Functional lung unit in the pig

To study the size of the vessels supplying the functional lung unit, polystyrene beads of uniform diameter were injected intravenously in anaesthetised pigs and subsequent gas exchange abnormalities were studied using the multiple inert gas elimination technique. Beads of different sizes, ranging from 63 to 262 microm, were used, each pig receiving beads of only one size. Successive 0.25 g boli of beads (cumulative dose 1.0-1.5 g) increased shunt (from 3% baseline to 20% of cardiac output) and pulmonary artery mean pressure (from 26 to 45 mmHg) and decreased arterial P(O(2)) (from 96 to 43 mmHg) and cardiac output from 2.8 to 2.2 L min(-1) with no differences according to bead sizes. The dispersion of the ventilation dist ribution (log SDV), normal at 0.39 before beads, increased progressively with bead size from 0.48 (63 microm to 0.91 (262 microm). The 63 microm beads were lodged in vessels associated with respiratory bronchioles and smaller airways, whereas larger beads were positioned in vessels associated with non-respiratory airways. A linear correlation analysis between log SDV and bead size showed that 59 microm beads produce a log SDV that is 2 SEM above mean baseline log SDV. These findings suggest that the functional lung unit in this species (with no collateral ventilation) is smaller than in a species of the same size and with collateral ventilation (dog) in whom occlusion of 124 microm or larger diameter vessels is required to increase log SDV.

Pulmonary blood flow distribution in lobar hypoxia--influence of cardiac output and nitric oxide inhalation

Inhaled NO is reported to be less effective in patients with ARDS if cardiac output is high (> 10 L/min). It has also been demonstrated that increased blood flow and increased shear stress cause an enhancement of endogenous NO production. In one-lung ventilation and regional hypoxia, nitric oxide (NO) delivered to the ventilated lung may decrease blood flow to the nonventilated lung and improve arterial oxygenation. So far, however, results have been divergent. The present study was performed with the hypothesis that inhaled NO would be less effective if cardiac output was increased. In the anaesthetized pig, hypoxia (5% O2) was induced in the left lower lobe. NO was delivered consecutively to the hypoxic lobe and to the other, oxygenated parts, of the lungs during continuous measurement of lobar blood flow and total lung blood flow. Bleeding and infusion of dextran caused variation in cardiac output. It was found that lobar hypoxia per se reduced lobar blood flow from 22.9+/-3.1% to 4.7+/-0.9% of cardiac output. An increase (3.2+/-0.3 L x min(-1)) and a decrease (2.2+/-0.2 L x min(-1)) in cardiac output did not alter the relative perfusion of the hypoxic lobe from baseline cardiac output (2.6+/-0.2 L x min(-1)) values. When NO was delivered to the hypoxic lobe, there was a marked increase in relative lobar perfusion to 19.0+/-2.9% during low cardiac output and 16.5+/-2.7% during high cardiac output without any significant difference between the two NO-induced increases of lobar perfusion. The increase in lobar perfusion tended to depend inversely on total pulmonary blood flow when cardiac output had been reduced by bleeding but without reaching statistical significance (r = -0.42, p > 0.05). The decrease in mean pulmonary artery pressure and PaO2 seen during NO inhalation to the hypoxic lobe did not correlate with the level of cardiac output. When NO was delivered to the oxygenated parts of the lungs, no significant effect on relative lobar perfusion or arterial oxygenation was observed, either at raised or at lowered cardiac output. The findings give no further evidence to show that variations in cardiac output alter the effect of NO inhalation.

Both inhaled histamine and hypertonic saline increase airway reactivity in non-sensitised rabbits

BACKGROUND

Asthmatics react with bronchoconstriction upon a variety of stimuli, i.e. exercise and hypertonic aerosol challenge. We have previously shown that hyperventilation with dry gas in a rabbit model resulted in a change of the ion content of the tracheal wall. This was followed by a hyperreactive response to histamine.

OBJECTIVE

We hypothesised that nebulisation with 3.6% hypertonic saline will be accompanied by a hyperreactive response to histamine in a rabbit model.

METHODS

Anaesthetised rabbits were given histamine after nebulisation with hypertonic saline. In addition, repeat nebulisation with hypertonic saline was given with or without histamine between these nebulisations.

RESULTS

There was a different response to histamine 10 mg x ml(-1) whether hypertonic saline had been given or not (p < 0.001). Histamine nebulisation, given after hypertonic saline, caused an increase from baseline in resistance of 65 +/- 12 cm H(2)O.litre(-1) x s (mean +/- SEM, p < 0. 001) and a decrease in compliance of 2.3 +/- 0.4 ml x cm H(2)O(-1) (p < 0.001). The corresponding values for the control animals were 10 +/- 4 cm H(2)O.litre(-1) x s (n.s.) and 1.7 +/- 0.2 ml x cm H(2)O(-1) (p < 0.001). At a second nebulisation with hypertonic saline, with a histamine challenge 30 min before, the resistance increased from baseline by 35 +/- 10 cm H(2)O x litre(-1) x s (p < 0.01). This was not observed when no histamine had been given between the hypertonic saline nebulisations.

CONCLUSIONS

This study in rabbits shows that hypertonic solutions cause an increase in the responsiveness to histamine and that histamine causes an increase in responsiveness to hypertonic saline. This is similar to the response of asthmatics to hypertonic saline.

Ventilation-perfusion relationships during exercise in standardbred trotters with red cell hypervolaemia

In order to evaluate the pulmonary gas exchange during exercise in Standardbred trotters with red cell hypervolaemia (RCHV), 12 horses with RCHV were compared with 9 normovolaemic (NV) horses. VO2 and VCO2 were determined with an open bias flow system. Cardiovascular and haemodynamic data were recorded during exercise at 4 different speeds on a treadmill. Pulmonary gas exchange was assessed by conventional blood gas variables (arterial and mixed venous blood gas tensions), and the ventilation-perfusion distribution VA/Q was estimated by the multiple inert gas elimination technique. VA and AaDO2 were calculated. Dispersions of perfusion and ventilation distribution (SDQ, SDV) were determined. HR, RR, Qt, VO2, VA, log SDV, C(a-åv)O2 and lactate did not differ between groups. The degree of hypoxaemia was more pronounced in the RCHV than in the NV (PaO2 = 54 and 59 mmHg; AaDO2 = 41 and 34 mmHg in RCHV and NV, respectively, at highest workload). Further, pH was lower in the RCHV and PaCO2 and VCO2 was significantly higher in the RCHV during the course of exercise (pH = 7.24 and 7.29; PaCO2 = 56 and 51 mmHg; VCO2 = 156 and 135 ml/kg x min in RCHV and NV, respectively, at highest workload). The PaO2 predicted from the VA/Q distribution was higher than actually measured in blood during heavy exercise which may suggest a certain diffusion limitation over the alveolar-capillary membranes in both groups but there was no difference between the 2 groups. The more pronounced hypoxaemia observed in RCHV trotters was mainly caused by increased VA/Q mismatch expressed as a significantly increased log SDQ (0.78 and 0.45 in RCHV and NV, respectively, at highest workload).

Gas exchange in acute respiratory failure

Acute respiratory failure is accompanied by a severe gas exchange impairment that is signified by a large shunt and no or only little of additional ventilation-perfusion mismatch. The shunt is caused by perfusion of collapsed and consolidated lung tissue that is mainly located in the lower, dependent lung region. The ventilation, on the other hand, is redistributed towards upper, non-dependent regions. By applying external PEEP, or producing an intrinsic PEEP by shortening the expiratory period, not only recruitment of collapsed or consolidated lung tissue may be achieved, but also a redistribution of inspired gas towards more dependent regions. Spontaneous breathing seems to improve gas exchange, and in proportion to its share of total ventilation, when added to mechanical ventilation. A shift from total mechanical ventilation to partial or fully spontaneous breathing may be the road of the future and should be tested further.

Thoracic epidural anesthesia as an adjunct to general anesthesia for cardiac surgery: effects on ventilation-perfusion relationships

OBJECTIVE

To determine the effects of thoracic epidural anesthesia (TEA) on ventilation-perfusion (VA/Q) relationships, atelectasis, and oxygenation before and after coronary artery bypass graft surgery (CABG).

DESIGN

Prospective, controlled, unblinded, randomized trial.

SETTING

Cardiothoracic clinic at a major university referral center.

PARTICIPANTS

Twenty-eight patients undergoing elective CABG.

INTERVENTIONS

Perioperative and postoperative TEA was added to general anesthesia (GA) in 14 patients, and 14 patients receiving GA alone served as controls.

MEASUREMENTS AND MAIN RESULTS

VA/Q relationships were measured by the multiple inert gas elimination technique, and, 20 hours postoperatively, atelectasis was assessed by computerized tomographic scans. Arterial and mixed venous blood gases and hemodynamic variables were measured by standard techniques. TEA per se caused no change in shunt, VA/Q matching, or oxygenation. Induction of GA in the control group and induction of TEA caused similar reductions in mean arterial pressure. The TEA patients needed less morphine analgesia postoperatively and were extubated earlier. Extubation caused significant improvement in VA/Q matching. On the first postoperative day, a slight reduction in PaCO2 was seen in the TEA group, but no differences in shunt, VA/Q matching, or oxygenation compared with the GA group. Both groups showed extensive bilateral atelectasis.

CONCLUSION

TEA can reduce respirator time and the need for morphine analgesics after CABG without negative effects on VA/Q matching, oxygenation, or atelectasis formation.

Hyperosmolarity reduces the relaxing potency of nitric oxide donors in guinea-pig trachea

1. Non-responders to inhaled nitric oxide treatment have been observed in various patient groups. The bronchodilatory effect of inhaled nitric oxide was attenuated when the airway lumen was rendered hyperosmolar in an in vivo study on rabbits. We used a guinea-pig tracheal perfusion model to investigate the effects of increased osmolarity (450 mOsm, NaCl added) on the relaxing potency of the nitric oxide donors sodium nitroprusside (SNP) and (+/-)-S-nitroso-N-acetylpenicillamine (SNAP). 2. Under iso-osmolar conditions SNP relaxed the carbachol (CCh, 1 microM) contracted trachea by 83+/-3%. After pretreatment with intraluminal hyperosmolarity SNP relaxed the CCh-contracted trachea by only 31+/-7% (P<0.05). When the trachea was contracted to the same extent under untreated and hyperosmolar conditions, the untreated trachea was completely relaxed by SNP but, after hyperosmolar pretreatment, SNP could no longer relax the trachea. 3. SNAP relaxed the CCh contracted trachea by 27+/-5%. After pretreatment with intraluminal hyperosmolarity, SNAP relaxed the trachea by 11+/-4%, which was less than in the iso-osmolar control (P<0.05). 4. Extraluminal hyperosmolarity did not affect carbachol elicited contraction, and SNP administered externally during extraluminal hyperosmolarity was able to relax the trachea (P<0.05). 5. The cell permeable guanosine 3'5'-cyclic monophosphate analogue 8-Br-cGMP relaxed the CCh contracted trachea in both iso-osmolar (P<0.05) and hyperosmolar conditions (P<0.05). 6. The relaxant effect of nitric oxide donors on tracheal smooth muscle is markedly reduced when the airway epithelium is exposed to hyperosmolar solution.

Impaired systolic and diastolic function and ventricular arrhythmia are common in normotensive healthy elderly men with left ventricular hypertrophy

BACKGROUND

The present study was designed to assess the impact of left ventricular hypertrophy (LVH) independent of hypertension on the presence of silent myocardial ischemia and arrhythmia, as well as on systolic and diastolic function in otherwise healthy elderly men.

METHODS AND RESULTS

Twenty apparently healthy normotensive subjects with LVH were compared with 18 hypertensive subjects with LVH and with 20 healthy subjects without LVH (controls)--all recruited from a health screening programme of 70-year-old men. All participants were free from known coronary heart disease and were evaluated by means of echocardiography with Doppler, a symptom-limited exercise test and 24 h ambulatory ECG monitoring. The healthy normotensive subjects with LVH showed impaired systolic function (ejection fraction 66 +/- 8 (SD)% versus 72 +/- 8% in controls, P < 0.03) and impaired diastolic function (E/A ratio 0.86 +/- 0.20 versus 1.12 +/- 0.30, P < 0.01) as well as an increased number of premature ventricular complexes in the exercise test and during the 24 h ECG monitoring (P < 0.05), when compared with the healthy group without LVH. The hypertensive subjects with LVH showed impaired diastolic function (P < 0.05) and a more pronounced ST depression in the exercise test (P < 0.05), when compared with the healthy group without LVH. In both of the LVH groups, more than 20% of the subjects evidenced ST-segment depression > or = 1 mm in the exercise test, compared with 5% of the healthy group without LVH.

CONCLUSIONS

In elderly men free from hypertension or other known disease, LVH at echocardiography was associated with impaired systolic and diastolic function as well as with ventricular arrhythmia. Thus, even in the absence of hypertension, LVH may be a harmful characteristic in the elderly.

Effects of ventilatory pattern on exhaled nitric oxide in mechanically ventilated rabbits

BACKGROUND

Nitric oxide [NOexp] is present in exhaled air in many species. During experiments on pressure-controlled inverse ratio ventilation (PCIRV) in rabbits, increased [NOexp] was observed during PCIRV. The present study was undertaken to clarify which component of PCIRV increased [NOexp].

METHODS

Three groups of six New Zealand White rabbits were anaesthetised and mechanically ventilated. Exhaled nitric oxide, lung mechanics and gas exchange were measured using an experimental protocol designed to assess the effects of variations in 1) flow profile, 2) inspiratory time and 3) time-weighted tidal volume. Ventilator settings used were volume and pressure control ventilation at I:E ratios of 1:2 and 4:1.

RESULTS

Constant and decelerating flow gave comparable [NOexp] levels (20.0 +/- 6.4 vs. 21.9 +/- 7.7 ppb, n.s.) when time-weighted tidal volume was kept constant. Using conventional (I:E 1:2) or inverted (I:E 4:1) I:E ratios in combination with decelerating flow and constant time-weighted tidal volumes did not alter [NOexp] (26.0 +/- 3.6 vs. 24.0 +/- 5.8 ppb, n.s.). An increased time-weighted tidal volume produced by pressure control with an I:E ratio of 4:1 increased [NOexp] (29.6 +/- 7.4) in comparison to constant (19.3 +/- 4.1, P < 0.05) and decelerating flow ventilation (19.6 +/- 3.6, P < 0.05) with I:E ratios of 1:2.

CONCLUSION

The exhaled NO concentration was affected by ventilator setting. Increased levels of [NOexp] were observed with increases in time-weighted tidal volume, whereas changes in flow pattern and inspiratory time did not seem to influence airway NO production or release.

Dynamics of re-expansion of atelectasis during general anaesthesia

A major cause of impaired gas exchange during general anaesthesia is atelectasis, causing pulmonary shunt. A 'vital capacity' (VC) manoeuvre (i.e. inflation of the lungs up to 40 cm H2O, maintained for 15 s) may re-expand atelectasis and improve oxygenation. However, such a manoeuvre may cause adverse cardiovascular effects. Reducing the time of maximal inflation may improve the margin of safety. The aim of this study was to analyse the change over time in the amount of atelectasis during a VC manoeuvre in 12 anaesthetized adults with healthy lungs. I.v. anaesthesia with controlled mechanical ventilation (VT 9 (SD 1) ml kg-1) was used. For the VC manoeuvre, the lungs were inflated up to an airway pressure (Paw) of 40 cm H2O. This pressure was maintained for 26 s. Atelectasis was assessed by analysis of computed x-ray tomography. The amount of atelectasis, measured at the base of the lungs, was 4.0 (SD 2.7) cm2 after induction of anaesthesia. The decrease in the amount of atelectasis over time during the VC manoeuvre was described by a negative exponential function with a time constant of 2.6 s. At an inspired oxygen concentration of 40%, PaO2 increased from 17.2 (4.0) kPa before to 22.2 (6.0) kPa (P = 0.013) after the VC manoeuvre. Thus in anaesthetized adults undergoing mechanical ventilation with healthy lungs, inflation of the lungs to a Paw of 40 cm H2O, maintained for 7-8 s only, may re-expand all previously collapsed lung tissue, as detected by lung computed tomography, and improve oxygenation. We conclude that the previously proposed time for a VC manoeuvre may be halved in such subjects.

Wall motion abnormalities in male elite orienteers are aggravated by exercise

During the period 1979-92, 16 (15 men and one woman) sudden unexpected cardiac deaths occurred among young Swedish orienteers. This finding indicated a sharp increase in the death rate of orienteers, and necropsy demonstrated that myocarditis was a common histopathological finding. Therefore, an extensive non-invasive cardiac investigation was performed. A total of 59 male élite orienteers (mean age 23 years) and 36 cross-country skiers and middle-distance runners (mean age 22 years), serving as controls, were examined by both echocardiography at rest and radionuclide ventriculography at rest and during exercise. Wall motion abnormalities were found in eight orienteers using echocardiography. The purpose of this study was to examine whether the group of orienteers with wall motion abnormalities found using echocardiography had a smaller increase in ejection fraction from rest to exercise using radionuclide ventriculography than the rest of the orienteers and the controls, indicating an aggravation of the wall motion abnormalities during exercise. There were no significant differences in the ejection fraction at rest between the groups. In the orienteers with wall motion abnormalities (group 1), 62% (five out of eight) had less than a 0.05 unit increase in left ventricular ejection fraction compared with 27% (14 out of 51) of the remaining orienteers (group 2) and 19% (7 out of 36) of the controls (group 3). A comparison of athletes in group 1 with those in groups 2 and 3 combined revealed a statistically significant difference (P < 0.05). The divergent response in left ventricular ejection fraction during exercise suggests an aggravation of the wall motion abnormalities with exercise. Both the echocardiographic and the radionuclide ventriculographic findings indicate that the orienteers in group 1 had concealed left ventricular damage.

Positive end-expiratory pressure prevents atelectasis during general anaesthesia even in the presence of a high inspired oxygen concentration

BACKGROUND

General anaesthesia impairs the gas exchange in the lungs, and moderate desaturation (SaO2 86-90%) occurred in 50% of anaesthetised patients in a blinded pulse oximetry study. A high FiO2 might reduce the risk of hypoxaemia, but can also promote atelectasis. We hypothesised that a moderate positive end-expiratory pressure (PEEP) level of 10 cmH2O can prevent atelectasis during ventilation with an FiO2 = 1.0.

METHODS

Atelectasis was evaluated by computed tomography (CT) in 13 ASA I-II patients undergoing elective surgery. CT scans were obtained before and 15 min after induction of anaesthesia. Then, recruitment of collapsed lung tissue was performed as a "vital capacity manoeuvre" (VCM, inspiration with Paw = 40 cmH2O for 15 s), and a CT scan was obtained at the end of the VCM. Thereafter, PEEP = 0 cmH2O was applied in group 1, and PEEP = 10 cmH2O in group 2. Additional CT scans were obtained after the VCM. Oxygenation was measured before and after the VCM.

RESULTS

Atelectasis (> 1 cm2) was present in 12 of the 13 patients after induction of anaesthesia. At 5 and 10 min after the VCM, atelectasis was significantly smaller in group 2 than group 1 (P < 0.005). A significant inverse correlation was found between PaO2 and atelectasis.

CONCLUSIONS

PEEP = 10 cmH2O reduced atelectasis formation after a VCM, when FiO2 = 1.0 was used. Thus, a VCM followed by PEEP = 10 cmH2O should be considered when patients are ventilated with a high FiO2 and gas exchange is impaired.

Increased airway osmolarity inhibits the action of nitric oxide in the rabbit

Inhalation of nitric oxide (NO) is known to dilate preconstricted airways. In asthmatics, there are large variations in the effect of NO on airway tone. One explanation of these variations may be different degrees of airway wall oedema. The effect of NO inhalation on methacholine (meth)-induced airway constriction was investigated in a rabbit model. Oedema and a change in osmolarity of the airways was achieved by hypertonic saline nebulization and hyperventilation with dry gas. There was an increase in resistance to meth at a concentration of 3 mg x mL(-1), of 86+/-14 cmH2O x L(-1) x s (mean+/-SEM) after oedema formation, compared with 46+/-16 cmH2O x L(-1) x s without oedema. Inhalation of 80 parts per million (ppm) NO failed to counter the increase in resistance due to meth, 92+/-14 cmH2O x L(-1) x s after hypertonic saline nebulization. After hyperventilation of dry gas, the increase in resistance due to meth at 1 mg x mL(-1) was 27+/-11 cmH2O x L(-1) x s with 80 ppm NO and 28+/- 5 cmH2O x L(-1) x s without NO. In conclusion, the relaxant effect of nitric oxide-inhalation on the airway smooth muscle can be blocked by an increase in the osmolarity of the airway surface liquid. The mechanism of this inhibition of nitric oxide remains to be established.

Airway closure, atelectasis and gas exchange during general anaesthesia

Airway closure and the formation of atelectasis have been proposed as important contributors to impairment of gas exchange during general anaesthesia. We have elucidated the relationships between each of these two mechanisms and gas exchange. We studied 35 adults with healthy lungs, undergoing elective surgery. Airway closure was measured using the foreign gas bolus technique, atelectasis was estimated by analysis of computed x-ray tomography, and ventilation-perfusion distribution (VA/Q) was assessed by the multiple inert gas elimination technique. The difference between closing volume and expiratory reserve volume (CV-ERV) increased from the awake to the anaesthetized state. Linear correlations were found between atelectasis and shunt (r = 0.68, P < 0.001), and between CV-ERV and the amount of perfusion to poorly ventilated lung units ("low Va/Q", r = 0.57, P = 0.001). Taken together, the amount of atelectasis and airway closure may explain 75% of the deterioration in PaO2. There was no significant correlation between CV-ERV and atelectasis. We conclude that in anaesthetized adults with healthy lungs, undergoing mechanical ventilation, both airway closure and atelectasis contributed to impairment of gas exchange. Atelectasis and airway closure do not seem to be closely related.

Position and shape of the diaphragm: implications for atelectasis formation

To evaluate diaphragm movement, 18 consecutive patients undergoing surgery under general anaesthesia were allocated to Group 1 (n = 9: no neuromuscular paralysis) or Group 2 (n = 9: neuromuscular paralysis achieved with pancuronium). Spiral computerised tomography was performed awake and during anaesthesia at end-expiratory level and, additionally, in four patients (Group 2) at end-inspiration for subsequent analysis. There was a significant cephalad displacement of the most cephalad point of the diaphragm dome at functional residual capacity, particularly in its dependent portion, in the pancuronium group. During anaesthesia with no persisting muscle paralysis, there was only a minor and insignificant cephalad shift of the diaphragm dome. However, regional analysis showed that the most dorsal part of the diaphragm was significantly displaced cephalad. Compared with conscious, spontaneous breathing, mechanical ventilation decreased the inspiratory displacement of the dependent part of the muscle. This minor movement of the diaphragm may play an additional role in atelectasis formation.

Effect of CPAP during cardiopulmonary bypass on postoperative lung function. An experimental study

BACKGROUND

Respiratory failure secondary to cardiopulmonary bypass (CPB) remains a major complication after cardiac surgery. We tested the hypothesis that post-CPB lung function impairment can be prevented by continuous positive airway pressure (CPAP) applied during the CPB.

METHODS

In 6 pigs, CPAP with 5 cmH2O pressure was applied during CPB. Six other pigs served as control, i.e. the lungs were open to the atmosphere during CPB. After median sternotomy, the right atrial appendage as well as the ascending aorta were cannulated. The total CPB duration was 90 min with 45 min cardioplegic arrest. Ventilation-perfusion distribution was measured with the multiple inert gas elimination technique and atelectasis by CT-scanning.

RESULTS

Large atelectasis appeared after CPB, corresponding to 14.5% +/- 5.5 (percent of the total lung area) in the CPAP group and 18.7% +/- 5.2 in the controls (P = 0.20). Intrapulmonary shunt increased and PaO2 decreased after the CPB in both groups.

CONCLUSIONS

We conclude that in this pig model post-CPB atelectasis is not effectively prevented by CPAP applied during CPB.

Effect of different pressure levels on the dynamics of lung collapse and recruitment in oleic-acid-induced lung injury

The effects of different inspiratory and expiratory airway pressures (Paw) on the dynamics of lung collapse and recruitment were studied in 14 anesthetized, mechanically ventilated, pigs with oleic-acid-induced lung injury. Repetitive CT scans of the same slice were obtained every 0.8 s during different inspiration and expiration hold procedures. The mean lung density and amount of atelectasis were measured in each scan. Inspiration to a Paw of 15 cm H2O above PEEP resulted in recruitment of collapsed lung tissue, mainly within 1.4 s. During expiration lung density increased rapidly and at an almost even rate within the first 1.4 s, whereas a rapid increase of atelectasis occurred after an initial delay period of 0.6 s with PEEP = 10 or 15 cm H2O. PEEP of 20 or 25 cm H2O almost prevented lung collapse during expiration. Thus, in order to avoid cyclic alveolar collapse during mechanical ventilation in oleic-acid-induced lung injury, a PEEP level >= 20 cm H2O or an expiration time <= 0.6 s is required. Long inspiratory time intervals, as used in inverse ratio ventilation, seem to be of minor importance for the recruitment of collapsed lung tissue in this experimental model.

Dynamics of lung collapse and recruitment during prolonged breathing in porcine lung injury

Oleic acid (OA) injection, lung lavage, and endotoxin infusion are three commonly used methods to induce experimental lung injury. The dynamics of lung collapse and recruitment in these models have not been studied, although knowledge of this is desirable to establish ventilatory techniques that keep the lungs open. We measured lung density by computed tomography during breath-holding procedures. Lung injury was induced with OA, lung lavage, or endotoxin in groups of six mechanically ventilated pigs. After a stabilization period, repetitive computed tomography scans of the same slice were obtained during prolonged expirations with and without positive end-expiratory pressure and during prolonged inspirations after 5 and 30 s of expiration. Lung collapse and recruitment occurred mainly within the first 4 s of breath-holding procedures in all three lung injury models, and some collapse and recruitment occurred even within 0.6 s. OA-injured lungs were significantly more unstable than lungs injured by bronchoalveolar lavage or endotoxin infusion. In this experimental setting, expiration times <0.6 s are required to avoid cyclic alveolar collapse during mechanical ventilation without extrinsic positive end-expiratory pressure.

Lung aeration and pulmonary gas exchange during lumbar epidural anaesthesia and in the lithotomy position in elderly patients

We investigated a total of 36 subjects with a mean (SD) age of 65 (13) years, during baseline conditions (supine, before any anaesthesia), and then during one of the following protocols: (1) lithotomy positioning (n = 12), (2) epidural anaesthesia (n = 12), (3) general anaesthesia in the supine position (n = 12). Lung aeration, ventilation/perfusion matching, gas exchange and functional residual capacity were measured. Lung aeration was normal during baseline assessment with almost no regions with poor aeration and no substantial dependent densities. Shunt and perfusion of poorly ventilated regions were minor. Lithotomy positioning did not reduce functional residual capacity and did not affect aeration of the lung or ventilation/perfusion matching. Epidural anaesthesia, in general, had no effect on aeration, ventilation/perfusion matching or gas exchange, regardless of whether the patient was in the supine or lithotomy position. General anaesthesia, however, caused significant increases in poorly aerated lung regions and in dependent densities (interpreted as atelectasis). In conclusion, no or little impairment of lung aeration and ventilation/perfusion matching was caused by the lithotomy position and/or epidural anaesthesia, contrary to the effects seen during general anaesthesia. However, our findings also suggest that being overweight is a factor that may cause impairment of lung aeration.

Atelectasis and gas exchange after cardiac surgery

BACKGROUND

Sometimes a high intrapulmonary shunt occurs after cardiac surgery, and impairment of lung function and oxygenation can persist for 1 week after operation. Animal studies have shown that postoperative shunt can be explained by atelectasis. In this study the authors tried to determine if atelectasis can explain shunt in patients who have had cardiac surgery.

METHODS

Nine patients having coronary artery bypass graft surgery and nine patients having mitral valve surgery were examined using the multiple inert gas elimination technique before and after operation. On the first postoperative day, computed tomography scans were made at three levels of the thorax.

RESULTS

Before anesthesia, the average shunt was low (2+/-3%; range, 0-13%), but on the first postoperative day shunt had increased to 12+/-60% (range, 3-28%). The computed tomography scans showed bilateral dependent densities in all patients but one. The mean area of the densities was 8+/-8% (range, 0-37%) of total lung area, corresponding to a calculated fraction of collapsed lung tissue of 20+/-14% (range, 0-59%). In the basal region, the calculated amount of collapsed tissue was 28+/-19% (range, 0-73%). One mitral valve patient was an outlier and had a large shunt both before and after the operation.

CONCLUSIONS

Large atelectasis in the dorsal part of the lungs was found on the first postoperative day after cardiac surgery. However, there was no clear correlation between atelectasis and measured shunt fraction.