Obesity hypoventilation syndrome
OHS is increasingly recognised as a significant public health issue, particularly in the context of the obesity epidemic that is occurring in many countries. However, its prevalence in Europe is unknown.
OHS is defined as the combination of obesity (BMI > 30 kg·m-2), hypercapnic (type II) respiratory failure (arterial carbon dioxide partial pressure greater than 45 mmHg or 6.5 kPa) and sleep disordered breathing when all other causes of type II respiratory failure have been excluded. This is unlike uncomplicated OSAS, in which the awake arterial carbon dioxide level is normal. The pathophysiology of OHS is complex, resulting from the interaction between OSA, decreased ventilatory drive and reduced compliance of the chest and abdominal walls caused by obesity.
The problem is under-recognised, with the corollary that the severe respiratory and cardio-metabolic consequences are not being adequately treated, which increases health-related costs and the risk of hospitalisation and death. There are very few well-conducted trials in the area, but the best treatment in terms of reducing mortality is noninvasive ventilation (NIV), which, like CPAP, is delivered via a face mask. Unlike CPAP, which provides almost constant pressure throughout the respiratory cycle, NIV provides higher inspiratory pressures than expiratory pressures, in order to assist ventilation; frequently, additional oxygen is also required.
Weight loss is an effective treatment for OHS but is often difficult to achieve without additional intervention such as bariatric surgery.
The limited evidence available suggests that early recognition, intervention and treatment saves lives and limits complications and costs to both the patient and society, but that this is occurring in a minority of instances, both in primary and secondary care settings.