American journal of respiratory and critical care medicine
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Am. J. Respir. Crit. Care Med. · Mar 1996
Comparative StudyEffects of assisted ventilation on the work of breathing: volume-controlled versus pressure-controlled ventilation.
During assisted ventilation, the same tidal volume can be delivered in different ways, with the possibility for the physician to vary the ventilatory target (pressure or volume) and the peak flow setting. We compared the effects on the respiratory work rate of assisted ventilation, delivered either with a square wave flow pattern (assist control ventilation [ACV]) or with a decelerating flow pattern and a constant pressure (assisted pressure-control ventilation [APCV]). In the first part of the study where seven patients were studied, inspiratory time and tidal volume were similar in the two modes of ventilation. ⋯ In the second part of the study where six additional patients were studied, tidal volume was kept constant at a moderate level (8 ml/kg), and we studied the effect of shortening inspiratory time and increasing mean inspiratory flow. At moderate VT and high inspiratory flow, no significant differences could be found between ACV and APCV, and although pressure-time index tended to be lower during APCV, absolute levels of effort were of small magnitude (56 +/- 55 versus 76 +/- 55 cm H2O.s). We conclude that at moderate VT and low flow rates only, inspiratory assistance delivered at a constant pressure reduces the respiratory work rate more effectively than assist control ventilation.
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Am. J. Respir. Crit. Care Med. · Mar 1996
Tracheal gas insufflation-pressure control versus volume control ventilation. A lung model study.
Tracheal gas insufflation (TGI) has been recommended as an adjunct to mechanical ventilation in the presence of elevated Pa CO2. Based on our initial clinical experience with continuous flow TGI and pressure control ventilation (PCV), we were concerned about elevation in peak airway pressure as TGI was applied. In a lung model, we evaluated the effects of continuous flow TGI during both PCV and volume control ventilation (VCV). ⋯ Auto-PEEP, VT, and peak alveolar and airway opening pressures increased as TGI and Ti increased, regardless of lung mechanics settings (p<0.01). All increases were greater with VCV than PCV (p<0.05). Continuous flow TGI with both PCV and VT-uncorrected VCV may result in marked increases in Vt and system pressures, especially at long TI.
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Am. J. Respir. Crit. Care Med. · Mar 1996
Case ReportsRefractory hypoxemia during liver cirrhosis. Hepatopulmonary syndrome or "primary" pulmonary hypertension?
We report an uncommon mechanism of severe hypoxemia in two cirrhotic patients under long-term beta-blocker therapy. Our patients presented with profound hypoxemia refractory to oxygen therapy, normal lung radiography and pulmonary function tests, and evidence of right-to-left anatomic shunt. Although these features are highly suggestive of hepatopulmonary syndrome, pulmonary hypertension was present, and a right-to-left shunt through a patent foramen ovale was demonstrated by contrast-enhanced echocardiography. ⋯ Pulmonary hypertension and intracardiac right-to-left shunt eventually regressed after discontinuation of beta-blocker therapy. We conclude that "primary" pulmonary hypertension associated with portal hypertension may because of severe hypoxemia during liver cirrhosis. Differential diagnosis of hepatopulmonary syndrome relies upon contrast-enhanced echocardiography and may be of critical importance because of possible therapeutic implications.
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Am. J. Respir. Crit. Care Med. · Mar 1996
Peripheral muscle weakness contributes to exercise limitation in COPD.
Recently, it was suggested that fatigue of peripheral muscles could contribute to exercise limitation in patients with chronic obstructive pulmonary disease (COPD). In order to quantify the role of peripheral muscle force, we restudied potential determinants of exercise capacity (6-min walking distance [6 MWD] and maximal oxygen consumption [V02max]) in 41 consecutive COPD patients (FEV1, 43 +/- 19% of predicted, TLCO, 56 +/- 25% of predicted) admitted to our pulmonary rehabilitation program. VO2max (incremental cycle ergometer test), 6 MWD (best of three), lung function (FEV1, FVC, TLC, FRC), diffusing capacity (TLCO), isometric quadriceps force (QF), hand grip force (HF), and maximal inspiratory (PImax) and expiratory (PEmax) pressures were measured. ⋯ In stepwise multiple regression analysis, the variables significantly contributing to 6 MWD were QF and Plmax. For VO2max, variables significantly contributing were TLCO, QF, and FEV1. We conclude that lung function and peripheral muscle force are important determinants of exercise capacity in COPD.