The contribution of the pulmonary microvascular pressure in the maintenance of an open lung during mechanical ventilation

Changes in pulmonary hemodynamics modify the mechanical properties of the lungs. The effects of alterations in pulmonary capillary pressure (Pc) were investigated on the airway and lung tissue mechanics during positive-pressure ventilation and following lung recruitment maneuvers. Isolated, mechanically normoventilated (PEEP 2.5 cmH(2)O) rat lungs were perfused with Pc set to 0 (unperfused), 5, 10 or 15 mmHg, in random sequence. The pulmonary input impedance (ZL) was measured at end-expiration before and after a 10-min long ventilation. After inflation of the lung to 30 cmH(2)O during P-V curve recordings, another set of ZL was measured to evaluate the degree of recruitment. The PEEP was then decreased to 0.5 cmH(2)O and the sequence was repeated. Airway resistance and parenchymal damping and elastance (H) were estimated from ZL by model fitting. From the P-V curves, elastance (E) and hysteresis indices were determined. Mechanical ventilation at both PEEP levels resulted primarily in elevations in the tissue parameters, with the greatest increases at the 0 Pc level (H changes of 27.8+/-4.2 and 61.3+/-3.7% at 2.5 and 0.5 cmH(2)O PEEP, respectively). The maintenance of physiological Pc (10 mmHg) led to a significantly lower elevation in H (11.6+/-1.5% versus 31.4+/-3.6%). The changes in the oscillatory mechanics were also reflected in E and the hysteresis of the P-V curves. These findings indicate that pulmonary hypoperfusion during mechanical ventilation forecasts a parenchymal mechanical deterioration. Physiological pressure in the pulmonary capillaries is therefore an important mechanical factor promoting maintenance of the stability of the alveolar architecture during positive-pressure mechanical ventilation.     

A practical protocol for titrating "optimal" PEEP in acute lung injury: recruitment maneuver and PEEP decrement

This study was conducted to evaluate the effectiveness and safety of a practical protocol for titrating positive end-expiratory pressure (PEEP) involving recruitment maneuver (RM) and decremental PEEP. Seventeen consecutive patients with acute lung injury who underwent PEEP titration were included in the analysis. After baseline ventilation, RM (continuous positive airway pressure, 35 cm H2O for 45 sec) was performed and PEEP was increased to 20 cmH2O or the highest PEEP guaranteeing the minimal tidal volume of 5 mL/kg. Then PEEP was decreased every 20 min in 2 cmH2O decrements. The "optimal" PEEP was defined as the lowest PEEP attainable without causing a significant drop (>10%) in PaO2. The "optimal PEEP" was 14.5 +/- 3.8 cmH2O. PaO2 /FI O2 ratio was 154.8 +/- 63.3 mmHg at baseline and improved to 290.0 +/- 96.4 mmHg at highest PEEP and 302.7 +/- 94.2 mmHg at "optimal PEEP", both significantly higher than baseline (p<0.05). Static compliance was significantly higher at "optimal" PEEP (27.2 +/- 10.4 mL/ cmH2O) compared to highest PEEP (22.3 +/- 7.7 mL/cmH2O) (p<0.05). Three patients experienced transient hypotension and one patient experienced atrial premature contractions. No patient had gross barotrauma. PEEP titration protocol involving RM and PEEP decrement was effective in improving oxygenation and was generally well-tolerated.     

Pressure controlled inverse ratio ventilation in acute respiratory distress syndrome patients

Appropriate ventilatory intervention is life saving in acute respiratory distress syndrome (ARDS). Pressure controlled inverse ratio ventilation (PC-IRV) is the likely mode of ventilation benefiting in extreme conditions of ARDS. However, guidelines when to start PC-IRV is not yet well defined. The ventilation-related dilemma, which we faced in two illustrative cases of ARDS are presented. The first patient presenting clinically with ARDS but with high peak airway pressure (PIP) and low dynamic lung compliance, PC-IRV helped in reducing PIP, improved haemodynamics and the oxygenation of blood. In second patient with similar clinical presentation of ARDS, where although PIP was high but the dynamic compliance was better, the PC-IRV caused deterioration in PaO2. Here, patient rather did better with high PEEP (15 cm H2O) and usual I: E ratio (1:2). It is probable that the dynamic lung compliance (< 20ml/cmH2O), PIP (> 50 cm H2O) at conventional I: E ratio (1:2) ventilation (10 ml/kg) with hypotension might form the basis to develop a scoring system for guidance to switch over to PC-IRV ventilation. Further randomised prospective controlled clinical trials will then be required to establish indication to start PC-IRV in ARDS.     

PEEP-induced changes in epithelial permeability in inbred mouse strains

Inbred mouse strains have demonstrated a range of susceptibilities to inhaled environmental irritants. C57Bl/6J mice are highly susceptible while C3H/HeJ mice are resistant to ozone exposures, as assessed by lavaged protein. However, lavaged protein reflects a loss of both the endothelial and epithelial barrier. To determine whether basal differences exist in the epithelial barrier, we measured soluble tracer ((99m)technetium-diethylenetriamine pentaacetic acid, (99m)Tc-DTPA) clearance from the lung in spontaneously breathing, anesthetized mice and mice ventilated with increased lung volume with applied positive end-expiratory pressure (PEEP; 1, 6, or 10cmH(2)O). Both strains showed more rapid clearance during ventilation with 10cmH(2)O PEEP compared with other ventilation pressures (p<0.001). There was a substantial difference in clearance between the two strains during ventilation with 10cmH(2)O PEEP (mean half time for C57Bl/6J mice=19+/-4min versus 34+/-3min for C3H/HeJ mice; p<0.001). Thus, when lung volume is increased, the susceptible C57Bl/6J strain shows a greater change in epithelial barrier than the resistant C3H/HeJ strain. These results may reflect fundamental differences in lung architecture.     

Positive end-expiratory pressure increases pulmonary clearance of inhaled 99mTc-DTPA in nonsmokers but not in healthy smokers

Positive end-expiratory pressure (PEEP) is widely used in the treatment of severe pulmonary oedema, although its effects on the clearance of water and small solutes from alveolus to blood are not well characterized. We studied the effect of the application of 10 cmH2O of PEEP on the flux of inhaled 99mTc-diethylene-triamine-penta-acetic acid (DTPA) from lung to blood in six healthy smoking and six nonsmoking subjects. The rate of flux was corrected for possible changes in pulmonary blood volume during PEEP by use of an intravenous injection of 99mTc-DTPA. The baseline clearance rate (K, % X min-1) for nonsmokers was 1.48 +/- 0.12 (mean +/- SE) and increased to 2.40 +/- 0.29 during PEEP (p less than 0.05). In contrast, the mean clearance rate for smokers was 3.26 +/- 0.82 at baseline and 3.03 +/- 0.82 during PEEP (p = NS). The application of positive end-expiratory pressure appears to increase alveolar solute flux in nonsmokers but not in smokers, suggesting that the pathway for solute clearance in smokers is governed by different rate-limiting steps to those of nonsmokers.     

双相气道正压结合反比通气应用于麻醉患者临床观察

目的观察双相气道正压结合反比通气应用于麻醉患者时对术中患者各项呼吸参数及血流动力学的影响,以探讨对正常肺如何实施肺保护策略。方法随机选择腹部肿瘤手术患者26例,其中男性15例,女性11例;年龄45～60岁,平均年龄45岁。美国麻醉学会（ASA）Ⅱ～Ⅲ级,术前无重大心肺疾病。全身麻醉,先以容量控制方式通气,吸呼比：1：2,90min后转换到双相气道正压结合反比通气,吸呼比=2：1。呼气终末正压（PEEP）从0．392kPa（4cmH2O）开始,吸气压（Pin）从0．686kPa（7cmH2O）开始,根据潮气量和呼气末CO2分压调整△P,保持与容量控制通气相同的潮气量。记录气道峰值压（Pmax）、平均气道压（Pmean）、呼气末CO2分压,血压、心率、脉搏氧饱和度变化,计算肺顺应性并在每一种通气方式90min稳定后抽取桡动脉血测血气分析。结果双相气道正压结合反比通气时,Pmean明显升高,肺顺应性明显升高,与容量控制通气相比,差异有显著统计学意义（P〈0．001）。凡。及动脉血CO2分压、氧分压、血流动力学差异无统计学意义。结论双相气道正压结合反比例通气应用于全身麻醉患者,‰。显著升高,肺顺应性明显改善,对血流动力学无显著影响,可安全用于全身麻醉患者术中机械通气维持。     

Extracorporeal CO2 removal in a lung lavage induced respiratory distress syndrome

Ten pigs with experimental respiratory distress syndrome were treated by extracorporeal CO2 removal (ECCO2-R) combined with low frequency positive pressure ventilation (LPPV). After lung damage had been induced by repeated lung lavages a PEEP trial was conducted in order to find the appropriate PEEP for the damaged lungs. This PEEP was then applied during the ECCO2-R/LPPV period. Blood gas values improved significantly on extracorporeal bypass within a short time (pre-bypass paO2: 54.2 +/- 3.7 vs 168.5 +/- 31.6 mmHg after 15 min on bypass, p less than 0.001) and were kept constant during the next 4 hours. Minute ventilation (MV) was reduced from 4.01 +/- 0.31 to 0.74 +/- 0.07 l/min (p less than 0.0001), FiO2 of the ventilator from 1.0 to 0.46 +/- 0.08 (p less than 0.0001) whereas FiO2 of the membrane lung (ML) was not changed significantly (FIO2ML 0.59 +/- 0.07 vs 0.53 +/- 0.06). During controlled mechanical ventilation (CMV), comparable adequate gas exchange was only achieved at a significantly higher mean airway pressure (Paw 14.1 +/- 0.08 vs 21.2 +/- 0.47 cmH20, p less than 0.0001). Hemodynamic variables did not change significantly during bypass time. ECCO2-R/LPPV driven by a simple renal perfusion system allows adequate gas exchange in experimental respiratory failure.     

Non-linear dependence of interstitial fluid pressure on joint cavity pressure and implications for interstitial resistance in rabbit knee

AIMS: Synovium retains lubricating fluid in the joint cavity. Synovial outflow resistance estimated as dPj/dQs (Pj, joint fluid pressure and Qs trans-synovial flow) is greater, however, than expected from interstitial glycosaminoglycan concentration. This study investigates whether subsynovial fluid pressure increases with intra-articular pressure, as this would reduce the estimated resistance estimate. METHODS: Interstitial fluid pressure (Pif) was measured as a function of distance from the joint cavity in knees of anaesthetized rabbits, using servo-null pressure-measuring micropipettes and using an external 'window'. Joint fluid pressure Pj was either endogenous (-2.4 +/- 0.4 cmH2O, mean +/- SEM) or held at approximately 4, 8 or 15.0 cmH2O by a continuous intra-articular saline infusion that matched the trans-synovial interstitial drainage rate. RESULTS: At endogenous Pj the peri-articular Pif was subatmospheric (-1.9 +/- 0.3 cmH2O, n = 19). At raised Pj the Pif values became positive. Gradient dPif /dx was approximately 20 times steeper across synovium than subsynovium. Pif close to the synovium-subsynovium border (Pif*) increased as a non-linear function of Pj to 1.4 +/- 0.2 cmH2O (n = 23) at Pj = 4.3 +/- 0.1 cmH2O : 2.3 +/- 0.2 cmH2O (n = 17) at Pj = 7.6 +/- 0.2 cmH2O: and 3.0 +/- 0.4 cmH2O (n = 26) at Pj = 15 +/- 0.2 cmH2O (P = 0.03, anova). CONCLUSIONS: Synovial resistivity is approximately 20x subsynovial resistivity. The increase in Pif*with Pj means that true synovial resistance d(Pj-Pif*)/dQs is overestimated 1.5x by dPj/dQs. This narrows but does not eliminate the gap between analysed glycosaminoglycan concentration, 4 mg ml(-1), and the net interstitial biopolymer concentration of 11.5 mg ml(-1) needed to generate the resistance.     

Superiority of nasal mask pressure over mouth pressure, as a surrogate of diaphragm twitch-related esophageal pressure, in healthy humans

Assessing diaphragm function is clinically and physiologically pertinent. It can rely on the measurement of pressure responses to phrenic stimulation. Combining mouth pressure (Pm) with cervical magnetic stimulation (CMS) is painless and easy to perform, but Pm-CMS poorly reflects esophageal pressure (Pes-CMS) because of poor pressure transmission across the airway. We reasoned that the mouth opening and neck flexion that are associated with the measurement of Pm-CMS would impair upper airway dynamics and further hinder pressure transmission. Therefore, we assessed the CMS-related pressure measured in a nasal mask (Pmask; mouth closed) without neck flexion as a possible surrogate of Pes-CMS, in 14 men and 3 women, age 24.5+/-2.2. Pes-CMS was 15.7+/-4.3 cmH2O, significantly higher than Pm-CMS (13.5+/-5.6 cmH2O, P<0.0001) but not different from Pmask-CMS (15.2+/-4.9 cmH2O). The concordance correlation coefficient was low (0.6808) between Pes-CMS and Pm-CMS. It was higher between Pes-CMS and Pmask-CMS (0.8730). Pm-CMS wrongly classified five subjects as abnormal (<10 cmH2O), versus 1 for Pmask and 5 for Pm (P=0.025). Passing and Bablok regressions found no difference between Pes-CMS and Pmask-CMS, but identified a systematic difference and a proportional error between Pes-CMS and Pm-CMS. We conclude that Pmask-CMS is a better surrogate of Pes-CMS than Pm-CMS.     

气道内高压力促进3种气道重塑相关因子的表达

目的探讨机械通气情况下气道内不同压力水平对气道重塑相关因子表达的影响。方法手术室经全麻行机械通气的42例慢性阻塞性肺疾病(COPD)作为COPD组和33例无基础肺疾病患者作为对照组。机械通气根据吸气峰压(PIP)水平又分为高、中、低压力组(分别为24、22和20 cm H_2O),呼气末正压均为5 cm H_2O。机械通气前及3 h后收集支气管肺泡灌洗液(BALF)。酶联免疫吸附法和Western blot法检测BALF中气道重塑相关因子成纤维生长因子2(FGF-2)、转化生长因子-β1(TGF-β1)和基质金属蛋白酶-9(MMP-9)蛋白表达水平。结果 1)机械通气前COPD组BALF中的FGF-2、TGF-β1和MMP-9蛋白水平明显高于对照组(P0.01)。2)机械通气后对照组在高压力刺激下FGF-2、TGF-β1和MMP-9表达水平升高(P0.05);而COPD组压力刺激下上述3种蛋白表达升高更明显(P0.05),且高压力组中及低压力组(P0.05)。3)相关性分析显示,COPD组BALF中FGF-2、TGF-β1、MMP-9表达水平与气道压力成正相关(P0.01)。结论机械通气时气道内的持续高压力可能通过作用于气道上皮细胞内压力敏感通道进而提高气道重塑因子FGF-2、TGF-β1、MMP-9的表达水平,COPD患者尤为显著。     

Trauma-adult respiratory distress syndrome

Sixteen trauma victims with adult respiratory distress syndrome were retrospectively examined. High injury severity score (mean: 44), massive transfusion requirements, and prolonged ventilator days characterize this group. Persistent intra-abdominal infection accounted for two of three deaths in this series. Most of these patients were managed without paralysis using intermittent mandatory ventilation and positive-end expiratory pressure (PEEP). High frequency jet ventilation was necessary in one subject. Two patients exhibited early ARDS reversal, ie, clinical improvement, better chest x-ray and decreased shunt (within 72 hours) when an intra-abdominal septic focus was eradicated. Principles of critical care for these patients remain: an FiO2 less than .40, limiting barotrauma, using PEEP for alveolar recruitment and close monitoring of O2 transport.     

Pulmonary clearance of inhaled [99Tcm]DTPA: effects of ventilation pattern

While a rise in lung volume is known to increase the pulmonary clearance of technetium-99m-labelled dietylene triamine pentaacetate ([99Tcm]DTPA), little interest has been focused on the effects of changes in ventilation frequency, tidal volume and airway pressure. We studied adult, anaesthetized and intubated rabbits during three ventilation patterns (VP) using pressure controlled ventilation (ServoVentilator 900C). VP was either deep slow (f = 20 min-1, tidal volume (VT) = 30 +/- 4 ml kg-1 and positive end-expiratory pressure (PEEP) = 0.2 kPa [VP 20/0.2, n = 8]) or rapid shallow (f = 80 min-1, VT = 11 +/- 2 ml kg-1 and PEEP = 0.2 or 0.4 kPa [VP 80/0.2, n = 6 and VP 80/0.4, n = 6]). The mean airway pressure was similar at VP 20/0.2 and VP 80/0.4. During administration of [99Tcm]DTPA aerosol all animals were ventilated under the same conditions (f = 40 min-1 and PEEP = 0.2 kPa). The pulmonary clearance rate expressed as the half-life time (T1/2) of [99Tcm]DTPA was at VP 80/0.2 = 113 +/- 31 min, at VP 80/0.4 = 70 +/- 24 min (P less than 0.01 compared to VP 80/0.2) and at VP 20/0.2 = 36 +/- 18 min (P less than 0.001 compared to VP 80/0.2 and P less than 0.01 compared to VP 80/0.4). We conclude that the pulmonary clearance of [99Tcm]DTPA increases (1) during rapid shallow ventilation when PEEP is increased from 0.2 to 0.4 kPa; (2) during deep slow ventilation relative to rapid shallow ventilation even when the mean airway pressure is similar.     

Complete respiratory support with AVCO2R and CPAP-mimic ventilation for total gas exchange in sheep

The altered respiratory mechanics in patients with chronic obstructive pulmonary disease (COPD) present unique challenges with regard to treatment during an acute exacerbation that often leads to respiratory support with mechanical ventilation. Alternative therapies are badly needed to reduce morbidity and mortality associated with mechanical ventilator use. We hypothesized that arteriovenous carbon dioxide removal (AVCO(2)R) coupled with continuous positive airway pressure (CPAP) would achieve total gas exchange eliminating the need for intubation/mechanical ventilation, thus reducing baro/volutrauma. This hypothesis was tested in six adult sedated apneic sheep with AVCO(2)R administered through a simple arteriovenous (AV) shunt for CO(2) removal. Because it is impractical to apply a CPAP mask to conscious sheep, the CPAP was mimicked in intubated/sedated sheep by positive end-expiratory pressure (PEEP) of 5-10 mmH(2)O with negligible ventilation. The AVCO(2)R and CPAP-mimic maintained Pa(o)(2) and Pa(co)(2) in the normal physiological ranges. The CO(2) removal was 120-150 ml/min through AVCO(2)R with AV blood flow of 1.1-1.5 L/min. A high fraction of inspired oxygen percentage (Fi(o)(2)) level (89 ± 3%) was required to achieve 40 ± 7% O(2) in the small bronchus. Thus, AVCO(2)R and CPAP-mimic achieved total gas exchange in anesthetized sheep and may be a potential option for acute COPD exacerbation in humans.     

Pulmonary vascular pressure profile in adult ferrets: measurements in vivo and in isolated lungs

We have determined the vascular pressure profile in lungs of adult ferrets utilizing an anaesthetized open chested preparation and have compared the pressure profile in vivo with that in isolated, perfused lungs. Ten adult ferrets, mean body weight 980 +/- 108 g, were studied. For in vivo measurements, five ferrets were anaesthetized, mechanically ventilated and the left chest wall resected. Pressures were measured in the pulmonary artery, left atrium and by micropuncture, in 20-50 microns diameter subpleural arterioles and venules. During micropuncture, ventilation was stopped for 1-2 min and the lungs kept distended at an airway pressure of 6 cmH2O. Left atrial pressure was raised to approximately 8 cmH2O with saline infusion so that lungs were in Zone 3. Cardiac output was measured by thermodilution. Lungs of five other ferrets were isolated and perfused with a steady flow roller pump. In these lungs blood flow was adjusted so that pulmonary artery pressure was similar to that in anaesthetized ferrets, with airway and left atrial pressures at 6 and 8 cmH2O respectively (Zone 3). Blood haematocrit (35 +/- 7%) was similar in the two groups. In lungs of anaesthetized ferrets total arteriovenous pressure drop was 12.1 +/- 1.9 cmH2O, with cardiac output being 210 +/- 80 ml kg body weight-1 min-1. Fractional resistance in arteries was 37%, 37% in microvessels and 26% in veins. In isolated ferret lungs, though blood flow was only 48 +/- 10 ml kg body wt-1 min-1 for the same total arteriovenous pressure drop as in vivo, the longitudinal distribution of vascular resistance was similar to that in live ferrets.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory response to inspiratory resistive load changes in patients with obstructive sleep apnea syndrome

Patients with OSA have many episodes of increased airway resistance because of repeated collapses of upper airways during night. The aim of this work was to evaluate respiratory response during chemical stimulation without and with added inspiratory resistive load (10 cmH2O/L/sec). The studies were performed during quiet breathing with air and during hypercapnic and hypoxic rebreathing tests without and with inspiratory resistive loading in 23 obese (BMI = 34.4 +/- 4.3 kg/m2) patients with OSA and in 10 healthy subjects with similar weight (BMI = 32.4 +/- 4.3 kg/m2). The measurements of respiratory responses (ventilation, mouth occlusion pressure) were performed with the use of computerized equipment. During quiet breathing in response to added load an increase of P0.1 in controls and in OSA patients was observed. During hypercapnic stimulation the ventilatory response with additional load decreased in patients as well as in controls. The slope of mouth occlusion pressure response increased significantly in controls (from 4.40 to 6.83 cmH2O/kPa, p < 0.001) and slightly weaker in OSA patients (from 4.21 to 5.43 cmH2O/kPa, p < 0.05). Although the difference between the slopes was not significant, we found that the absolute increase of P0.1 measured at point 8 kPa of PEtCO2 during loaded breathing was significantly smaller in OSA patients in comparison to controls. (2.1 vs. 10.3 cm H2O; p < 0.001). During hypoxic stimulation occlusion pressure responses were similar in both examined groups. In conclusion we postulate that OSA patients have impaired respiratory compensation of additional inspiratory load, what was demonstrated during hypercapnic rebreathing test.     

Elevations in plasma ADH levels during PEEP ventilation in the dog: mechanisms involved

In an attempt to define more precisely the various mechanisms involved in antidiuretic hormone (ADH) release during positive end-expiratory pressure ventilation (PEEP), experiments were performed on seven groups of dogs. PEEP-10 and PEEP-15 cmH2O caused significant elevations of plasma ADH from basal values of 24.9 +/- 5.2 pg/ml (mean +/- SE) to 64.6 +/- 14.2 and 106.0 +/- 20.6, respectively (P < 0.02, P < 0.005). The ADH levels returned to basal values after cessation of PEEP. This rise in ADH levels was prevented by an infusion of dextran prior to PEEP. The fall in blood pressure and cardiac output that occurred during PEEP was also prevented by the dextran infusion. Changes in ADH levels were unrelated to lung volume, left transmural pressure, and serum osmolality. Bilateral vagotomy and carotid sinus denervation was followed by an attenuated rise in ADH levels in terms of the percent rise above base line, but it did not significantly alter the absolute rise in ADH during PEEP. ADH levels were, however, reduced significantly by decreasing intracranial pressure by the removal of cerebrospinal fluid during PEEP. Propranolol administration prior to PEEP completely blocked plasma renin activity. Although the peak ADH levels were unaffected by propranolol, the rise was delayed. The results obtained indicate that a number of physiological factors may affect plasma ADH levels during PEEP. These include the carotid body and aortic arch baroreceptors as wells as sensors of intracranial pressure.     

Auto-adjusting versus fixed positive pressure therapy in mild to moderate obstructive sleep apnoea

STUDY OBJECTIVES: To determine if auto-adjusting positive airway pressure (APAP) would be better tolerated on the basis of delivering a lower mean pressure in patients with mild to moderate obstructive sleep apnoea syndrome (OSAS). DESIGN: Patients spent 8 weeks on continuous positive airway pressure (CPAP) and 8 weeks on APAP in a randomized crossover design. SETTING: Respiratory Sleep Disorders Unit in a University Hospital and the patient's home. PARTICIPANTS: Twenty-nine patients with newly diagnosed mild to moderate OSAS (apnoea-hypopnoea frequency of 5-30 events/hour) were studied. INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: Overnight polysomnography and Epworth Sleepiness Scale were recorded at baseline and at the end of each treatment period in addition to patient preference for device, side effects, and objective compliance. No differences were found in polysomnographic variables or Epworth Sleepiness Scale scores between the 2 treatment modes, but all variables were significantly improved from baseline values. Mean APAP pressure levels were significantly lower than CPAP (6.3 +/- 1.4 vs 8.1 +/- 1.7 cm H2O, p < .001). Patient compliance was similar with both treatments. More patients requiring higher fixed pressure (> or = 8cm H2O) preferred APAP, whereas those requiring lower pressure (< 8 cm H2O) preferred CPAP (p = .03). Follow-up after 18 months of therapy indicated that 76% of subjects continued to be compliant, with a nightly use of 5.8 +/- 1.9 hours per night, despite high levels of minor side effects. CONCLUSIONS: APAP and CPAP are equally effective in managing patients with mild to moderate OSAS, but device preference may be influenced by fixed pressure requirements.     

Comparison of automatic (AUTO-CPAP)and "manual" CPAP pressure titration in patients with obstructive sleep apnea

Auto-CPAP gives an opportunity to decrease costs of evaluating patient with OSA, replacing manual titration of pressure during PSG. The aim of this study was to compare automatic (auto-CPAP) and manual CPAP pressure titration in patients with OSA. We studied 50 obese patients (BMI--35 +/- 6 kg/m2), mean age 52.4 +/- 9.4 years with severe OSA, mean: AHI--62.9 +/- 22.1, mean overnight SaO2--89.1 +/- 3.7%, T90--54.4 +/- 29.6%. Two polysomnographies were performed: first when patient slept with CPAP and pressure was titrated manually by a technician and second on auto-CPAP device. Both methods had similar efficacy in reduction of AHI (< 10/h) and hypoxaemia, despite lower pressure established during auto-CPAP mode preventing apnoeas and hypopnoes during 90% of sleep time (8.2 +/- 1.7 cm H2O) compared to manual CPAP titration (9.2 +/- 1.7 cm H2O) (p < 0.05). CONCLUSION: Auto-CPAP seems to be a reliable alternative to manual titration of the therapeutic pressure in patients with OSA. This may help to cut a waiting list for PSG of patients suspected of OSA.     

The influence on the breathing pattern in man of moderate levels of continuous positive and negative airway pressure and of positive end-expiratory pressure during air and CO2 inhalation

The purpose of this study was to determine in man the effect on the breathing pattern of continuous positive (CPAP), continuous negative (CNAP) and positive end-expiratory (PEEP) airway pressure during air breathing and CO₂ inhalation. Six subjects were exposed to CPAP, CNAP and PEEP 0.5 kPa, while five subjects were exposed to CPAP and CNAP 0.8 kPa. End-expiratory lung volume increased during CPAP 0.8 kPa and decreased during CNAP 0.8 kPa. CPAP induced more extensive changes in the ventilatory pattern, and the changes in each parameter were larger than observed during CNAP and PEEP at the same pressure level. In contrast to previous reports we found the effect of CO₂ inhalation combined with the effect of pressure breathing to be not stronger than additive. Even moderate CPAP induced alveolar hyperventilation with marked reduction in arterial PCO₂ (PaCO₂) when breathing air. With increasing fraction of CO₂ in the inspiratory gas, the difference in PaCO₂ between CPAP and no CPAP disappeared. PEEP also affected the breathing pattern in that it induced an increase in mean inspiratory flow and mean expiratory flow and a reduction in inspiratory duration. Occurrence of ventilatory pauses depended on whether mouthpiece or facemask was used. CPAP and CNAP did not influence the occurrence of pauses, while PEEP prolonged post-expiratory pauses. We conclude that CPAP, CNAP and PEEP induce active ventilatory responses in man and that strong mechanisms are involved during CPAP since PaCO₂ is markedly reduced.