Comparison of two flow generators with a noninvasive ventilator to deliver continuous positive airway pressure: a test lung study

Objective To compare the performance of two continuous flow generators with a ventilator designed for noninvasive positive pressure ventilation (NPPV) to deliver continuous positive airway pressure (CPAP). The performance of flow generators using different oxygen pressure supplies was also compared.Design and setting Experimental study using a mechanical lung model in a university research laboratory.Measurements Two flow generators supplied at 100, 200, and 300 kPa and an NPPV ventilator were compared at CPAP of 5, 10, and 15 cmH₂O in: (a) area under the adjusted CPAP level during inspiration, (b) capacity to attain the preset CPAP, and (c) tidal volume.Results The NPPV ventilator attained the preset CPAP better than flow generators, but its area under adjusted CPAP was similar to or higher than that of flow generators when these were adjusted to their better pressure supply. Both flow generators had better performance with an output flow around 100 l/min, which was achieved at 100 kPa with one flow generator and 300 with the other. Flow generators and the NPPV ventilator generated similar tidal volumes.Conclusions Flow generators performance showed large variations among different devices and oxygen pressure supplies. Adjusted to their better pressure supply, flow generators had a similar or better capacity to maintain the CPAP level, but the NPPV ventilator was more reliable to attain the preset CPAP. Flow generators could be an alternative to provide CPAP in low-income areas, usually with scarce medical equipment availability.     

Pattern of spontaneous breathing: potential marker for weaning outcome

OBJECTIVE: To quantitatively assess the spontaneous breathing (SB) pattern, during minimal ventilatory support, of patients who pass or fail weaning trials from mechanical ventilation. DESIGN: A prospective, clinical trial. SETTING: Intensive care unit of a university teaching hospital. PATIENTS: Fifty-two tracheally intubated and hemodynamically stable patients who were judged clinically ready for extubation. METHODS: Using a computerized respiratory profile monitor, continuous respiratory parameters were obtained while patients were receiving four or less synchronized intermittent mandatory (SIMV) breaths and during CPAP trials. Coefficients of variation (CV) of spontaneous tidal volumes and flows during SIMV trials as well as the entropies and dimensions of the breathing patterns during CPAP trials were used to assess the dynamical breathing behaviors of the patients who passed or failed weaning trials. MEASUREMENTS AND RESULTS: Thirty-nine extubations were successful and 13 were not. The CV of the spontaneous tidal volumes (VT) and the spontaneous peak inspiratory flows (PF), the Kolmogorov entropy and the dimension of the SB patterns were compared in the two groups. The CV of VT (9.13 +/- 4.11 vs 26.07 +/- 6.94), the CV of PF (11.63 +/- 4.18 vs 29.88 +/- 12.07), the Kolmogorov entropy (0.09 +/- 0.03 bits/cycle vs 0.39 +/- 0.09 bits/cycle), and the dimension of the SB pattern (1.33 +/- 0.07 vs 3.93 +/- 0.47) were all significantly smaller (P < 0.05) in the successfully extubated group versus the group that failed extubation. CONCLUSION: The spontaneous breathing pattern during minimal mechanical ventilatory support is more chaotic in patients who failed extubation trials compared to patients who passed extubation trials. Thus, we speculate that characterizing the SB pattern during minimal ventilatory support might be a useful tool in differentiating between extubation success and failure.     

Advantages and disadvantages of different nasal CPAP systems in newborns

Objective To compare three different systems of continuous positive airway pressure (CPAP): the naso-pharyngeal tube and two-prong systems in newborns, focusing on duration of CPAP, side effects and cost.Design Randomized clinical study.Patients Between July 2000 and September 2001 newborns were randomized to three different CPAP systems. Forty infants in two weight groups (>2500 g and 1250–2500 g; 20 patients in each group) were included.Results In the group >2500 g the median duration of CPAP was 1.1 days (range 0.25–14.3 days). The median time on a naso-pharyngeal CPAP was 1 day (range 0.25–14.3 days), on Hudson prongs 1.6 days (range 0.5–3.3 days) and on the Infant Flow system 0.7 days (range 0.3–13.6 days; p>0.05 for comparison between groups, Fishers exact test). With naso-pharyngeal CPAP, 2 patients developed moderate nasal injuries. On Hudson, 2 patients developed moderate and three mild nasal injuries. One patient on the Infant Flow showed mild and one moderate nasal injuries. In the weight group 1250–2500 g the median duration of CPAP was 1.1 days (range 0.1–7.0 days). The median time on the naso-pharyngeal tube was 0.9 days (range 0.1–7 days), on Hudson prongs 1.1 days (range 0.7–6.6 days) and on the Infant Flow system 1.3 days (range 0.25–5.9 days; p>0.05 for comparison between groups, Fishers exact test). With a naso-pharygeal tube, one infant developed mild and one moderate nasal injuries. On Hudson prongs, two had moderate nasal injuries. On Infant Flow, one newborn showed a severe nasal injury and two mild injuries. None of the patients developed a pneumothorax.Conclusion The naso-pharyngeal tube is an easy, safe and economical CPAP system usable with every common ventilator. For very low birth weight newborns, a prong system may have advantages.     

Continuous positive airway pressure facilitates spontaneous breathing in weaning chronic obstructive pulmonary disease patients by improving breathing pattern and gas exchange

OBJECTIVE: To elucidate the effects of continuous positive airway pressure (CPAP) on breathing pattern, gas exchange and the ability to sustain spontaneous breathing (SB) in chronic obstructive pulmonary disease (COPD) patients with dynamic hyperinflation. DESIGN: Prospective study with two randomised trials of SB without and with CPAP in each patient. SETTING: Medical intensive care units (ICUs) in two university hospitals. PATIENTS: Nine dynamically hyperinflated, intubated COPD patients recuperating from acute exacerbation. INTERVENTIONS: One SB trial with CPAP (5-7.5 cmH2O), one without (control) in each patient. MEASUREMENTS: airway opening pressure, gas flow and thus breathing pattern, oxygen uptake, carbon dioxide excretion, arterial blood gases, dyspnoea and respiratory drive (P100). RESULTS: With CPAP, intrinsic positive end-expiratory pressure (PEEPi) fell from 11.4 to 6.3 cm H2O (p < 0.05). Eight patients sustained SB with CPAP for the maximum time planned (30 min), one failed after 18 min. In contrast, only four patients successfully completed the control trial, the others failing after 5-18 min (p < 0.05). Dyspnoea-gauged on a visual analogue scale by five patients--was less severe or occurred later with CPAP. Breathing with CPAP tended to be slower (18.9 vs 22.2 min(-1), p < 0.05) and deeper (tidal volume 370 vs 323 ml). At the end of the control run, PaCO2 was higher (60 vs 55 mmHg, p < 0.05) and still rising while being stable at the end of the CPAP trial. CONCLUSION: CPAP helps severely ill COPD patients sustain SB. Apparently it does so by promoting slower, deeper breathing and thus facilitating carbon dioxide elimination.     

Airway pressure release ventilation in a neonatal lamb model of acute lung injury

OBJECTIVE: To determine if airway pressure release ventilation (APRV) is feasible in a neonatal animal model with acute lung injury. DESIGN: Nonrandomized, repeated, bracketed measures. SETTING: University research laboratory. SUBJECTS: Seven neonatal sheep (5.6 +/- 0.6 kg), less than 10 days of age. INTERVENTIONS: Acute lung injury was induced by oleic acid infusion and cardiorespiratory profiles were compared during spontaneous ventilation at ambient airway pressure, continuous positive airway pressure (CPAP), APRV, and conventional positive-pressure ventilation (PPV). MEASUREMENTS AND RESULTS: Oleic acid resulted in acute lung injury with stable cardiorespiratory status during the 3-hr study period. Mean airway pressure (Paw) was comparable for all three positive-pressure modes (CPAP 13.4 +/- 1.5, APRV 13.5 +/- 1.4, PPV 13.9 +/- 1.4 cm H2O, NS). After acute lung injury, CPAP increased arterial oxygenation compared with spontaneous ventilation (77.3 +/- 6.9 vs. 57.7 +/- 4.2 torr [10.3 +/- 0.9 vs. 7.7 +/- 0.6 kPa], p less than .05), and this increase was maintained during APRV (73.3 +/- 5.6 vs. 77.3 +/- 6.9 torr [9.8 +/- 0.7 vs. 10.3 +/- 0.9 kPa], NS). Alveolar ventilation was increased by APRV compared with CPAP (PaCO2 29 +/- 1 vs. 41 +/- 2 torr [3.9 +/- 0.1 vs. 5.4 +/- 0.3 kPa], p less than .05) without impairment of cardiovascular performance (cardiac output 1.18 +/- 0.16 vs. 1.20 +/- 0.17 L/min, NS). To achieve ventilation equivalent to APRV during PPV, peak Paw was greater (36.4 +/- 3.2 vs. 19.7 +/- 1.7 cm H2O, p less than .05) and cardiac output (0.94 +/- 0.11 vs. 1.18 +/- 0.16 L/min, p less than .05) and mean arterial pressure (91 +/- 7 vs. 96 +/- 6 mm Hg, p less than .05) were decreased during PPV compared with APRV. CONCLUSIONS: In this neonatal laboratory model of acute lung injury, APRV maintained oxygenation and augmented alveolar ventilation compared with CPAP. Compared with PPV, APRV provided similar ventilation and oxygenation, but at lower peak Paw than PPV, without compromising cardiovascular performance.     

持续气道正压通气、T管及压力支持通气等条件下患者呼吸功的比较性研究

目的：探讨从呼吸功角度评价呼吸机脱机方式优劣的可能性及其意义。方法：通过ＢｉｃｏｒｅＣＰ１００呼吸监测仪测定２２例患者在压力支持通气（ＰＳＶ）、持续气道内正压（ＣＰＡＰ）通气、Ｔ管及拔管后２小时等条件下的呼吸功的变化。结果：ＣＰＡＰ０．４９ｋＰａ（１ｋＰａ＝１０．２０ｃｍＨ２Ｏ）、Ｔ管、ＰＳＶ０．４９ｋＰａ时，患者呼吸功依次逐渐降低。ＣＰＡＰ０．４９ｋＰａ时呼吸功（９．９８Ｊ／ｍｉｎ）比ＰＳＶ０．４９ｋＰａ时高２３．７％（Ｐ＜０．００１），比拔管后２小时高４８．５％（Ｐ＜０．０１），与Ｔ管时比较无显著性差异。Ｔ管时呼吸功（９．３１Ｊ／ｍｉｎ）比ＰＳＶ０．４９ｋＰａ时高１５．４％（Ｐ＜０．０５），比拔管后２小时高３８．５％（Ｐ＜０．０１）。结论：患者呼吸功因脱机方式不同而显著不同，ＰＳＶ０．４９ｋＰａ比ＣＰＡＰ０．４９ｋＰａ和Ｔ管更有利于脱机。     

Continuous positive airway pressure breathing in supine and upright postures

Continuous positive airway pressure (CPAP) breathing of 10 cmH2O was applied to 10 healthy conscious subjects both in supine and 70 degree head-up postures. CPAP increased the end-expiratory lung volume from 2.44 +/- 0.66 (S.D.) to 3.07 +/- 0.68 liters in supine subjects, and from 3.01 +/- 0.57 to 3.64 +/- 0.57 liters in head-up subjects. During CPAP, rib cage motion became predominant when compared to abdominal motion in upright posture. Among the respiratory parameters, inspiratory duration was significantly shortened during CPAP in either posture, but was changed less significantly by individual posture change alone. Mean inspiratory flow decreased during CPAP only in upright posture. Abdominal muscles were recruited for expiration during CPAP in either posture, and the EMG recorded from the lower intercostal space developed inspiratory activity during CPAP in head-up posture. All of 10 subjects felt most dyspneic during CPAP with the head-up. We conclude that the mechanism of increase in lung volume during CPAP is mediated by different pathway from that of posture change, and that CPAP in upright posture can alter the respiratory pattern of the subject.     

Measurement of changes in respiratory mechanics during partial liquid ventilation using jet pulses

OBJECTIVE: To compare the changes in respiratory mechanics within the breathing cycle in healthy lungs between gas ventilation and partial liquid ventilation using a special forced-oscillation technique. DESIGN: Prospective animal trial. SETTINGS: Animal laboratory in a university setting. SUBJECTS: A total of 12 newborn piglets (age, <12 hrs; mean weight, 725 g). INTERVENTIONS: After intubation and instrumentation, lung mechanics of the anesthetized piglets were measured by forced-oscillation technique at the end of inspiration and the end of expiration. The measurements were performed during gas ventilation and 80 mins after instillation of 30 mL/kg perfluorocarbon PF 5080. MEASUREMENTS AND MAIN RESULTS: Brief flow pulses (width, 10 msec; peak flow, 16 L/min) were generated by a jet generator to measure the end-inspiratory and the end-expiratory respiratory input impedance in the frequency range of 4-32 Hz. The mechanical variables resistance, inertance, and compliance were determined by model fitting, using the method of least squares. At least in the lower frequency range, respiratory mechanics could be described adequately by an RIC single-compartment model in all piglets. During gas ventilation, the respiratory variables resistance and inertance did not differ significantly between end-inspiratory and end-expiratory measurements (mean [sd]: 4.2 [0.7] vs. 4.1 [0.6] kPa x L(-1) x sec, 30.0 [3.2] vs. 30.7 [3.1] Pa x L(-1) x sec2, respectively), whereas compliance decreased during inspiration from 14.8 (2.0) to 10.2 (2.4) mL x kPa(-1) x kg(-1) due to a slight lung overdistension. During partial liquid ventilation, the end-inspiratory respiratory mechanics was not different from the end-inspiratory respiratory mechanics measured during gas ventilation. However, in contrast to gas ventilation during partial liquid ventilation, compliance rose from 8.2 (1.0) to 13.0 (3.0) mL x kPa(-1) x kg(-1) during inspiration. During expiration, when perfluorocarbon came into the upper airways, both resistance and inertance increased considerably (mean with 95% confidence interval) by 34.3% (23.1%-45.8%) and 104.1% (96.0%-112.1%), respectively. CONCLUSIONS: The changes in the respiratory mechanics within the breathing cycle are considerably higher during partial liquid ventilation compared with gas ventilation. This dependence of lung mechanics from the pulmonary gas volume hampers the comparability of dynamic measurements during partial liquid ventilation, and the magnitude of these changes cannot be detected by conventional respiratory-mechanical analysis using time-averaged variables.     

呼气保持法测定机械通气患者呼气末肺动脉楔压的方法研究

目的 联合应用呼吸机上的"呼气保持"功能与监护仪上的"肺动脉楔压(PAWP)回顾"软件,寻找一种既准确又方便快捷的测量呼气末PAWP(eePAWP)的方法.方法 采用前瞻性自身前后对照研究方法.选择放置肺动脉导管的机械通气患者12例,随机选取50例次的测量值,21例次为单纯正压通气,29例次为正压通气混有自主呼吸.先根据呼吸变异度(RV)分为＜8 mm Hg(1 mm Hg=0.133 kPa)或≥8 mm Hg两组,再根据PAWP测量方法分为监护仪自动显示组和呼气保持法测量组.比较每例次自动测量法和呼气保持法所测PAWP值的差异.结果 21例次单纯正压通气患者中,12例次RV＜8 mm Hg者中自动显示组PAWP(mm Hg)数值多高于呼气保持组(12～16比9～14),但两组数值差距较小,甚至偶有相等的情况;9例次RV≥8 mm Hg者中自动显示组PAWP(mm Hg)数值均高于呼气保持组(13～20比9～15),两组数据差距较大;但不论RV多少,两组间PAWP数值比较差异均有统计学意义(均P＜0.01).29例次正压通气混有自主呼吸的患者中,RV＜8 mm Hg者(13例次)和RV≥8 mm Hg者(16例次)中自动显示组大部分PAWP数值高于呼气保持组(11～18比10～17),4例次低于呼气保持组(11～20比14～23),但两组间各PAWP数值差异均无统计学意义(均P＞0.05).结论 无论单纯正压通气还是正压通气混有自主呼吸的患者,呼气保持法均能较好地识别eePAWP波形,更准确、快速地反映患者真实的血流动力学状态.

Abstract：

Objective To find an accurate and convenient method of measuring end-expiratory pulmonary artery wedge pressure (eePAWP) by "expiration holding" function of ventilator and "pulmonary artery wedge pressure (PAWP) Review" software of monitor. Methods Twelve patients with introduction of pulmonary artery catheter and undergoing mechanical ventilation were selected. Fifty measurements were randomly selected for the comparison of the differences between automatic measurement and expiration holding method in each patient. There were 21 cases underwent single positive pressure ventilation and 29 cases with positive pressure ventilation mixed with spontaneous breathing. All measurements were first divided into ＜8 mm Hg (1 mm Hg=0. 133 kPa) or ≥8 mm Hg groups according to respiratory variability (RV). They were then divided into automatic measurement group and expiration holding group according to PAWP measurement, and the difference in the results between two groups were recorded. Results In 21 cases with single positive pressure ventilation, in 12 cases PAWP (mm Hg) of automatic measurement group was higher than that of expiration holding group (12 - 16 vs. 9 - 14) when RV＜8 mm Hg, but the difference between two groups was not obvious, and measurements were similar occasionally. In automatic measurement group PAWP (mm Hg) was higher than that of expiration holding group (13 - 20 vs. 9 - 15) in 9 cases when RV≥8 mm Hg, the difference was obvious. Neither RV＜8 mm Hg nor RV≥8 mm Hg, the statistical difference was significant (all P＜0. 01). In 29 cases, when positive pressure ventilation was mixed with spontaneous breathing, RV＜ 8 mm Hg (n = 13), RV≥ 8 mm Hg (n=16), most of the results in automatic measurement group were higher than those of expiration holding group (11 - 18 vs. 10 - 17), and only 4 of them were lower than expiration holding group (11 - 20 vs. 14 - 23). There was no statistically significant difference between two groups (all P＞0. 05). Conclusion Expiration holding measurement is a better method that can identify the eePAWP, and it reflects the true hemodynamic status more accurately and quickly whether positive pressure ventilation only or positive pressure ventilation mixed with spontaneous breathing is given.     

Effects of nasal continuous positive airway pressure (NCPAP) on breathing pattern in spontaneously breathing premature newborn infants

OBJECTIVE: The aim of the study was to assess the influence of nasal continuous positive airway pressure (NCPAP) on breathing pattern in preterm newborns. DESIGN: Prospective study. SETTING: Neonatal intensive care unit. PATIENTS: Ten premature newborn infants on NCPAP (gestational age range from 27 to 32 weeks, mean birth weight 1300+/-460 g) admitted in our neonatal intensive care unit (NICU) for respiratory distress syndrome. METHODS: Breathing patterns and changes in lung volumes level were obtained using respiratory inductive plethysmography (RIP), at random CPAP levels (0, 2, 4, 6 and 8 cmH2O). Raw data were analysed for end-expiratory lung volume level (EELV-level), tidal volume (Vt), respiratory rate, phase angle and labour breathing index (LBI). RESULTS: CPAP increased EELV-level by 2.1+/-0.3xVt from 0 to 8 cmH2O ( p<0.01). Vt increased by 43% from CPAP of 0 cmH2O to CPAP of 8 cmH2O ( p<0.01). We also found that CPAP lowered the phase angle (from 76+/-21 degrees at CPAP of 0 cmH2O to 30+/-15 degrees at CPAP of 8 cmH2O; p<0.01 ) and LBI (from 1.7+/-0.8 at CPAP of 0 cmH2O to 1.2+/-0.3 at CPAP of 8 cmH2O; p<0.05). CONCLUSION: NCPAP improves the breathing strategy of premature infants with respiratory failure, as reflected by improved thoraco-abdominal synchrony, increased Vt and reduction of the LBI. This effect is associated with an increase in EELV-level with CPAP level. However, further investigations are necessary to establish the best CPAP level that ensures both safety and efficiency.     

Effects of the inspiratory pressure waveform during patient-triggered ventilation on pulmonary stretch receptor and phrenic nerve activity in cats

OBJECTIVE: To examine the effects of square wave, sinusoidal, and linear inspiratory pressure waveforms during pressure-controlled assist/control ventilation on the firing pattern of pulmonary stretch receptors and phrenic nerve activity. DESIGN: Experimental, comparative study. SETTING: Research laboratory at a university biomedical center. SUBJECTS: Nine anesthetized, endotracheally intubated young cats (2.5-3.4 kg). INTERVENTION: With interposed periods of continuous positive airway pressure (0.2 kPa), each cat was exposed to periods of assist/control ventilation with three different pressure waveforms, where the peak inspiratory pressure (0.74 +/- 0.13 kPa), end-expiratory pressure (0.2 +/- 0.02 kPa), and tidal volume (14.9 +/- 5.22 mL/kg) were kept constant. Preset controlled ventilator rate was set below the rate of spontaneous breathing, and the mechanical inflation time equaled the inspiratory time during spontaneous breathing on continuous positive airway pressure. MEASUREMENTS AND MAIN RESULTS: Respiratory rate and arterial blood gases did not change between the three pressure waveforms during assist/control ventilation. Peak pulmonary stretch receptor activity was lower and mean phrenic nerve activity higher during continuous positive airway pressure than during assist/control ventilation (p <.05). Peak inspiratory pulmonary stretch receptor activity was the same with all three pressure waveforms (82 +/- 17 impulses.sec-1) but occurred earlier with square wave than with sinusoidal or linear pressure waveforms (p <.05). The total number of impulses in the phrenic nerve activity burst was smaller with square wave than with the other two pressure waveforms (0.21 +/- 0.17 vs. 0.33 +/- 0.27 and 0.42 +/- 0.30 arbitrary units; p <.05), and the phrenic nerve activity burst duration was shorter with square wave (1.10 +/- 0.45 vs. 1.54 +/- 0.36 and 1.64 +/- 0.25 secs; p <.05). CONCLUSION: Square wave pressure waveform during pressure-controlled assist/control ventilation strongly inhibits spontaneous inspiratory activity in cats. One mechanism for this inhibition is earlier and sustained peak pulmonary stretch receptor activity during inspiration. These findings show that differences in inspiratory pressure waveforms influence the spontaneous breathing effort during assist/control ventilation in cats.     

Continuous positive airway pressure delivered with a "helmet": effects on carbon dioxide rebreathing

OBJECTIVE: The "helmet" has been used as a novel interface to deliver noninvasive ventilation without applying direct pressure on the face. However, due to its large volume, the helmet may predispose to CO2 rebreathing. We hypothesized that breathing with the helmet is similar to breathing in a semiclosed environment, and therefore the PCO2 inside the helmet is primarily a function of the subject's CO2 production and the flow of fresh gas through the helmet. DESIGN: Human volunteer study. SETTING: Laboratory in a university teaching hospital. SUBJECTS: Eight healthy volunteers. INTERVENTIONS: We delivered continuous positive airway pressure (CPAP) with the helmet under a variety of ventilatory conditions in a lung model and in volunteers. MEASUREMENTS AND MAIN RESULTS: Gas flow and CO2 concentration at the airway were measured continuously. End-tidal PCO2, CO2 production, and ventilatory variables were subsequently computed. We found that a) when CPAP was delivered with a ventilator, the inspired CO2 of the volunteers was high (12.4 +/- 3.2 torr [1.7 +/- 0.4 kPa]); b) when CPAP was delivered with a continuous high flow system, inspired CO2 of the volunteers was low (2.5 +/- 1.2 torr [0.3 +/- 0.2 kPa]); and c) the inspired CO2 calculated mathematically for a semiclosed system model of CO2 rebreathing was highly correlated with the values measured in a lung model (r = .97, slope = 0.92, intercept = -1.17, p < .001) and in the volunteers (r = .94, slope = 0.96, intercept = 0.90, p < .001). CONCLUSIONS: a) The helmet predisposes to CO2 rebreathing and should not be used to deliver CPAP with a ventilator; b) continuous high flow minimizes CO2 rebreathing during CPAP with the helmet; and c) minute ventilation and Pco2 should be monitored during CPAP with the helmet.     

Laboratory and clinical evaluation of the MAX transport ventilator

Transport of critically ill, mechanically ventilated patients from intensive care units for diagnostic and therapeutic procedures has become common in the last decade. Maintenance of adequate oxygenation and ventilation during transport is mandatory. We evaluated the Hamilton MAX transport ventilator in the laboratory and in the clinical arena to determine its usefulness during in-hospital transport. METHODS: In the laboratory, we determined the MAX's ability to assure tidal volume (VT) delivery in the face of decreasing compliance of a test lung, and we tested the alarm system. Using a two-compartment lung model modified to simulate spontaneous breathing, we also evaluated the responsiveness of the demand valve. The clinical evaluation was accomplished by comparing arterial blood gases and ventilator settings in the intensive care unit to those during transport. RESULTS: As lung compliance was reduced from 0.1 to 0.02 L/cm H2O [1.0 to 0.20 L/kPa], delivered VT fell significantly at each set VT. The alarm systems performed according to manufacturer's specifications. The demand valve triggered appropriately without positive end-expiratory pressure (PEEP), but as PEEP was increased, triggering became more difficult. The demand valve is referenced to ambient pressure and cannot compensate for elevated end-expiratory pressures. During patient transport, arterial blood gases were comparable to those achieved in the ICU. Because an inspired oxygen concentration of 1.0 was used during transport, arterial oxygenation (PaO2) was significantly greater (123 +/- 75 vs 402 +/- 85 torr [16.4 +/- 10 vs 53.6 +/- 11 kPa]). A higher ventilator rate was required during transport to prevent tachypnea (7 +/- 3 vs 12 +/- 6 breaths/min), and peak inspiratory pressure (PIP) was higher during transport (40 +/- 8 vs 52 +/- 11 cm H2O [3.9 +/- 0.8 vs 5.1 +/- 1.1 kPa]). CONCLUSIONS: The MAX is a reliable transport ventilator, capable of maintaining adequate ventilation and oxygenation in a majority of mechanically ventilated patients. Care should be taken to assure adequate VT delivery at high PIP, and ventilator rate may require adjustment to prevent tachypnea associated with triggering the non-PEEP-compensated demand valve when PEEP greater than 8 cm H2O [0.8 kPa] is used.     

Laboratory and clinical evaluation of the impact Uni-Vent 750 portable ventilator

BACKGROUND: Transportation of critically ill, mechanically ventilated patients from intensive care units for diagnostic and therapeutic procedures has become common in the last decade. Maintenance of adequate oxygenation and ventilation during transport is essential. We evaluated the Impact Uni-Vent 750 portable ventilator in the laboratory and in the clinical arena to determine its usefulness during inhospital transport. MATERIALS & METHODS: In the laboratory, we determined the Uni-Vent 750's ability to assure tidal volume (VT) delivery in the face of decreasing compliance of a test lung and tested the alarm systems. Using a two-compartment lung model modified to simulate spontaneous breathing, we also evaluated the responsiveness of the demand valve. The clinical evaluation was accomplished by comparing arterial blood gas values and ventilator settings in the intensive care unit before transport to those during transport. RESULTS: As lung compliance was reduced from 0.1 to 0.02 mL/cm H2O [1.0 to 0.20 L/kPa], a slight, statistically insignificant decrease in delivered tidal volume was observed. All alarm systems operated according to manufacturer's specifications. The demand valve triggered appropriately with PEEP from 0 to 20 cm H2O [0 to 1.96 kPa]. Sensitivity settings less than -6 cm H2O [-0.59 kPa] sometimes resulted in inability to trigger the demand valve. During patient transport, arterial blood gas values and ventilator settings were comparable to those observed in the ICU. Because an FIO2 of 1.0 was used during transport, mean (SD) PaO2 was significantly greater 89 (26) vs 341 (78) [11.8 (3.5) vs 45.3 (10.4) kPa]. CONCLUSIONS: The Uni-Vent 750 is a reliable transport ventilator, capable of maintaining adequate oxygenation and ventilation in a majority of mechanically ventilated patients. The Uni-Vent 750's ability to (1) provide CMV, AMV, and SIMV; (2) provide low and high pressure alarms; and (3) provide PEEP compensation is unique among portable ventilators.     

A comparison of the CPAP performance characteristics of the Puritan-Bennett 7200a and a prototype continuous pressure-regulating ventilator

BACKGROUND: A prototype demand-flow medical ventilator for intensive care unit (ICU) applications has been developed with the ability to maintain continuous pressure regulation of proximal airway pressure during both inspiratory and expiratory respiratory phases. The performance of this system was investigated in laboratory tests of continuous positive airway pressure (CPAP) mode, a ventilatory mode in which airway pressure regulation is strongly challenged. MATERIALS & METHODS: Comparative tests of the pressure-regulating ventilator (PRV) prototype and a Puritan-Bennett 7200a (PB7200a) ventilator were made in three performance categories: pressure-volume product error, peak pressure error during inspiration, and peak pressure error during expiration. Testing was performed on a Bio-Tek VT-2 test lung modified to simulate spontaneous breathing, at CPAP levels of 0, 10, and 20 cm H2O. Results were obtained at a lung compliance of 50 mL/cm H2O and at three parabolic airway resistance levels (Rp10, Rp20, and Rp50 cm H2O.L-2.s2 per ANSI Z79.7-1976). RESULTS: The PRV prototype achieved reductions in pressure-volume product error of more than 0.111 joules (43%) for all test conditions, relative to the PB7200a. Peak pressure errors were reduced by at least 4.0 cm H2O (55%) during inspiration and 4.9 cm H2O (38%) during expiration. CONCLUSIONS: An ICU ventilator utilizing continuous regulation of proximal airway pressure has been shown to provide statistically significant improvements in CPAP performance relative to the PB7200a in laboratory tests. Further testing will be required to quantify the clinical significance of these results.     

Prospective, randomized, controlled clinical trial comparing traditional versus reduced tidal volume ventilation in acute respiratory distress syndrome patients.

OBJECTIVE: To assess the safety and potential efficacy of a mechanical ventilation strategy designed to reduce stretch-induced lung injury in acute respiratory distress syndrome. DESIGN: Prospective, randomized, controlled clinical trial. SETTING: Eight intensive care units in four teaching hospitals. PATIENTS: Fifty-two patients with acute respiratory distress syndrome. INTERVENTIONS: Traditional tidal volume patients: tidal volume 10-12 mL/kg ideal body weight, reduced if inspiratory plateau pressure was > 55 cm H2O (7.3 kPa). Small tidal volume patients: tidal volume 5-8 mL/kg ideal body weight, to keep plateau pressure < 30 cm H2O (4.0 kPa). MEASUREMENTS AND MAIN RESULTS: Mean tidal volumes during the first 5 days in traditional and small tidal volume patients were 10.2 and 7.3 mL/kg, respectively (p < .001), with mean plateau pressure = 30.6 and 24.9 cm H2O (3.3 kPa), respectively (p < .001). There were no significant differences in requirements for positive end-expiratory pressure or FIO2, fluid intakes/outputs, requirements for vasopressors, sedatives, or neuromuscular blocking agents, percentage of patients that achieved unassisted breathing, ventilator days, or mortality. CONCLUSIONS: The reduced tidal volume strategy used in this study was safe. Failure to observe beneficial effects of small tidal volume ventilation treatment in important clinical outcome variables may have occurred because a) the sample size was too small to discern small treatment effects; b) the differences in tidal volumes and plateau pressures were modest; or c) reduced tidal volume ventilation is not beneficial.     

Early prediction of successful weaning during pressure support ventilation in chronic obstructive pulmonary disease patients.

OBJECTIVE: The aim of this study was to examine variables for early prediction of successful weaning in chronic obstructive pulmonary disease (COPD) patients during pressure support ventilation weaning. DESIGN: Thirteen COPD patients were prospectively studied to compare the respiratory pattern (inspiratory time, expiratory time, total breath cycle duration, tidal volume, respiratory rate, minute ventilation), the respiratory drive (airway occlusion pressure at 0.1 sec, tidal volume/inspiratory time), and blood gases after 30 mins of pressure support weaning. SETTING: The study was performed in the 20-bed General Critical Care Unit of the Rome "La Sapienza" University Hospital. PATIENTS: We evaluated 13 consecutive COPD patients fulfilling the standard weaning criteria (including clinical status, blood gases, forced vital capacity, maximum inspiratory pressure, and spontaneous respiratory rate after a 30-min T-piece trial) in which we compared respiratory pattern, respiratory drive, and blood gases after 30 mins of pressure support weaning. MEASUREMENTS AND MAIN RESULTS: After 30 mins of pressure support ventilation weaning (pressure support level 20 cm H2O), we measured respiratory pattern (airway pressure and airflow tracing), airway occlusion pressure at 0.1 sec (occluding the inspiratory line during expiration with a rubber balloon), tidal volume/inspiratory time, maximal inspiratory pressure, and blood gases. According to the result of the weaning trial, the patients were divided into two groups (not weaned and weaned), and the statistical difference between the evaluated variables was analyzed in weaned and not weaned groups. We did not observe a significant difference in breathing pattern data and arterial blood gases between weaned and not weaned patients. By contrast, airway occlusion pressure at 0.1 sec and maximum inspiratory pressure measured after 30 mins of weaning trial appeared significantly (p less than .001) different in patients in whom the weaning trial succeeded or failed. Considering maximum inspiratory pressure, we could not separate weaned from not weaned patients, while all patients showing values of airway occlusion pressure at 0.1 sec less than 4.5 cm H2O were easily weaned. CONCLUSIONS: This study confirms that conventional weaning criteria are often inadequate in predicting successful weaning of COPD patients, while airway occlusion pressure at 0.1 sec during the first phase of pressure support ventilation weaning can represent a good weaning predictor.     

The effect of continuous positive airway pressure versus positive end-expiratory pressure on the diaphragm

Continuous positive airway pressure (CPAP) and positive end-expiratory pressure (PEEP) both increase lung volume and hence may compromise diaphragm function. However, the effects of these two positive airway pressure modalities on inspiratory work of breathing are conflicting. In this study, we compared the effect of CPAP versus PEEP on diaphragm function in spontaneously breathing anesthetized dogs. Eight sodium pentobarbital-anesthetized dogs were randomly exposed to various levels of CPAP and PEEP. Measurements of diaphragmatic shortening, transdiaphragmatic pressure swings, and diaphragmatic electromyogram (EMG) were made. The change in lung volume and diaphragm length was similar at equivalent airway pressures during PEEP or CPAP. Therefore, expiratory muscle recruitment in the two conditions was equivalent. However, tidal diaphragmatic EMG and transdiaphragmatic pressure swings increased markedly during PEEP compared with CPAP. At a PEEP of 18 cm H₂O, crural and costal EMG activities were 185% ± 16% and 163% ± 8% of control, respectively, whereas during CPAP the EMG activity was 66% ± 11 % of control for both the costal and the crural diaphragms (±SE). During PEEP, the duration of neural inspiration (T_IEMG) was greater than the duration of inspiration as measured by airflow (T_IV). On the other hand, during CPAP, T_IEMG was less than T_IV. We conclude that although expiratory muscle recruitment is comparable and tidal volume greater during CPAP, the inspiratory activation of the diaphragm decreases with CPAP but increases markedly with PEEP.     

Effects of the components of positive airway pressure on work of breathing during bronchospasm

Introduction

Partial assist ventilation reduces work of breathing in patients with bronchospasm; however, it is not clear which components of the ventilatory cycle contribute to this process. Theoretically, expiratory positive airway pressure (EPAP), by reducing expiratory breaking, may be as important as inspiratory positive airway pressure (IPAP) in reducing work of breathing during acute bronchospasm.

Method

We compared the effects of 10 cmH₂O of IPAP, EPAP, and continuous positive airwaypressure (CPAP) on inspiratory work of breathing and end-expiratory lung volume (EELV) in a canine model of methacholine-induced bronchospasm.

Results

Methacholine infusion increased airway resistance and work of breathing. During bronchospasm IPAP and CPAP reduced work of breathing primarily through reductions in transdiaphragmatic pressure per tidal volume (from 69.4 ± 10.8 cmH₂O/l to 45.6 ± 5.9 cmH₂O/l and to 36.9 ± 4.6 cmH₂O/l, respectively; P < 0.05) and in diaphragmatic pressure–time product (from 306 ± 31 to 268 ± 25 and to 224 ± 23, respectively; P < 0.05). Pleural pressure indices of work of breathing were not reduced by IPAP and CPAP. EPAP significantly increased all pleural and transdiaphragmatic work of breathing indices. CPAP and EPAP similarly increased EELV above control by 93 ± 16 ml and 69 ± 12 ml, respectively. The increase in EELV by IPAP of 48 ± 8 ml (P < 0.01) was significantly less than that by CPAP and EPAP.

Conclusion

The reduction in work of breathing during bronchospasm is primarily induced by the IPAP component, and that for the same reduction in work of breathing by CPAP, EELV increases more.     

Effects of breathing exercises on breathing patterns in obese and non-obese subjects

Chest physiotherapy in connection with abdominal surgery includes different deep-breathing exercises to prevent post-operative pulmonary complications. The therapy is effective in preventing pulmonary complications, especially in high-risk patients such as obese persons. The mechanisms behind the effect is unclear, but part of the effect may be explained by the changes in breathing patterns. The aim of this study was therefore to describe and to analyse the breathing patterns in obese and non-obese subjects during three different breathing techniques frequently used in the treatment of post-operative patients. Twenty-one severely obese [body mass index (BMI) > 40] and 21 non-obese (BMI 19–25) subjects were studied. All persons denied having any lung disease and were non-smokers. The breathing techniques investigated were: deep breaths without any resistance (DB), positive expiratory pressure (PEP) with an airway resistance of approximately +15 cmH₂O (1·5 kPa) during expiration, inspiratory resistance positive expiratory pressure (IR-PEP) with a pressure of approximately –10 cmH₂O (–1·0 kPa) during inspiration. Expiratory resistance as for PEP. Volume against time was monitored while the subjects were sitting in a body plethysmograph. Variables for volume and flow during the breathing cycle were determined. Tidal volume and alveolar ventilation were highest during DB, and peak inspiratory volume was significantly higher than during PEP and IR-PEP in the group of obese subjects. The breathing cycles were prolonged in all techniques but were most prolonged in PEP and IR-PEP. The functional residual capacity (FRC) was significantly lower during DB than during PEP and IR-PEP in the group of obese subjects. FRC as determined within 2 min of finishing each breathing technique was identical to before the breathing manoeuvres.