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??Abstract??Mechanical ventilation is still one of the main treatment measures of acute respiratory distress syndrome (ARDS).From the traditional high tidal volume ventilation (10 ~ 15 mL/kg) to the current use of lung protective ventilation strategies??low tidal volume ventilation (VT)??positive end-expiratory pressure (PEEP)??airway pressure release ventilation (APRV)??bilevel positive airway pressure(BIPAP)??considerable progress has been made.In addition to the well-known conventional mechanical ventilation modes and methods??there are many non-standard mechanical ventilation modes and methods??such as prone position ventilation??neurally adjusted ventilatory assist (NAVA)??extracorporeal membrane oxygenation (ECMO)??high-frequency ventilation and etc.All these measures produce unique effects on the treatment of ARDS.     

Do all mechanically ventilated pediatric patients require continuous capnography?

With most patients in modern ICUs requiring mechanical ventilation, any technology that may lead to more optimal ventilatory strategies would be invaluable in the management of critically ill patients. The focus of most ventilator strategies is protecting the lung from the deleterious effects of mechanical ventilation. Every effort is made to minimize the duration of mechanical ventilation while optimizing the potential for successful extubation. A concise organized plan based on objective criteria that is adjusted to meet changes in patient status is clearly recommended. Continuous capnographic monitoring provides clinicians with clear, precise, objective data that may prove beneficial in the design and implementation of mechanical ventilatory strategies. There are no clear-cut methods for achieving the optimal ventilator strategy for a specific patient. Although guidelines and management theories exist throughout the medical literature, in practice, they often merely serve as loose guidelines. The dynamic properties of an acutely ill patient make the management of mechanical ventilation an ongoing process requiring clinical assessment and planning by multidisciplinary members of the patient care team. Comprehensive evaluation of ventilatory management strategies and patient responses must be made by a collaborative effort of physicians, respiratory care practitioners, and nurses. An objective, consistent approach to the overall management is essential. Although still controversial, it is the authors' opinion that volumetric capnograph provides the data necessary to establish adequate gas delivery, optimal PEEP, and effective ventilation with the least amount of mechanical assistance, regardless of clinician or institutional preferences.     

Lung recruitment maneuvers in acute respiratory distress syndrome

In the experimental setting, repeated derecruitments of the lungs of ARDS models accentuate lung injury during mechanical ventilation, whereas open lung concept strategies can attenuate the injury. In the clinical setting, recruitment manuevers that use a continuous positive airway pressure of 40 cmH2O for 40 secs improve oxygenation in patients with early ARDS who do not have an impairment in the chest wall. High intermittent positive end-expiratory pressure (PEEP), intermitent sighs, or high-pressure controlled ventilation improves short-term oxygenation in ARDS patients. Both conventional and electrical impedance thoracictomography studies at the clinical setting indicate that high PEEP associated with low levels of pressure control ventilation recruit the collapsed portions of the ARDS lungs and that adequate PEEP levels are necessary to keep the ARDS lungs opened allowing a more homogenous ventilation. High PEEP/low tidal volume ventilation was seen to reduce inflammatory mediators in both bronchoalveolar lavage and plasma, compared to low PEEP/high tidal volume ventilation, after 36 hours of mechanical ventilation in ARDS patients. Recruitment maneuvers that used continuous positive airway pressure levels of 35-40 cmH2O for 40 secs, with PEEP set at 2 cmH2O above the lower inflection point of the pressure-volume curve, and tidal volume < 6 mL/kg were associated with a 28-day intensive care unit survival rate of 62%. This contrasted with a survival rate of only 29% with conventional ventilation (defined as the lowest PEEP for acceptable oxygenation without hemodynamic impairment with a tidal volume of 12 mL/kg), without recruitment manuevers (number needed to treat = 3; p < 0.001). In the near future, thoracic computed tomography associated with high-performance monitoring of regional ventilation may be used at the bedside to determine the optimal mechanical ventilation of the ARDS keeping an opened lung with a homogenous ventilation.     

Hypercapnic encephalopathy syndrome: a new frontier for non-invasive ventilation?

According to the classical international guidelines, non-invasive ventilation is contraindicated in hypercapnic encephalopathy syndrome (HES) due to the poor compliance to ventilatory treatment of confused/agitated patients and the risk of aspirative pneumonia related to lack of airways protection. As a matter of fact, conventional mechanical ventilation has been recommended as "golden standard" in these patients. However, up to now there are not controlled data that have demonstrated in HES the advantage of conventional mechanical ventilation vs non-invasive ventilation. In fact, patients with altered mental status have been systematically excluded from the randomised and controlled trials performed with non-invasive ventilation in hypercapnic acute respiratory failure. Recent studies have clearly demonstrated that an initial cautious NPPV trial in selected HES patients may be attempt as long as there are no other contraindications and the technique is provided by experienced caregivers in a closely monitored setting where ETI is always readily available. The purpose of this review is to report the physiologic rationale, the clinical feasibility and the still open questions about the careful use of non-invasive ventilation in HES as first-line ventilatory strategy in place of conventional mechanical ventilation via endotracheal intubation.     

Expiratory washout versus optimization of mechanical ventilation during permissive hypercapnia in patients with severe acute respiratory distress syndrome.

The aim of this study was to compare three ventilatory techniques for reducing PaCO2 in patients with severe acute respiratory distress syndrome treated with permissive hypercapnia: (1) expiratory washout alone at a flow of 15 L/min, (2) optimized mechanical ventilation defined as an increase in the respiratory frequency to the maximal rate possible without development of intrinsic positive end- expiratory pressure (PEEP) combined with a reduction of the instrumental dead space, and (3) the combination of both methods. Tidal volume was set according to the pressure-volume curve in order to obtain an inspiratory plateau airway pressure equal to the upper inflection point minus 2 cm H2O after setting the PEEP at 2 cm H2O above the lower inflection point and was kept constant throughout the study. The three modalities were compared at the same inspiratory plateau airway pressure through an adjustment of the extrinsic PEEP. During conventional mechanical ventilation using a respiratory frequency of 18 breaths/min, respiratory acidosis (PaCO2 = 84 +/- 24 mm Hg and pH = 7.21 +/- 0.12) was observed. Expiratory washout and optimized mechanical ventilation (respiratory frequency of 30 +/- 4 breaths/min) had similar effects on CO2 elimination (DeltaPaCO2 = -28 +/- 11% versus -27 +/- 12%). A further decrease in PaCO2 was observed when both methods were combined (DeltaPaCO2 = -46 +/- 7%). Extrinsic PEEP had to be reduced by 5.3 +/- 2.1 cm H2O during expiratory washout and by 7.3 +/- 1.3 cm H2O during the combination of the two modes, whereas it remained unchanged during optimized mechanical ventilation alone. In conclusion, increasing respiratory rate and reducing instrumental dead space during conventional mechanical ventilation is as efficient as expiratory washout to reduce PaCO2 in patients with severe ARDS and permissive hypercapnia. When used in combination, both techniques have additive effects and result in PaCO2 levels close to normal values.     

Volume-assured pressure support ventilation (VAPSV). A new approach for reducing muscle workload during acute respiratory failure.

This study reports the preliminary clinical evaluation of a new mode of ventilation--volume-assured pressure support ventilation (VAPSV)--which incorporates inspiratory pressure support (PSV) with conventional volume-assisted cycles (VAV). This combination optimizes the inspiratory flow during assisted/controlled cycles, reducing the patient's respiratory burden commonly observed during VAV. Different from conventional PSV, VAPSV assures precise control of tidal volume (VT) in unstable patients. Eight patients with acute respiratory failure (ARF) were submitted to assisted ventilation under VAV and VAPSV. Patient's ventilatory workload (evaluated through the pressure-time product, mechanical work per liter of ventilation, and work per minute) and patient's ventilatory drive (occlusion pressure--P0.1) were significantly reduced during VAPSV. This "relief" was more evident among the most distressed patients (p < 0.001), allowing a reduction of more than 60 percent in muscle load, without the need of increasing peak tracheal pressure. Mean inspiratory flow (VT/TI), VT, and effective dynamic compliance were significantly increased during VAPSV, whereas the effective inspiratory impedance decreased. These mechanical advantages of VAPSV allowed a reduction of intrinsic PEEP, whenever it was present. Blood gas values were similar in both periods. We concluded that VAPSV is a promising form of ventilatory support. At the same time that it was able to safely assure a minimum preset VT, VAPSV reduced patient workload and improved synchrony between the patient and the ventilator during ARF.     

呼气末正压通气治疗重型颅脑损伤后神经源性肺水肿的疗效观察

目的探讨呼气末正压机械通气（positive end-expiratory pressure,PEEP）在救治重型颅脑损伤后神经源性肺水肿（neurogenic pulmonary edema,NPE）中的作用。方法对28例重型颅脑损伤并发神经源性肺水肿患者进行常规机械通气,并设对照组;应用PEEP治疗后设置为治疗组,进行肺部啰音、RR、PaO2、SaO2、PaCO2比较。结果 PEEP治疗2 h后,26例患者双肺湿啰音明显减少甚至消失,RR、PaO2、SaO2改善具有极显著性差异（P〈0.001）。结论机械通气加用PEEP治疗重型颅脑损伤后NPE能在短时间内明显提高PaO2,是目前治疗NPE最有效的方法之一。     

Respiratory system mechanics in acute respiratory distress syndrome

Respiratory mechanics research is important to the advancement of ARDS management. Twenty-eight years ago, research on the effects of PEEP and VT indicated that the lungs of ARDS patients did not behave in a manner consistent with homogenously distributed lung injury. Both Suter and colleagues] and Katz and colleagues reported that oxygenation continued to improve as PEEP increased (suggesting lung recruitment), even though static Crs decreased and dead-space ventilation increased (suggesting concurrent lung overdistension). This research strongly suggested that without VT reduction, the favorable effects of PEEP on lung recruitment are offset by lung overdistension at end-inspiration. The implications of these studies were not fully appreciated at that time, in part because the concept of ventilator-associated lung injury was in its nascent state. Ten years later. Gattinoni and colleagues compared measurements of static pressure-volume curves with FRC and CT scans of the chest in ARDS. They found that although PEEP recruits collapsed (primarily dorsal) lung segments, it simultaneously causes overdistension of non-dependent, inflated lung regions. Furthermore, the specific compliance of the aerated, residually healthy lung tissue is essentially normal. The main implication of these findings is that traditional mechanical ventilation practice was injecting excessive volumes of gas into functionally small lungs. Therefore, the emblematic low static Crs measured in ARDS reflects not only surface tension phenomena and recruitment of collapsed airspaces but also overdistension of the remaining healthy lung. The studies reviewed in this article support the concept that lung injury in ARDS is heterogeneously distributed, with resulting disparate mechanical stresses, and indicate the additional complexity from alterations in chest wall mechanics. Most of these studies, however, were published before lung-protective ventilation. Therefore, further studies are needed to refine the understanding of the mechanical effects of lung-protective ventilation. Although low-VT ventilation is becoming a standard of care for ARDS patients, many issues remain unresolved; among them are the role of PEEP and recruitment maneuvers in either preventing or promoting lung injury and the effects of respiratory rate and graded VT reduction on mechanical stress in the lungs. The authors believe that advances in mechanical ventilation that may further improve patient outcomes are likely to come from more sophisticated monitoring capabilities (ie, the ability to measure P1 or perhaps Cslice) than from the creation of new modes of ventilatory support.     

Recruiting collapsed lung through collateral channels with positive end-expiratory pressure.

We studied reinflation of collapsed parts in excised normal human lungs through both the ordinary bronchial route and through collateral channels. A model of atelectasis was made either by simple collapse or by applying a positive pressure to the pleura and a negative to the airway. Five different ventilatory patterns were used for reinflation: simulated normal breathing with and without continuous positive airway pressure (CPAP), simulated deep breathing and mechanical ventilation with and without positive end-expiratory pressure (PEEP). All methods, except normal breathing without CPAP, reinflated the collapsed part with pressures well within the range used clinically. The most effective way of re-expanding collapsed lung was the application of CPAP during simulated normal breathing or PEEP during mechanical ventilation, which required smaller transpulmonary pressure swings than the other methods. A comparison between CPAP and PEEP showed CPAP to be preferable. Collateral reinflation occurred just as readily as normal reinflation and the results suggest that collateral reinflation is the primary choice. This route of reexpansion also has a potential secretion clearing effect in that pressure is built up distal to an obstruction.     

ARDSnet ventilatory protocol and alveolar hyperinflation: role of positive end-expiratory pressure

RATIONALE: In patients with acute respiratory distress syndrome (ARDS), a focal distribution of loss of aeration in lung computed tomography predicts low potential for alveolar recruitment and susceptibility to alveolar hyperinflation with high levels of positive end-expiratory pressure (PEEP). OBJECTIVES: We tested the hypothesis that, in this cohort of patients, the table-based PEEP setting criteria of the National Heart, Lung, and Blood Institute's ARDS Network (ARDSnet) low tidal volume ventilatory protocol could induce tidal alveolar hyperinflation. METHODS: In 15 patients, physiologic parameters and plasma inflammatory mediators were measured during two ventilatory strategies, applied randomly: the ARDSnet and the stress index strategy. The latter used the same ARDSnet ventilatory pattern except for the PEEP level, which was adjusted based on the stress index, a monitoring tool intended to quantify tidal alveolar hyperinflation and/or recruiting/derecruiting that occurs during constant-flow ventilation, on a breath-by-breath basis. MEASUREMENTS AND MAIN RESULTS: In all patients, the stress index revealed alveolar hyperinflation during application of the ARDSnet strategy, and consequently, PEEP was significantly decreased (P < 0.01) to normalize the stress index value. Static lung elastance (P = 0.01), plasma concentrations of interleukin-6 (P < 0.01), interleukin-8 (P = 0.031), and soluble tumor necrosis factor receptor I (P = 0.013) were significantly lower during the stress index as compared with the ARDSnet strategy-guided ventilation. CONCLUSIONS: Alveolar hyperinflation in patients with focal ARDS ventilated with the ARDSnet protocol is attenuated by a physiologic approach to PEEP setting based on the stress index measurement.     

Successful treatment of a critically ill patient with acute respiratory distress syndrome from COVID-19 using mechanical ventilation strategy with low levels of positive end-expiratory pressure: A CARE-compliant case report

Introduction:Acute respiratory distress syndrome (ARDS) secondary to COVID-19 is different from the ARDS caused by other infections. Conventional mechanical ventilation strategies using high levels of PEEP may not be beneficial and can even be harmful to patient with ARDS from COVID-19. So the ventilation strategies should be adjusted in order to improve the pulmonary ventilation function and oxygenation status, and outcomes of the patient.Patient concerns:Herein, we present a 76-year-old male patient with ARDS secondary to COVID-19. We describe our experience with mechanical ventilation strategy and the changes in respiratory mechanics in the patient during treatment.Diagnosis:The patient had tested positive for coronavirus (COVID-19) in nucleic acid test. Chest CT showed multiple ground glass shadows in both lungs.Interventions:The patient received mechanical ventilation with low tidal volume and low PEEP.Outcomes:After treatment, the patients condition, as well as oxygenation status was improved, and he tested negative for the coronavirus several times.Conclusion:This case demonstrated that the low tidal volume with low levels of PEEP ventilation strategy may be more suitable for ARDS from COVID-19.     

Assisted ventilation in patients with preexisting cardiopulmonary disease. The effect on systemic oxygen consumption, oxygen transport, and tissue perfusion variables

We have evaluated systemic oxygen consumption (VO2), systemic oxygen transport, and tissue perfusion variables in 30 patients with preexisting cardiac and underlying pulmonary disease during continuous positive-pressure ventilation and positive end-expiratory pressure [PEEP], during intermittent mandatory ventilation (IMV and PEEP), and during spontaneous ventilation (continuous positive airway pressure [CPAP]), with end-expiratory pressure held constant during all ventilatory modes. Using radionuclide angiography together with invasive determinations of pressure and flow, we also measured left and right ventricular ejection fractions and calculated the end-systolic (ESVI) and end-diastolic (EDVI) volume indices of both ventricles. We found that oxygen transport was significantly greater during CPAP (583 +/- 172 ml/min/M2)(mean +/- SD) than during either IMV and PEEP (543 +/- 151 ml/min/sq; p less than 0.01) or CPPV and PEEP (526 +/- 159 ml/min/M2; p less than 0.01); however, we found no significant change in systemic VO2 with conversion from CPPV and PEEP to CPAP. The increase in oxygen transport was related to a greater cardiac index and, more specifically, to a higher heart rate during CPAP (CPAP, 106 +/- 16 beats per minute; CPPV and PEEP, 97 +/- 14 beats per minute) (p less than 0.01). Enhanced oxygen transport during CPAP was also associated with an increase in mixed venous oxygenation and a decrease in arterial lactate. Although neither the mean left ventricular EDVI nor ESVI changed from CPPV and PEEP to CPAP, the mean pulmonary capillary wedge pressure increased (CPPV and PEEP, 12 +/- 5 mm Hg; CPAP, 14 +/- 7 mm Hg) (p less than 0.01), suggesting the possibility of a decrease in left ventricular compliance with the spontaneous ventilatory mode. This study suggests that in the absence of ventilatory failure, spontaneous ventilation provides for better systemic oxygen transport and overall tissue perfusion than either controlled ventilation or IMV; however, this benefit of enhanced oxygen delivery with spontaneous ventilation may potentially be offset by a decrease in left ventricular compliance.     

Inverse ratio ventilation in ARDS. Rationale and implementation

Conventional ventilatory support of patients with the adult respiratory distress syndrome (ARDS) consists of volume-cycled ventilation with applied positive end-expiratory pressure (PEEP). Unfortunately, recent evidence suggests that this strategy, as currently implemented, may perpetuate lung damage by overinflating and injuring distensible alveolar tissues. An alternative strategy--termed inverse ratio ventilation (IRV)--extends the inspiratory time, and, in concept, maintains or improves gas exchange at lower levels of PEEP and peak distending pressures. There are two methods to administer IRV: (1) volume-cycled ventilation with an end-inspiratory pause, or with a slow or decelerating inspiratory flow rate; or (2) pressure-controlled ventilation applied with a long inspiratory time. There are several real or theoretical problems common to both forms of IRV: excessive gas-trapping; adverse hemodynamic effects; and the need for sedation in most patients. Although there are many anecdotal reports of IRV, there are no controlled studies that compare outcome in ARDS patients treated with IRV as opposed to conventional ventilation. Nonetheless, clinicians are using IRV with increasing frequency. In the absence of well-designed clinical trials, we present interim guidelines for a ventilatory strategy in patients with ARDS based on the literature and our own clinical experience.     

Respiratory failure in the elderly patient

Respiratory failure frequently complicates the care of elderly patients with or without chronic lung disease. Recent advances in techniques and applications of noninvasive ventilation provide an exceedingly useful means of managing respiratory compromise, and the clinical utilization of noninvasive mechanical ventilation has transformed the prognosis of acute and chronic respiratory failure in this age group. The majority of elderly patients can recover from an acute respiratory failure episode if adequate support is provided, although some may require long-term ventilatory assistance. Such assistance may be provided in the home setting if an adequate support system is available. As the size of the elderly population grows, an increased number of elderly patients with multifactorial respiratory failure will undoubtedly require episodic or sustained ventilatory assistance, and noninvasive ventilation can be provided for various forms of acute and chronic respiratory failure, including advanced chronic obstructive pulmonary disease, other parenchymal lung disease, and chest wall deformities. Health care organizations must incorporate long-term care facilities with ventilatory support capabilities into their health management strategies.     

Adverse ventilatory strategy causes pulmonary-to-systemic translocation of endotoxin

Accumulating evidence strongly suggests that ventilatory strategy has an important impact on development of lung injury and patient outcome. Adverse ventilatory strategies have been shown to cause release of pulmonary-derived cytokines and may permit bacterial translocation from the lung to the systemic circulation. Because endotoxin is a potent and clinically important stimulant of cytokine-mediated systemic inflammatory responses that can lead to multiorgan failure, we investigated the effects of ventilatory strategy on lung-to-systemic translocation of endotoxin. We studied the effects of protective (tidal volume [VT] 5 ml. kg(-)(1), positive end-expiratory pressure [PEEP] 10 to 12.5 cm H(2)O) versus nonprotective (VT 12 ml. kg(-)(1), PEEP zero) ventilatory strategy on translocation of endotracheally instilled endotoxin. Anesthetized New Zealand White rabbits were subjected to saline lung lavage, and 32 were randomized to one of four groups: PS (protective ventilation + instilled saline); PE (protective ventilation + instilled endotoxin); NS (nonprotective ventilation + instilled saline); NE (nonprotective ventilation + instilled endotoxin), and ventilated for 3 h. Plasma endotoxin levels increased significantly in the NE group, and remained low and unchanged in the other groups. Peak levels of plasma tumor necrosis factor-alpha (TNF-alpha) were higher in NE versus other groups. Pa(O(2)) and mean arterial pressure (Pa) were lowest, and requirement for pressor and bicarbonate support greatest, in the NE group. Finally, plasma endotoxin levels were significantly greater in eventual nonsurvivors than survivors. These data provide convincing evidence for pulmonary translocation of lung-derived endotoxin. This translocation depends on ventilatory strategy, and suggests a pathophysiologic link between ventilatory strategy and outcome.     

Intraoperative mechanical ventilation strategies for obese patients: a systematic review and network meta‐analysis

Several intraoperative ventilation strategies are available for obese patients. However, the same ventilation interventions have exhibited different effects on PaO₂/FIO₂ concerning obese patients in different trials, and the issue remains controversial. Therefore, we conducted a network meta‐analysis to identify the optimal mechanical ventilation strategy. We searched the Cochrane Central Register of Controlled Trials (CENTRAL) in the Cochrane Library, Embase, MEDLINE, CINAHL and Web of Science for studies published up to June 2014, and the PaO₂/FIO₂ in obese patients given different mechanical ventilation strategies was assessed. We assessed the studies for eligibility and extracted data and then pooled the data and used a Bayesian fixed‐effect model to combine direct comparisons with indirect evidence. Eligible studies evaluated different ventilation strategies for obese patients and reported the intraoperative PaO₂/FIO₂ ratio, atelectasis and pulmonary compliance. Thirteen randomized controlled trials were included for network meta‐analysis, including 476 patients who received 1 of 12 ventilation strategies. Volume‐controlled ventilation with higher PEEP plus single recruitment manoeuvres (VCV + higher PEEP + single RM) was associated with the highest PaO₂/FiO₂ ratio, improving intraoperative pulmonary compliance and reducing the incidence of intraoperative atelectasis.     

Proportional Assist Ventilation and Neurally Adjusted Ventilatory Assist-Better Approaches to Patient Ventilator Synchrony?

Understanding the regulation of breathing in the critical care patient is multifaceted, especially in ventilator-dependent patients who must interact with artificial respiration. Mechanical ventilation originally consisted of simple, manually-driven pump devices, but it has developed into advanced positive pressure ventilators for continuous support of patients in respiratory failure. This evolution has resulted in mechanical ventilators that deliver assist intermittently, attempting to mimic natural breathing. Recently, modes of mechanical ventilation that synchronize not only the timing, but also the level of assist to the patient's own effort, have been introduced. This article describes the concepts related to proportional assist ventilation and neurally adjusted ventilatory assist, and how they relate to conventional modes in terms of patient-ventilator synchrony.     

Pressure-volume curves in the management of acute respiratory distress syndrome

The interpretation of P-V curves is uncertain for several reasons: the influence of chest wall compliance, differences in regional lung compliance and intrapulmonary gas distribution, lung volume history, lung recruitment beyond the LIP, peripheral airway fluid movement, expiratory-flow limitation, differences between inflation and deflation limb characteristics, and interobserver variability in curve analysis. In addition, many studies of acute lung injury have constructed P-V curves following disconnection from the ventilator. The inevitable lung volume changes that occur may alter the elastic and viscoelastic behavior so that the resulting P-V curve characteristics may not accurately reflect conditions during mechanical ventilation. More extensive research seems to be required before P-V curves are used as a routine guide for mechanical ventilation therapy in ARDS. Furthermore, this article suggests that titrating PEEP or VT according to the inflation-limb P-V curve should be done with caution, because the mechanical significance of this information is open to question. Current research suggests the possibility that PEEP could be targeted according to the slope of deflation-limb compliance, because this measure may more accurately reflect global alveolar closing pressures. This type of analysis can be done only by transferring data into software programs that can perform sophisticated curve fitting, and such programs are not readily available to most clinicians. From a practical standpoint, there is no compelling clinical evidence that adjusting mechanical ventilation according to the P-V curve improves mortality or morbidity in ARDS as much or more than can be achieved simply by decreasing the VT and Pplat.     

Effect of general anaesthesia on respiratory function

This review examines the possible mechanisms for the impairment in pulmonary gas exchange and ventilatory control associated with general anaesthesia. Venous admixture and physiological dead space are both increased during anaesthesia. These changes result from increased ventilation-perfusion inequality, an effect apparently mainly attributable to alteration in the intrapulmonary distribution of ventilation. Concomitantly, with these changes in pulmonary gas exchange anaesthesia is known to alter the mechanics of the respiratory system and, in particular, to decrease functional residual capacity in recumbent subjects. Recent research has renewed interest in the finding that anaesthesia also alters chest wall shape and motion. This review attempts to synthesize the available and increasing evidence which supports the hypothesis that anaesthesia-induced alterations in chest wall behavior are responsible for the associated changes in lung function and consequent impairment of pulmonary gas exchange. Finally, the important finding that anaesthetic agents depress the ventilatory response to hypercapnia and hypoxia is discussed.     

Sequential alterations of tracheal mechanical properties in the neonatal lamb: effect of mechanical ventilation

Alterations in neonatal airway mechanics resulting from ventilatory therapies are implicated in airway collapse and chronic disease. Quantifying the functional impact of mechanical ventilation (MV) on the neonatal airway and elucidating the time course of these changes will support development of protective therapies. The objective of this study was to test the hypothesis that conventional MV would result in decreased static and dynamic elastance of an isolated tracheal segment and thinning of the muscle (trachealis) region of the tracheal wall in a time dependent manner. Tracheal segments were isolated in newborn lambs spontaneously breathing through the distal trachea; segments were MV (n = 7; PIP/PEEP = 35/5 cmH2O; 40 breaths/min) or instrumented, non-ventilated (SHAM; n = 7; PIP/PEEP = 0/0 cmH2O) for 4 hr. At baseline and hourly, tracheal segments were filled with saline, and static pressure-volume curves were constructed as the pressure response to stepwise volume infusions. Then, cross-sectional ultrasound images were captured at 0 cmH2O on SHAM, and at 0 cmH2O, peak inspiratory pressure (PIP) and positive end expiratory pressure (PEEP), on MV tracheae for subsequent dimensional analysis. Tracheal elasticity indices were derived from static pressure-volume data, and during dynamic ventilation using ultrasound images to calculate the stress-strain relationships. Over 4 hr of MV, tracheal internal diameter (ID) increased (14%; P < 0.05). Markers of tracheal mechanical properties indicated a decrease in elasticity under both static (bulk modulus; 28%; P < 0.05) and dynamic (elastic modulus; 282 %; P < 0.05) conditions, indicating a significant alteration in elastic components. No time dependent changes were identified in dimensions or mechanical properties in the SHAM group. CONCLUSIONS: MV results in dimensional alterations that increased anatomical dead space and reduced static and dynamic elastance of the neonatal trachea.