The causes and evaluation of chronic hypercapnea

The obesity hypoventilation syndrome

The obesity hypoventilation syndrome, which is defined as a combination of obesity and chronic hypoventilation, utimately results in pulmonary hypertension, cor pulmonale, and probable early mortality. Since the classical description of this syndrome nearly fifty years ago, research has led to a better understanding of the pathophysiologic mechanisms involved in this disease process, and to the development of effective treatment options. However, recent data indicate the obesity hypoventilation syndrome is under-recognized, and under-treated. Because obesity has become a national epidemic, it is critical that physicians are able to recognize and treat obesity-associated diseases. This article reviews current definitions of the obesity hypoventilation syndrome, clinical presentation and diagnosis, present understanding of the pathophysiology, and treatment options.

Hypercapnia and oxygen therapy in older asthmatic patients

Excessive oxygen administration in hypercapnic chronic obstructive pulmonary disease predisposes to worsening respiratory failure during intercurrent respiratory illness. Chronic hypercapnia is thought to downregulate carbon dioxide chemoreceptor sensitivity, adversely affecting respiratory function/mechanics and worsening ventilation-perfusion inequality. These patients are dependent on hypoxic drive to maintain adequate spontaneous respiration. Whether an analogous situation occurs in asthma in older adults is unknown. These conditions may be difficult to differentiate clinically, and both may respond adversely to the administration of excessive oxygen in the presence of chronic hypercapnia. Although unrestricted oxygen is beneficial and safe in children and young adults with asthma, it may lead to progressive hypercapnia in older patients with asthma, a potential risk highlighted by this case. To avert progressive hypercapnia, oxygen therapy that is carefully adjusted to achieve adequate, but not maximal, tissue oxygenation may be a safer strategy than unrestricted oxygen use in older asthmatic patients. However, the correction of hypoxia overrides strategies to avert oxygen-related hypercapnia.

CO2 retention in lung disease; could there be a pre-existing difference in respiratory physiology

Some patients with lung disease retain CO(2), while others with similar lung function do not. This could be explained if CO(2) retainers had a pre-existing low hypercapnic ventilatory response (HCVR) and, from this, a tendency to retain CO(2). To test if such a phenomenon exists in healthy people, we determined the change in end-tidal P(CO(2)) (deltaPET(CO(2))) produced by the addition of a dead-space (DS), during wakefulness and sleep, and related this to the HCVR measured awake. The group mean (n=14) HCVR slope was 2.2+/-1.1 (S.D.) L min(-1) mmHg(-1). The deltaPET(CO(2)) with the application of DS was 1.6+/-1.6 mmHg awake and 2.6+/-2.2 mmHg asleep. During wakefulness the deltaPET(CO(2)) with DS did not correlate with the HCVR slope. However, during sleep, four subjects had a marked increase in the deltaPET(CO(2)) (3.7, 4.3, 6.2, 8.0 mmHg) and a relatively low HCVR (slope 1.5, 1.7, 1.5, 1.7 L min(-1) mmHg(-1), respectively). We speculate that such individuals, should they develop lung disease, may be predisposed to retain CO(2).

Hypercapnic Respiratory Failure in Systemic Sclerosis

Respiratory failure from causes exclusive of intrinsic lung disease is rare in systemic sclerosis. We report an unusual case of a young woman with diffuse systemic sclerosis who presented with proximal muscle weakness, dyspnea, weight loss, and nasal regurgitation. On physical examination, she had normal breath sounds but severe limitation of chest wall expansion. Pulmonary function tests (PFTs) were consistent with restrictive lung disease. Pulmonary pressures were normal on right heart catheterization and chest radiography and high-resolution computed tomography showed no evidence of intrinsic lung disease. Quadriceps muscle biopsy was consistent with type II atrophy, without any element of inflammatory myositis. After a meal, the patient aspirated, developed severe persistent hypercapnia, and required ventilatory support. Therapy with corticosteroids and intravenous gamma globulin failed to improve her condition, and the patient subsequently expired. Hypercapnic respiratory failure on the basis of chest wall involvement from systemic sclerosis may occur in the absence of intrinsic lung disease, and the prognosis, as in this case, may be grave.

Chronic respiratory failure due to bilateral vocal cord paralysis managed with nocturnal nasal positive pressure ventilation

A patient with bilateral vocal cord paralysis developed chronic respiratory failure. Treatment with nocturnal inspiratory positive airway pressure via nasal mask improved symptoms and reduced hypercapnia.