Dependent Lung Zones Research Articles

Distribution of ventilation and oxygenation in surgical obese patients ventilated with high versus low positive end-expiratory pressure: A substudy of a randomised controlled trial.

BACKGROUNDIntra-operative ventilation using low/physiological tidal volume and positive end-expiratory pressure (PEEP) with periodic alveolar recruitment manoeuvres (ARMs) is recommended in obese surgery patients.OBJECTIVESTo investigate the effects of PEEP levels and ARMs on ventilation distribution, oxygenation, haemodynamic parameters and cerebral oximetry.DESIGNA substudy of a randomised controlled trial.SETTINGTertiary medical centre in Geneva, Switzerland, between 2015 and 2018.PATIENTSOne hundred and sixty-two patients with a BMI at least 35 kg per square metre undergoing elective open or laparoscopic surgery lasting at least 120 min.INTERVENTIONPatients were randomised to PEEP of 4 cmH2O (n = 79) or PEEP of 12 cmH2O with hourly ARMs (n = 83).MAIN OUTCOME MEASURESThe primary endpoint was the fraction of ventilation in the dependent lung as measured by electrical impedance tomography. Secondary endpoints were the oxygen saturation index (SaO2/FIO2 ratio), respiratory and haemodynamic parameters, and cerebral tissue oximetry.RESULTSCompared with low PEEP, high PEEP was associated with smaller intra-operative decreases in dependent lung ventilation [-11.2%; 95% confidence interval (CI) -8.7 to -13.7 vs. -13.9%; 95% CI -11.7 to -16.5; P = 0.029], oxygen saturation index (-49.6%; 95% CI -48.0 to -51.3 vs. -51.3%; 95% CI -49.6 to -53.1; P < 0.001) and a lower driving pressure (-6.3 cmH2O; 95% CI -5.7 to -7.0). Haemodynamic parameters did not differ between the groups, except at the end of ARMs when arterial pressure and cardiac index decreased on average by -13.7 mmHg (95% CI -12.5 to -14.9) and by -0.54 l min-1 m-2 (95% CI -0.49 to -0.59) along with increased cerebral tissue oximetry (3.0 and 3.2% on left and right front brain, respectively).CONCLUSIONIn obese patients undergoing abdominal surgery, intra-operative PEEP of 12 cmH2O with periodic ARMs, compared with intra-operative PEEP of 4 cmH2O without ARMs, slightly redistributed ventilation to dependent lung zones with minor improvements in peripheral and cerebral oxygenation.TRIAL REGISTRATIONNCT02148692, https://clinicaltrials.gov/ct2

Revealing the dilemma in COVID-19 pneumonia: use of the prone thorax CT imaging in differentiation of opacificities due to dependant zone and pneumonic consolidation.

In December 2019, a disease called Coronavirus disease 2019 (COVID-19) caused by the novel coronavirus called severe acute respiratory syndrome coronavirus 2 (SARS- COV-2) emerged in Wuhan, China. The disease was declared as a pandemic by the WHO on May 11, 2020. The gold standard to diagnose is a Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) test. Additionally, computed tomography (CT) is also very useful in certain cases. A 62-year-old female patient was admitted to the emergency room with fever and joint pain for five days. She had a history of contact with COVID- 19 (+) patient. Unenhanced lung CT performed in the prone position clearly showed nodular infiltrations in the subpleural/peripheral lower lung areas in the right lung. Another 73-year-old female patient was admitted to the emergency room with tiredness and loss of taste for a few days. She had no history of contact with COVID-19 (+) patients. Unenhanced lung CT performed while the patient was in supine position, diffuse nodular opacities in the lower regions of both lungs could hardly be detected. The combined throat nose swab test was positive in both patients. In conclusion, lung CT in prone position is a very valuable method of imaging to show COVID-19 pneumonia; it can distinguish true COVID-19 pneumonia from dependent lung zones in the supine position.

Incidence and Location of Atelectasis Developed During Bronchoscopy Under General Anesthesia: The I-LOCATE Trial

Despite the many advances in peripheral bronchoscopy, its diagnostic yield remains suboptimal. With the use of cone-beam CT imaging we have found atelectasis mimicking lung tumors or obscuring them when using radial-probe endobronchial ultrasound (RP-EBUS), but its incidence remains unknown. What are the incidence, anatomic location, and risk factors for developing atelectasis during bronchoscopy under general anesthesia? We performed a prospective observational study in which patients undergoing peripheral bronchoscopy under general anesthesia were subject to an atelectasis survey carried out by RP-EBUS under fluoroscopic guidance. The following dependent segments were evaluated: right bronchus 2 (RB2), RB6, RB9, and RB10; and left bronchus 2 (LB2), LB6, LB9, and LB10. Images were categorized either as aerated lung ("snowstorm" pattern) or as having a nonaerated/atelectatic pattern. Categorization was performed by three independent readers. Fifty-seven patients were enrolled. The overall intraclass correlation agreement among readers was 0.82 (95%CI, 0.71-0.89). Median time from anesthesia induction to atelectasis survey was 33min (range, 3-94 min). Fifty-one patients (89%; 95%CI, 78%-96%) had atelectasis in at least one of the eight evaluated segments, 45 patients (79%) had atelectasis in at least three, 41 patients (72%) had atelectasis in at least four, 33 patients (58%) had atelectasis in at least five, and 18 patients (32%) had atelectasis in at least six segments. Right and left B6, B9, and B10 segments showed atelectasis in > 50%of patients. BMI and time to atelectasis survey were associated with increased odds of having more atelectatic segments (BMI: OR, 1.13 per unit change; 95%CI, 1.034-1.235; P= .007; time to survey: OR, 1.064 per minute; 95%CI, 1.025-1.105; P= .001). The incidence of atelectasis developing during bronchoscopy under general anesthesia in dependent lung zones is high, and the number of atelectatic segments is greater with higher BMI and with longer time under anesthesia. ClinicalTrials.gov; No.: NCT03523689; URL: www.clinicaltrials.gov.

Lung Ultrasonography for the Assessment of Perioperative Atelectasis: A Pilot Feasibility Study

Few diagnostic tools are available to anesthesiologists when confronted with intraoperative hypoxemia. Lung ultrasonography is a safe and accurate bedside imaging modality. The aim of this study was to evaluate the feasibility of lung ultrasonography during the perioperative period and assess its ability to detect intraoperative respiratory complications and oxygenation changes resulting from perioperative atelectasis. In this prospective observational pilot study, 30 consecutive patients scheduled for laparoscopic surgery were recruited. Mechanical ventilation was standardized. Lung ultrasonography was performed at 5 predefined time points: before induction of general anesthesia (GA), after induction of GA, after pneumoperitoneum insufflation, on arrival in the recovery room, and before recovery room discharge. For each echographic examination, 12 pulmonary quadrants were imaged. From these, a semiquantitative score, the lung ultrasound (LUS) score, was calculated to assess lung aeration at each time point. Lung ultrasonography was possible in all patients. Changes in the LUS score between the postinduction period and arrival in the recovery room were correlated with changes in oxygenation (Spearman r = -0.43, P = .018). Induction of GA was associated with an increase in the LUS score, which gradually worsened at all time points until recovery room discharge. This increase was significantly worse in the basal and dependent lung zones. Lung ultrasonography helped in the detection of 2 capnothoraces, 1 endobronchial intubation, and 1 episode of subclinical pulmonary edema. Lung ultrasonography in the perioperative period is feasible, allows tracking of perioperative atelectasis, and facilitates the diagnosis of respiratory complications. The evolution of aeration loss correlates moderately with changes in oxygenation.

Real-time in-vivo imaging of pulmonary capillary perfusion using probe-based confocal laser scanning endomicroscopy in pigs: An interventional laboratory study.

Little is known about real-time in-vivo microscopy of pulmonary capillary perfusion because current microscopy requires direct access to lung tissue with surgical intervention such as the thoracic-window technique and open-lung model. To evaluate if probe-based confocal laser scanning endomicroscopy (pCLE) via the trachea allows for real-time in-vivo visualisation of pulmonary capillary density and red blood cell (RBC) velocity in pigs. An interventional animal study. European University Hospital. Nine female domestic pigs (50 to 60 kg) were used. A pCLE probe was positioned in non-dependent, central and dependent lung zones in nine anaesthetised pigs (Alveoflex, Cellvizio, Maunakea, France). After intravenous administration of fluorescein isothiocyanate dextran as contrast agent repetitive pCLE videos were recorded during pressure-controlled ventilation (PCV) or continuous positive airway pressure for 3 min each. Using fluorescein isothiocyanate-labelled RBC erythrocyte velocities in pulmonary capillaries were quantified. Data are expressed as mean ± SD or median with interquartile range (IQR). Capillary density was greater in dependent and central as compared with non-dependent lung zones [[32 (29 to 34) %] and 32 (30 to 34) % vs. 28 (26 to 28) %, respectively, P < 0.05]. During PCV, RBC velocities were higher in larger lung capillaries [diameter >20 μm, 309 μm s(-1) (209 to 397)] than intermediate [diameter 10.1 to 20 μm, 146 μm s(-1) (118 to 235)] and small [diameter <10 μm, 153 μm s(-1) (117 to 236), P < .05]. During continuous positive airway pressure of 1.5 kPa, RBC velocities in dependent lung areas decreased to 47 μm s(-1) (30 to 82) compared with 198 μm s(-1) (148 to 290) during PCV (P < 0.05). pCLE allows endoscopic real-time in-vivo imaging of pulmonary capillary morphology and perfusion. Alterations in pulmonary capillary blood flow induced by different ventilator regimens can be detected. This minimally invasive approach via the endotracheal route is feasible in an experimental setting and may help to understand changes in regional pulmonary capillary perfusion.

Solitary Lung Masses Due to Occult Aspiration

BackgroundAspiration occurs commonly, at times clinically occult, and is recognized to cause a widening spectrum of lung disorders. Presentation of aspiration as a lung mass has not been described. MethodsAmong cases of aspiration-related pulmonary diseases diagnosed at Mayo Clinic (Rochester, Minn) from 2007 to 2013, 3 patients were identified to have presented with a solitary lung mass lesion. ResultsThe age of 3 patients, all men with a history of gastroesophageal reflux disease, ranged from 53 to 65 years. All patients presented with dyspnea, cough, and intermittent fevers. Chest computed tomography in each patient demonstrated malignant-appearing solitary lung mass, cavitated in 2 patients. Two patients underwent positron emission tomography, which showed intense fluorodeoxyglucose uptake in the lung mass for both. Surgical lung resection revealed acute and organizing pneumonia with giant cell reaction to foreign material, consistent with aspiration in all 3 patients. None of these lung masses were located in the “dependent” (posterior or basal) lung zones. These patients were managed with antireflux medical therapy; 1 patient underwent a Nissen fundoplication procedure for recurrent symptoms. No additional aspiration-related complications occurred during the follow-up period ranging from 24 to 84 months. ConclusionsAspiration-related pulmonary complications can present as a solitary lung mass that may not be located in dependent lung zones, which have traditionally been associated with aspiration-related pulmonary diseases.

Higher Levels of Spontaneous Breathing Induce Lung Recruitment and Reduce Global Stress/Strain in Experimental Lung Injury

Spontaneous breathing (SB) in the early phase of the acute respiratory distress syndrome is controversial. Biphasic positive airway pressure/airway pressure release ventilation (BIPAP/APRV) is commonly used, but the level of SB necessary to maximize potential beneficial effects is unknown. Experimental acute respiratory distress syndrome was induced by saline lung lavage in anesthetized and mechanically ventilated pigs (n = 12). By using a Latin square and crossover design, animals were ventilated with BIPAP/APRV at four different levels of SB in total minute ventilation (60 min each): (1) 0% (BIPAP/APRV0%); (2) greater than 0 to 30% (BIPAP/APRV>0-30%); (3) greater than 30 to 60% (BIPAP/APRV>30-60%); and (4) greater than 60% (BIPAP/APRV>60%). Gas exchange, hemodynamics, and respiratory variables were measured. Lung aeration was assessed by high-resolution computed tomography. The distribution of perfusion was marked with Ga-labeled microspheres and evaluated by positron emission tomography. The authors found that higher levels of SB during BIPAP/APRV (1) improved oxygenation; (2) decreased mean transpulmonary pressure (stress) despite increased inspiratory effort; (3) reduced nonaerated lung tissue, with minimal changes in the distribution of perfusion, resulting in decreased low aeration/perfusion zones; and (4) decreased global strain (mean ± SD) (BIPAP/APRV0%: 1.39 ± 0.08; BIPAP/APRV0-30%: 1.33 ± 0.03; BIPAP/APRV30-60%: 1.27 ± 0.06; BIPAP/APRV>60%: 1.25 ± 0.04, P < 0.05 all vs. BIPAP/APRV0%, and BIPAP/APRV>60% vs. BIPAP/APRV0-30%). In a saline lung lavage model of experimental acute respiratory distress syndrome in pigs, levels of SB during BIPAP/APRV higher than currently recommended for clinical practice, that is, 10 to 30%, improve oxygenation by increasing aeration in dependent lung zones without relevant redistribution of perfusion. In presence of lung recruitment, higher levels of SB reduce global stress and strain despite an increase in inspiratory effort.

Therapeutic Range of Spontaneous Breathing during Mechanical Ventilation

In the 4th century before the common era, the Greek philosopher and physician Hippocrates expressed his wisdom on the value of activity to the sick and the healthy by stating that: “The sick will of course profit to a great extent from gymnastics with regard to the restoration of their health and the healthy will profit with regard to its maintenance” (On Regimen in Acute Diseases, 3, 400 BCE). In contemporary critical care medicine, driven by the goal of protecting our patients from self-inflicted injury, pain, and anxiety – and in contrast to Hippoctrates’ suggestions – we have built a culture of immobilizing our patients: we prescribe high doses of opioids, sedatives, anxiolytics, and anti-pychotics, and with the best intentions, place bed-rest and restraints orders. In patients with severe respiratory failure, we frequently use immobilizing ventilator settings such as volume control ventilation. Recent data have been increasingly challenging this tenet in relation to the fields of neuropsychiatry, rehabilitation, and respiratory medicine1–5. In this issue of Anesthesiology, two papers bring new information on why the concept of muscle activity is also relevant to lung biochemistry and regional function. Guldner et al.6 and Bruells et al.7 provide important experimental data on the relationship between the dose of diaphragmatic activity (spontaneous contribution to breathing during mechanical ventilation) and the resulting response in terms of lung mechanical stress, gas exchange, and markers of muscle deconditioning. As the main respiratory muscle, the diaphragm contributes 72% for tidal breathing8 and its role in respiratory mechanics and gas exchange goes well beyond this global number. One of the reasons is its curvature during spontaneous breathing in the supine position, which facilitates expansion of dependent lung regions9, optimizes the regional distribution of lung ventilation, and prevents loss of dependent lung aeration and increase in shunt observed when muscle paralysis is produced10. Accordingly, modes of ventilation proposed since the 1970s tried to explore those advantages in patients with acute respiratory failure. The beneficial effects of maintaining continuous spontaneous breathing during mechanical ventilation of patients with acute respiratory failure have been described by Putensen and coworkers in two well-designed studies11,12. However, in patients with severe respiratory failure, barriers to spontaneous breathing during mechanical ventilation including patient-ventilator asynchrony, high oxygen consumption of the respiratory pump muscles, and the risk of barotrauma, have hindered a clear determination of the value of spontaneous breathing during mechanical ventilation.

Weak link in the lung

The transition from peripheral airway disease to COPD is characterized by three sequential stages: stage I, during which the closing volume (CV) eventually exceeds the FRC; stage II, during which tidal expiratory flow limitation (EFL) is eventually exhibited; and stage III, during which dynamic hyperinflation (DH) progressively increases leading to dyspnoea and exercise limitation. Smoking is a condition which accelerates the age-related impairment of lung function. Indeed, McCarthy et al.1 who studied 35 smokers with FEV and FEV/FVC within normal limits, found that in almost all instances CV was higher than predicted normal both sitting and supine. More importantly, while in all 66 control non-smokers whose age was less than 65 years the FRC exceeded the CV in the sitting position, this was not the case in eight of the 35 smokers. In the supine position, the age-corrected prevalence of peripheral airway closure during tidal breathing was even higher in the smokers. Thus, during stage I the extent of peripheral airway closure increases progressively in smokers until tidal EFL is present not only supine but also sitting. Tidal FL implies sequential closure of the peripheral airways during expiration and reopening during inspiration, with concurrent risk of peripheral airway injury, which is characterized by denuded epithelium in the respiratory and membranous bronchioles. Furthermore, there is rupture of the airway-alveolar attachments.2–4 Thus, the peripheral airways represent a weak link in the lung because airway closure in the tidal volume range is a common phenomenon reflecting either a decreased FRC (e.g. obesity, left heart failure) or an increased CV (e.g. smoking, asthma). The smoking-related increase of CV accelerates the age-related decrease of maximal flow (Vmax) at low lung volume.5 Indeed, such a pattern is exhibited by all COPD patients and is due to the combined effect of age and smoke on gas trapping. As a result of gas trapping the RV increases. Peripheral airway closure is usually found in the dependent lung zones where the transpulmonary pressure is lowest. Thus, airway closure is a regional phenomenon, whose extent and location varies with body posture. Most normal subjects do not exhibit tidal EFL even during maximal exercise. In contrast, in many COPD patients EFL may be present even at rest.6 Two main mechanisms promote the occurrence of tidal EFL, namely a reduction of available flow and/or an increase in ventilatory requirements. Since smoking causes a progressive reduction of Vmax in the tidal volume range of susceptible subjects, tidal EFL eventually occurs even in the sitting position. Tidal EFL implies heterogeneous sequential dynamic compression of the peripheral airways during expiration and re-expansion during inspiration, with concurrent risk of peripheral airway injury.7 During stage II, both tidal EFL and peripheral airway closure may concur in eliciting airway injury. In stage II, EFL during tidal breathing occurs initially at local levels but it eventually extends to all pulmonary airways, that is, the tidal expiratory flow rates are maximal even during quiet breathing. Thus, during stage II, EFL progresses from a local to a generalized phenomenon. Both promote DH because, in order to generate an adequate pulmonary ventilation, subjects with tidal EFL must breathe at a higher volume than the relaxed FRC. In normal individuals at rest, the end-expiratory lung volume (FRC) corresponds to the relaxation volume (Vr) of the respiratory system, that is, the lung volume at which the elastic recoil pressure of the respiratory system is zero. Pulmonary hyperinflation, which is defined as an increase in FRC above the predicted normal range, maybe due to increased Vr as a result of loss of lung recoil (e.g. emphysema) or dynamic pulmonary hyperinflation, which is said to be present when the FRC exceeds Vr. DH exists when the duration of expiration is insufficient to allow the lungs to deflate to Vr before the next inspiration. This may occur when expiratory flow is impeded (e.g. increased airway resistance) and/or expiratory time is shortened (e.g. increased breathing frequency). In COPD patients DH is common and is mainly due to tidal EFL.6 It is the main factor causing dyspnoea and exercise limitation.8

Respiratory function in morbidly obese subjects

Morbidly obese subjects are characterized by important changes in respiratory function both during spontaneous breathing as well as during general anesthesia and mechanical ventilation. The most characteristic abnormalities consist of decreased functional residual capacity (FRC), reduced expiratory reserve volume, decreased compliance and increased resistance of the respiratory system. Breathing at low lung volume promotes airway closure in the dependent lung zones with consequent gas exchange abnormalities. Furthermore, the decreased expiratory reserve as a result of decreased FRC and the higher ventilatory requirements of these patients due to increased metabolic demands may promote expiratory flow limitation (EFL) in the tidal volume range. The presence of peripheral airway closure and EFL during tidal breathing promotes peripheral airway injury and may accelerate the abnormalities of lung function. The risk of injury is expected to be higher during mechanical ventilation due to the high-pressure transients, which develop under this condition. Consequently, external positive end-expiratory pressure (PEEP) must be applied to these subjects in order to increase the end-expiratory lung volume above the closing volume as well as the flow limitation volume and thus decrease the risk of peripheral airway injury.

Acute Effects of Upright Position on Gas Exchange in Patients With Acute Respiratory Distress Syndrome

Patients with acute respiratory distress syndrome (ARDS) have dorsal atelectasis of the lungs. This is probably caused by several mechanisms: compression on dependent lung zones, purulent secretions in alveoli, and upward shift of the diaphragm. An upright position (UP) of the patient (the whole body in a straight line at 40 to 45 degrees) can theoretically ameliorate these mechanisms. The objective was to evaluate whether there was an improvement of gas exchange during UP of ARDS patients and to evaluate the hemodynamic effects. A prospective interventional study was performed in the surgical and medical ICUs and the burn unit of the Ghent University Hospital, a tertiary care center. Included were ARDS patients with onset of ARDS within 48 hours before start of the study. Patients were excluded when there was hemodynamic instability or when the PaO2/FiO2 ratio deteriorated during the 2 hours preceding UP. After a 2-hour observation period in a semirecumbent position, patients were put in UP for 12 hours. Respiration and hemodynamic data were recorded at the start and end of the 2-hour observation period, and after 1, 4, and 12 hours in UP. Eighteen patients were included in the study. There was a significant increase of the PaO2/FiO2 ratio during UP (P < .001). Except for the need for volume resuscitation in 5 patients (27.8%), there was no significant change in the hemodynamic profile of the patients. Upright positioning of patients with ARDS, a relatively simple maneuver, resulted in an improvement of gas exchange and was tolerated hemodynamically relatively well during a 12-hour observation period.

Respiratory function in morbidly obese subjects

AbstractMorbidly obese subjects are characterized by important changes in respiratory function both during spontaneous breathing as well as during general anesthesia and mechanical ventilation. The most characteristic abnormalities consist of decreased functional residual capacity (FRC), reduced expiratory reserve volume, decreased compliance and increased resistance of the respiratory system. Breathing at low lung volume promotes airway closure in the dependent lung zones with consequent gas exchange abnormalities. Furthermore, the decreased expiratory reserve as a result of decreased FRC and the higher ventilatory requirements of these patients due to increased metabolic demands may promote expiratory flow limitation (EFL) in the tidal volume range. The presence of peripheral airway closure and EFL during tidal breathing promotes peripheral airway injury and may accelerate the abnormalities of lung function. The risk of injury is expected to be higher during mechanical ventilation due to the high-pressu...

Alterations in regional ventilation, perfusion, and shunt after smoke inhalation measured by PET.

Regional changes in ventilation and perfusion occurring in the early hours after smoke inhalation injury were evaluated through the use of positron emission tomography. Five lambs were imaged before and 1, 2, and 4 h after receiving 100 breaths of cotton smoke. Utilizing a recently developed model of (13)N tracer kinetics (3), we evaluated changes in ventilation, perfusion, shunt, and regional gas content in nondependent, middle, and dependent lung zones. The data demonstrated a progressive development of regional shunt in dependent (dorsal) regions in which perfusion remained the highest throughout the study. These findings, together with decreasing regional ventilation and fractional gas content in the dependent regions, correlated with decreasing arterial Pa(O(2)) values over the course of the study. A negative correlation between regional shunt fraction and regional gas content in dependent and middle regions suggests that shunt was caused by progressive alveolar derecruitment or flooding.

Regional VA, Q, and VA/Q during PLV: effects of nitroprusside and inhaled nitric oxide.

Partial liquid ventilation (PLV) with high-specific-weight perfluorocarbon liquids has been shown to improve oxygenation in acute lung injury, possibly by redistributing perfusion from dependent, injured regions to nondependent, less injured regions of the lung. Our hypothesis was that during PLV in normal lungs, a shift in perfusion away from dependent lung zones might, in part, be due to vasoconstriction that could be reversed by infusing sodium nitroprusside (NTP). In addition, delivering inhaled NO during PLV should improve gas exchange by further redistributing blood flow to well-ventilated lung regions. To examine this, we used a single transverse-slice positron emission tomography camera to image regional ventilation and perfusion at the level of the heart apex in six supine mechanically ventilated sheep during five conditions: control, PLV, PLV + NTP, and PLV + NO at 10 and 80 ppm. We found that PLV shifted perfusion from dependent to middle regions, and the dependent region demonstrated marked hypoventilation. The vertical distribution of perfusion changed little when high-dose intravenous NTP was added during PLV, and inhaled NO tended to shift perfusion toward better ventilated middle regions. We conclude that PLV shifts perfusion to the middle regions of the lung because of the high specific weight of perflubron rather than vasoconstriction.

PARTIAL LIQUID VENTILATION FOR ACUTE RESPIRATORY DISTRESS SYNDROME

PLV represents an intriguing alternative paradigm in the approach to the patient with ALI. Within the past decade, substantial information has become available regarding this technique. Clearly, PLV is feasible in patients with ALI and ARDS, and it appears to be safe with respect to short-term effects on hemodynamics and lung physiology, as well as long-term toxicity (although further research in this area is warranted). Although PLV has not yet been proven to be superior to traditional mechanical ventilation for patients with ALI or ARDS, PLV possesses an intriguing combination of physical, physiologic, and biologic effects: "Liquid PEEP" effect--e.g., more effective recruitment of dependent lung zones than achieved by gas ventilation Anti-inflammatory effects Lavage of alveolar debris Mitigation of ventilator-induced lung injury Direct anti-inflammatory effects--e.g., decreased macrophage release of proinflammatory cytokines, etc. Prevention of nosocomial pneumonia Combination with other modalities--e.g., exogenous surfactant replacement, inhaled NO, prone position Enhanced delivery of drugs or gene vectors into the lung. The results of ongoing and future clinical trials will be necessary to establish whether PLV improves clinical outcomes in patients with ALI or ARDS, or specific subgroups of such patients. Significant work also remains to be done to define the optimum dose level of PLV and the most appropriate ventilatory strategies.

Closing volume influences the postural effect on oxygenation in unilateral lung disease.

In normal adults, both blood flow and ventilation are distributed preferentially to the dependent lung zones. In adults with unilateral lung disease, arterial oxygenation improves when they are positioned with their good lung down because of improved matching of ventilation and perfusion. When the closing volume is increased, dependent airways are closed during tidal breathing, so that reduced ventilation-perfusion ratio and hypoxia develops and ventilation is preferentially distributed to the upper lung zones. We undertook an observational study on the effects of lateral recumbency on arterial oxygenation in adult patients with unilateral lung disease and tested the hypothesis that oxygenation in lateral recumbency might be influenced by an increase in closing volume. Arterial blood gases were analyzed in the supine, right and left lateral decubitus positions and the AaPO(2) was calculated in 44 randomly selected patients 49.9 +/- 18.7 yr of age with unilateral pneumonia (23 cases) or pulmonary tuberculosis (21 cases). In 26 patients, individual Pa(O(2)) with the normal lung in the dependent position was higher than that with the diseased lung; the opposite was true for 18 patients. The difference in Pa(O(2)) and AaPO(2) between the two positions was statistically significant in both groups. In 16 patients (10 men and six women 49.2 +/- 18.2 yr of age), we measured closing volume and determined the fractional ventilation to each lung by (133)Xe lung scan in the three positions. In these 16 patients, the difference in Pa(O(2)) between the normal and the diseased lung in the dependent position was related significantly to the difference in the fractional ventilation going to the normal lung between the dependent and the supine position (r = 0.642, p = 0. 007). The latter was related significantly to the % predicted closing volume (CV/VC) (r = -0.597, p = 0.015). This study has shown that closing volume, as well as posture, might be involved in determining oxygenation in lateral recumbency in patients with unilateral lung disease.

Computer tomographic assessment of perfluorocarbon and gas distribution during partial liquid ventilation for acute respiratory failure.

The average in vivo chest computed tomographic (CT) attenuation number (air = -1,000, soft tissue = 0, perflubron = +2,300 Hounsfield units [HU]) of 10 ventrodorsal-oriented lung segments was calculated to assess the distribution of gas and perflubron in 14 oleic acid lung-injured adult sheep during partial liquid ventilation (PLV, n = 7) or gas ventilation (GV, n = 7). Partial liquid ventilation was associated with a significant decrease in shunt fraction (PLV = 40 +/- 12%, GV = 76 +/- 12%, p = 0.004). Computed tomographic attenuation data during expiration (HUexp) demonstrated minimal gas aeration in GV animals in the dependent (segments 6-10) lung zones (HUexp = -562 +/- 108 for segments 1-5, HUexp = -165 +/- 104 for segments 6-10, p = 0.015). During PLV, perflubron was predominantly distributed to the dependent lung regions (HUexp = 579 +/- 338 for segments 1-5, HUexp = 790 +/- 149 for segments 6-10, p = 0.04). The ratio of the inspiratory to expiratory HU (HUinsp/exp) was greater in dependent than nondependent regions (mean HUinsp/exp segments 1-5 = 0.56, segments 6-10 = 0.81, p = 0.01), indicating that during inspiration relatively more gas than perflubron was distributed to the nondependent lung regions. We conclude that during PLV in this lung injury model, (1) gas exchange is improved when compared with gas ventilation, (2) perflubron is distributed predominantly to the dependent regions of the lung, and (3) although gas is distributed throughout the lung with each inspiration, more gas than perflubron goes to the nondependent lung regions.

Airway closure and intraoperative hypoxaemia: twenty-five years later.

The literature describing the pulmonary mechanisms of increased PA-PaO2 during general anaesthesia was examined to define the role of airway closure and sub-radiological atelectasis. A Medline search was designed to include articles dealing with the stated purpose, which is thus selective rather than a meta-analysis. The MeSH consisted of the following words: Anesthesia: general/inhalational; Pulmonary gas exchange; Ventilation:perfusion ratio; Lung Physiology; Lung Volume measurements; Closing Volume/Capacity; Functional Residual Capacity; Atelectasis; Diaphragm. Also, Dr H. Rothen and Prof. G. Hedenstierna supplied raw data. Changes in shape and dimensions of the thorax and abdomen immediately after induction of anaesthesia result in marked alterations in the efficiency of oxygenation. Three pathways can be described: increased effects of airway closure, increased low ventilation: perfusion in dependent lung zones, and frank atelectasis. The magnitude of the alterations is determined by the patients' age and body habitus. Some of the changes may carry-over into the postoperative period. The data suggest that increasing tidal volume during anaesthesia will reduce the effects of airway closure and that vital capacity breaths will re-expand atelectatic areas. Airway closure and atelectasis contribute equally to the increased ventilation: perfusion mismatching that occurs during general anaesthesia.

Perioperative functional residual capacity

The literature dealing with the magnitude, mechanism and effects of reduced FRC in the perioperative period is reviewed. During general anaesthesia FRC is reduced by approximately 20%. The reduction is greater in the obese and in patients with COPD. The most likely mechanism is the loss of inspiratory muscle tone of the muscles acting on the rib cage. Gas trapping is an additional mechanism. Lung compliance decreases and airways resistance increases, in large part, due to decreased FRC. The larynx is displaced anteriorly and elongated, making laryngoscopy and intubation more difficult. The change in FRC creates or increases intrapulmonary shunt and areas of low ventilation to perfusion. This is due to the occurrence of compression atelectasis, and to regional changes in mechanics and airway closure which tend to reduce ventilation to dependent lung zones which are still well perfused. Abdominal and thoracic operations tend to increase shunting further. Large tidal volume but not PEEP will improve oxygenation, although both increase FRC. Both FRC and vital capacity are reduced following abdominal and thoracic surgery in a predictable pattern. The mechanism is the combined effect of incisional pain and reflex dysfunction of the diaphragm. Additional effects of thoracic surgery include pleural effusion, cooling of the phrenic nerve and mediastinal widening. Postoperative hypoxaemia is a function of reduced FRC and airway closure. There is no real difference among the various methods of active lung expansion in terms of the speed of restoration of lung function, or in preventing postoperative atelectasis/pneumonia. Epidural analgesia does not influence the rate of recovery of lung function, nor does it prevent atelectasis/pneumonia.

Does airway closure affect lung sound generation?

The purpose of this experiment was to test the hypothesis that airway closure impedes the production of lung sounds at low lung volume. We recorded breath sounds in three healthy men during inspiratory vital capacity manoeuvres in upright, head-down and lateral decubitus postures. We then compared the rate of increase of breath sound intensity (BSI) between the dependent and non-dependent lung zones. Closing volumes, measured separately, were normal in the upright but increased in the head-down postures. The data revealed no consistent lag in the rate of increase of BSI over the dependent lung zones in any of the postures. Our data suggest that airway closure does not influence lung sound generation. If true, this implies that lung sounds are produced proximally to the site of physiologic airway closure.