Faculty Bibliography

Compensation for inspiratory flow-resistive loading was compared during progressive hypercapnia and incremental exercise to determine the effect of changing the background ventilatory stimulus and to assess the influence of the interindividual variability of the unloaded CO2 response on evaluation of load compensation in normal subjects. During progressive hypercapnia, ventilatory response was incompletely defended with loading (mean unloaded delta VE/delta PCO2 = 3.02 +/- 2.29, loaded = 1.60 +/- 0.67 1.min-1.Torr-1 CO2, where VE is minute ventilation and PCO2 is CO2 partial pressure; P less than 0.01). Furthermore the degree of defense of ventilation with loading was inversely correlated with the magnitude of the unloaded CO2 response. During exercise, loading produced no depression in ventilatory response (mean delta VE/delta VCO2 unloaded = 20.5 +/- 1.9, loaded = 19.2 +/- 2.5 l.min-1.l-1.min-1 CO2 where VCO is CO2 production; P = NS), and no relationship was demonstrated between degree of defense of the exercise ventilatory response and the unloaded CO2 response. Differences in load compensation during CO2 rebreathing and exercise suggest the presence of independent ventilatory control mechanisms in these states. The type of background ventilatory stimulus should therefore be considered in load compensation assessment

The mechanisms of hypercapnia in eight patients with the 'Pickwickian' syndrome and obstructive sleep apnea (OSAS) were evaluated pretherapy and posttherapy (tracheostomy in seven patients and chronic nocturnal use of nasal CPAP in one). Four patients (correctors) became eucapnic within two weeks of therapy. Four others (noncorrectors) remained hypercapnic. Neither residual apneas, changes in pulmonary function, change in anatomic dead space, nor changes in ventilatory chemoresponsiveness differentiated the two groups, nor did the last three factors account for return to eucapnia in the correctors. The results indicated two separate mechanisms exist for chronic hypercapnia in OSAS: a critical balance between the ventilation during the time spent awake and hypoventilation due to apneas, a mechanism removed by treatment for obstructive apnea; and sustained hypoventilation independent of the apnea phenomenon and therefore not correctible. The subset of patients with the second mechanism appears to represent the true 'Pickwickian' syndrome and can be identified before therapy by measuring a low level of ventilation in the sustained awake state

Seven patients with acquired immunodeficiency syndrome (AIDS) and Pneumocystis carinii pneumonia were studied to define the pathophysiology of their respiratory failure. The patients had fever, cough, dyspnea, hypoxemia, and diffuse infiltrates on chest x-ray. Biopsies revealed a spectrum of alveolar filling, interstitial edema and infiltration, and fibrosis. The patients were studied on mechanical ventilation to assess the effect of positive end-expiratory pressure (PEEP) and supplemental oxygen on shunt fraction. Mean anatomic shunt (measured on 100% oxygen) was 34 +/- 8%, which increased significantly (p less than .001) to 43 +/- 9% when the FIO2 was decreased to 40% to 60% (physiologic shunt), indicating ventilation/perfusion (V/Q) imbalance or impaired diffusion. Increasing PEEP by 9 +/- 2 cm H2O reduced the anatomic shunt to 30 +/- 7% (p less than .01) and the physiologic shunt to 37 +/- 7% (p less than .02). There was a similar decrease in anatomic and physiologic shunts in five studies, a greater decrease in physiologic shunt in four, and a greater decrease in anatomic shunt in two. Evidence of alveolar recruitment with PEEP, measured by an increase in static thoracic compliance, was found in only one study. There was no correlation between the effect of PEEP on compliance and its effect on shunt. The data suggest that in patients with AIDS and P. carinii pneumonia, PEEP can decrease shunt by reducing the anatomic shunt, improving V/Q imbalance, and converting areas of anatomic shunt to areas of low V/Q. P. carinii pneumonia in patients with AIDS can produce a clinical and pathophysiologic pattern similar to that described in the adult respiratory distress syndrome