura. Pleural physiology and movement of pleural fluid are directly related to the particular structures of the pleura.

Resorption of gas trapped in body cavities: comparison of alveolar and pleural space with inner ear and paranasal sinuses

This paper describes our attempt to devise a short text aimed at improving students' understanding of gas resorption in body cavities. Students are expected to understand the mechanisms behind paranasal sinusitis, otitis media, closed pneumothorax, and atelectasis of collapsed lung tissue, all used as examples. On the basis of the interpretation that during pneumothorax resorption, gas diffuses down pressure gradients into the blood, students are encouraged to calculate tables of pressure gradients for the above-mentioned pathological conditions. After answering a few questions, students need to analyze and eventually accept the following conclusion: in cases of air trapping in collapsible body cavities, all gases will be fully reabsorbed without pain. Air trapping in bone cavities leads only to partial reabsorption of gases and results in subatmospheric intracavity pressure. Partial vacuum causes painful mucosal edema and free fluid secretion.

Manual vibration increases expiratory flow rate via increased intrapleural pressure in healthy adults: an experimental study

QUESTION

What is the relationship between vibration of the chest wall and the resulting chest wall force, chest wall circumference,intrapleural pressure, and expiratory flow rate? Is the change in intrapleural pressure during vibration the sum of the intrapleural pressure due to recoil of the lung, chest wall compression, and chest wall oscillation?

DESIGN

Randomised, within-subject,experimental study.

PARTICIPANTS

Seven experienced cardiopulmonary physiotherapists and three healthy adults.

INTERVENTION

Vibration (compression + oscillation), compression alone, and oscillation alone were applied manually to the chest walls of healthy participants during passive exertion and compared with passive expiration alone.

OUTCOME MEASURES

Chest wall force, chest wall circumference, intrapleural pressure, and expiratory flow rate.

RESULTS

During vibration, coherence was high(r2 > 0.97) between external chest wall force, chest wall circumference, intrapleural pressure, and expiratory flow. The mean change in intrapleural pressure during vibration was 9.55 cmH2O (SD 1.66), during chest compression alone was 8.06 cmH2O(SD 1.65), during oscillation alone was 7.93 cmH2O (SD 1.57), and during passive expiration alone was 6.82 cmH2O (SD 1.51). During vibration, compression contributed 13% of the change in intrapleural pressure, oscillation contributed 12%, and lung recoil contributed the remaining 75%.

CONCLUSIONS

During vibration the chest behaves as a highly linear system. Changes in intrapleural pressure occurring during vibration appear to be the sum of changes in pressure due to lung recoil and the compressive and oscillatory components of the technique, which suggests that all three components are required to optimise expiratory flow.

Breath-to-breath analysis of abdominal and rib cage motion in surfactant-depleted piglets during high-frequency oscillatory ventilation

OBJECTIVE

To assess the value of monitoring abdominal and rib cage tidal displacement as an indicator of optimal mean airway pressure (Paw) during high-frequency oscillatory ventilation (HFOV).

DESIGN AND SETTING

Prospective observational study in a university research laboratory.

ANIMALS

Eight piglets weighing 12.0+/-0.5 kg, surfactant depleted by lung lavage.

INTERVENTIONS

Compliance of the respiratory system (C(rs)) was calculated from a quasistatic pressure volume loop. After initiation of HFOV lung volume was recruited by increasing Paw to 40 cmH(2)O. Then mean Paw was decreased in steps until PaO(2)/FIO(2) was below 100 mmHg. Proximal pressure amplitude remained constant.

MEASUREMENTS AND RESULTS

Abdominal and rib cage tidal displacement was determined using respiratory inductive plethysmography. During HFOV there was maximum in tidal volume (Vt) in seven of eight piglets. At maximal mean Paw abdominal and rib cage displacement were in phase. Phase difference between abdominal and rib cage displacement increased to a maximum of 178+/-28 degrees at minimum mean Paw. A minimum in abdominal displacement and a maximum of Vt was found near the optimal mean Paw, defined as the lowest mean Paw where shunt fraction is below 0.1.

CONCLUSIONS

During HFOV abdominal and rib cage displacement displayed mean Paw dependent asynchrony. Maximal Vt and minimal abdominal displacement coincided with optimal C(rs), oxygenation, and ventilation, suggesting potential clinical relevance of monitoring Vt and abdominal displacement during HFOV.

Dog peritoneal and pleural cavities as bioreactors to grow autologous vascular grafts

OBJECTIVE

The purpose of this study was to grow "artificial blood vessels" for autologous transplantation as arterial interposition grafts in a large animal model (dog).

METHOD AND RESULTS

Tubing up to 250 mm long, either bare or wrapped in biodegradable polyglycolic acid (Dexon) or nonbiodegradable polypropylene (Prolene) mesh, was inserted in the peritoneal or pleural cavity of dogs, using minimally invasive techniques, and tethered at one end to the wall with a loose suture. After 3 weeks the tubes and their tissue capsules were harvested, and the inert tubing was discarded. The wall of living tissue was uniformly 1-1.5 mm thick throughout its length, and consisted of multiple layers of myofibroblasts and matrix overlaid with a single layer of mesothelium. The myofibroblasts stained for alpha-smooth muscle actin, vimentin, and desmin. The bursting strength of tissue tubes with no biodegradable mesh scaffolds was in excess of 2500 mm Hg, and the suture holding strength was 11.5 N, both similar to that in dog carotid and femoral arteries. Eleven tissue tubes were transplanted as interposition grafts into the femoral artery of the same dog in which they were grown, and were harvested after 3 to 6.5 months. Eight remained patent during this time. At harvest, their lumens were lined with endothelium-like cells, and wall cells stained for alpha-actin, smooth muscle myosin, desmin and smoothelin; there was also a thick "adventitia" containing vasa vasorum.

CONCLUSION

Peritoneal and pleural cavities of large animals can function as bioreactors to grow myofibroblast tubes for use as autologous vascular grafts.

Pleural mechanics and fluid exchange

The pleural space separating the lung and chest wall of mammals contains a small amount of liquid that lubricates the pleural surfaces during breathing. Recent studies have pointed to a conceptual understanding of the pleural space that is different from the one advocated some 30 years ago in this journal. The fundamental concept is that pleural surface pressure, the result of the opposing recoils of the lung and chest wall, is the major determinant of the pressure in the pleural liquid. Pleural liquid is not in hydrostatic equilibrium because the vertical gradient in pleural liquid pressure, determined by the vertical gradient in pleural surface pressure, does not equal the hydrostatic gradient. As a result, a viscous flow of pleural liquid occurs in the pleural space. Ventilatory and cardiogenic motions serve to redistribute pleural liquid and minimize contact between the pleural surfaces. Pleural liquid is a microvascular filtrate from parietal pleural capillaries in the chest wall. Homeostasis in pleural liquid volume is achieved by an adjustment of the pleural liquid thickness to the filtration rate that is matched by an outflow via lymphatic stomata.

Static lung compliance and body pressures in Tupinambis merianae with and without post-hepatic septum

The surgical removal of the post-hepatic septum (PHS) in the tegu lizard, Tupinambis merianae, significantly reduces resting lung volume (V(Lr)) and maximal lung volume (V(Lm)) when compared with tegus with intact PHS. Standardised for body mass (M(B)), static lung compliance was significantly less in tegus without PHS. Pleural and abdominal pressures followed, like ventilation, a biphasic pattern. In general, pressures increased during expiration and decreased during inspiration. However, during expiration pressure changes showed a marked intra- and interindividual variation. The removal of the PHS resulted in a lower cranio-caudal intracoelomic pressure differential, but had no effect on the general pattern of pressure changes accompanying ventilation. These results show that a perforated PHS that lacks striated muscle has significant influence on static breathing mechanics in Tupinambis and by analogy provides valuable insight into similar processes that led to the evolution of the mammalian diaphragm.

Intermittent or continuous carbon dioxide insufflation for de-airing of the cardiothoracic wound cavity? An experimental study with a new gas-diffuser

Insufflation of carbon dioxide into the chest wound is used in open-heart surgery to de-air the heart and great vessels. In a cardiothoracic wound model, we compared the degree of air displacement achieved by a new insufflation device, a gas-diffuser, with that of a thin open-ended tube during steady-state and with carbon dioxide flows of 2.5, 5, 7.5, and 10 L/min. We also studied air displacement at the start of and after discontinuation of carbon dioxide insufflation with the gas-diffuser and evaluated the influence of an open pleura. During steady state, the gas-diffuser produced efficient air displacement in the wound cavity model at carbon dioxide flows of > or = 5 L/min (< or = 0.65% remaining air), whereas the open-ended tube was inefficient (> or = 82% remaining air) at all studied carbon dioxide flows (P < 0.001). An open pleural cavity prolonged the time needed to obtain a high degree of air displacement in the wound cavity (P = 0.001). Carbon dioxide insufflation of the cardiothoracic wound cavity should be initiated at a carbon dioxide flow of 10 L/min at least 1 min before the incision of the heart and great vessels and should be continued at a carbon dioxide flow of at least 5 L/min until surgical closure.

Heterogeneity of intrapleural pressures during exercise

We determined whether the caudodorsal region of the intrapleural space in exercising horses experiences larger pressure fluctuations than other regions and whether systematic phase-shifting of peak intrapleural pressures along the length of the thorax suggests the existence of locomotor-induced intrapleural pressure waves. We utilised percutaneous introducers and solid-state pressure-tip transducers implanted along the dorsal aspect of the thorax, mid-thorax or oesophagus to measure regional intrapleural pressures while 3 horses galloped on a flat treadmill at 13-14 m/s, then recorded pressures from the same catheters when horses exercised intensely (heart rate 170-190 beats/min) while swimming with no ground concussion. Pressure excursions in the caudodorsal region did not vary systematically from other regions during galloping or swimming, nor more than a few torr between different locations. During swimming, peak expiratory pressures were higher than during galloping (68-79 vs. 26-32 torr), and horses breathed explosively at frequencies 5 times slower than while galloping (28 vs. 120/min). During galloping, individual catheter locations registered locomotor concussion; however, this was variable and did not indicate a systematic pressure wave passing through the lung or intrapleural space.