Monday, February 15, 2010

The Pickwickian syndrome

Today we look into an interesting syndrome in which obesity decreases ventilation!

The pickwickian syndrome, or obesity hypoventilation syndrome, was first described by Burwell and associates in 1956 and named for a character described in Charles Dickens' The Posthumous Papers of the Pickwick Club. Marked obesity, somnolence, cyanosis, periodic breathing, secondary erythrocytosis, and right ventricular heart failure were the initially described clinical characteristics of this syndrome.
Morbid obesity decreases total lung capacity (TLC), functional residual capacity (FRC), and tidal volume because of increased adipose tissue in the chest wall and elevation of the diaphragm caused by increased intra-abdominal adipose tissue. The reduced lung volumes lead to atelectasis, which alters ventilation-perfusion (V/Q) matching and causes hypoxia. There is also evidence that respiratory muscles become less effective in morbid obesity. Most patients also have OSA, which further increases ventilatory work in a system already predisposed to hypoventilation. Finally, hypoxic and hypercapnic responses are diminished in most patients, either secondary to chronic hypoxia and hypercapnia or, perhaps, congenitally. Even though the exact sequence of events is not always clear, it is easy to conceptualize that affected patients with reduced lung volumes, atelectasis, noncompliant chest walls, ineffective respiratory muscles, upper airway obstruction,and altered metabolic control of breathing are prone to develop hypoventilation, especially during sleep.

Nasal continuous positive airway pressure (nCPAP) is now clearly established as the most effective therapy for OSA. nCPAP is also effective in the majority of patients with obesity hypoventilation, not only resolving upper airway obstruction during sleep, but also increasing the ventilatory response to CO2 while awake and improving awake hypercapnia. However, there are many patients with severe OSA and hypercapnia who are only partially responsive to CPAP alone and continue to demonstrate sleep associated hypoventilation and elevated PaCO2 during wakefulness.

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