Obesity Hypoventilation Syndrome

A 66 year old man was brought by ambulance to ED after becoming acutely dyspnoeic at home, and his wife had struggled to wake him after an afternoon sleep. He had a moderate smoking history of 20 pack years and quit 20 years ago, and drank approximately 20 units of alcohol as beer per week. There were no preceding prodromal respiratory or infective symptoms reported. On examination, he was obese with a weight of 120 kg. He was snoring, which eased with a chin-lift or jaw thrust, but he would not tolerate an airway adjunct. His respiratory rate was 8 and shallow, producing SpO2 84% on high flow oxygen. He was flushed and veno-dilated peripherally, with a tachycardia of 110 bpm sinus rhythm. Most notably, he was difficult to rouse, with a GCS of 9 (E2 M5 V2). Arterial blood sampling revealed a profound hypercapnoeic respiratory acidosis with no degree of compensation. Chest X-ray showed poorly expanded lungs with bilateral basal atelectasis and prominent pulmonary vasculature. Empiric naloxone was ineffective and he was commenced on non-invasive ventilation (NIV). The working diagnosis was an acute non-infective exacerbation of previously undiagnosed chronic obstructive pulmonary disease (COPD). Treatment involved bronchodilators, steroids and bilevel pressure support non- invasive ventilation.

One hour after NIV had been established, the patient’s respiratory acidosis was worse and his clinical picture was unchanged. He was intubated then transferred to Intensive Care. CTPA ruled out significant pulmonary emboli but was suggestive of pulmonary hypertension. Ventilation was not difficult, with near normal inspiratory pressures, and his minimal wheeze resolved quickly. As his respiratory acidosis normalised, his GCS rapidly improved to the point of safe extubation after just 48 hours. Given his obesity he was extubated to non-invasive pressure support ventilation immediately. After a further 24 hours, his gas exchange began to deteriorate again. Increasing inspiratory pressure improved his tidal volumes but his intrinsic rate of breathing slowed such that his minute ventilation remained static. He eventually found a stable equilibrium using nocturnal bilevel pressure support with a high mandatory backup rate (pressure control), which maintained a normal minute ventilation. Although he developed hypercapnoea each night, this was mild and eventually compensated. With the NIV, he did not obstruct, have hypopnoeic events or desaturated overnight, which markedly improved his daytime somnolence and effort of breathing.

What are the clinical features and approaches to management of obesity hypoventilation syndrome?

Duncan Chambler

Obesity Hypoventilation Syndrome (OHS) has significant overlap with Obstructive Sleep Apnoea-Hypopnea Syndrome (OSAH) and similarities to Chronic Obstructive Pulmonary Disease (COPD). The American Academy of Sleep Medicine defined it (1) as:

• BMI≧30kgm-2;and
• Awake PaCO2 > 45 mmHg (6 kPa); and
• Sleep-related breathing disorder, demonstrated by polysomnography.

The latter can be obstructive sleep apnoea hypopnoea, with > 5 apnoea episodes per hour whilst asleep, or sleep hypoventilation syndrome, as demonstrated by an increase in PaCO2 > 10 mmHg compared to an awake measurement or oxygen desaturation unexplained by apnoeic episodes. This complex and technical description was defined to allow standardised research into the field, but clinically a mixed picture often exists. Experts have observed that a subset of patients with OSAH also have daytime hypercapnoea (2). The important difference is whether the hypercapnoea resolves with effective relief of the obstructive cause (i.e. with CPAP therapy), or persists regardless (i.e. related to obesity regardless of airway obstruction). (3)

In this case, the “knee jerk” response was to treat him for COPD. In retrospect it is clear many features where inconsistent with that diagnosis, but it is a common presentation of acute respiratory distress. The overlap syndrome, as described in a case series (4), exists where COPD and OHS are present. Supplemental oxygen may worsen hypercapnoea and hypoventilation. This feature is not common in isolated COPD unless the FEV1 < 1 litre (severe disease).

Presentation of OHS is usually insidious, featuring the physical and psychological problems from sleep disturbance. Secondary polycythaemia, pulmonary hypertension and cor pulmonale develop later. Acute presentations of decompensation require an additional trigger, such as sedating drugs or alcohol, bacterial or viral infections, or acute coronary events. In this case, alcohol may have been a contributing factor, but no specific, reversible, exacerbating trigger was identified.

An interesting theory has been suggested to explain OHS ventilatory patterns (5). Normal changes in obesity up-regulate ventilation control to increase the work of breathing, which overcomes the increased lung compliance. Animal studies and human data suggest leptin is involved in this process. It is possible that OHS suffers are deficient or resistant to adipose produced leptin, resulting in hypoventilation proportional to their requirement.

Treatment consists of weight loss and non-invasive ventilatory support. Surgical programmes for weight loss can be advocated for patients with OHS, given the secondary organ damage. Ventilatory support takes two forms: continuous positive airway pressure is excellent for those with elements of OSAH Syndrome; bilevel or volume cycled mechanical ventilation may be required for those with pure hypoventilation syndrome. Rarely, when non-invasive ventilation is critically essential but not tolerated, a tracheostomy may be necessary.

In this case, bilevel mandatory ventilation was required and was effective. It was interesting to see how his ventilation control had chronically established a new PaCO2 threshold. As we supported his ventilation, he simply breathed less frequently. By forcing his ventilation to a greater minute volume, normocapnoea and oxyaemia was achieved. He went home with a bilevel machine to support this in his long-term.

Lessons learnt

This case illustrated a basic issue: incorrect initial diagnosis and management. It reminded me that clinicians must regularly revisit the working diagnosis and to keep in mind appropriate differentials.

Exploring the literature has educated me on Obesity Hypoventilation Syndrome, a problem that is likely to increase in prevalence. I shall consider it whenever Obstructive Sleep Apnoea-Hypopnoea Syndrome is identified, as the required ventilatory mode is subtly different but critically important.

References

1 The Report of an American Academy of Sleep Task Force. Sleep- related breathing disorders in adults: Recommendations for syndrome definition and measurement techniques in clinical research. Sleep 1999; 22: 667– 89.

2 Piper AJ, Sullivan CE. Effects of short-term NIPPV in the treatment of patients with severe obstructive sleep apnea and hypercapnia. Chest 1994; 105: 434 – 40.

3 Rabec CA. Obesity hypoventilation syndrome: what’s in a name? Chest 2002; 122(4): 1498. 4 Flenley DC.Sleep in chronic obstructive lung disease. Clinics in Chest Med 1985; 6: 651– 61.

5 Olson AL and Zwillcih O. The obesity hypoventilation syndrome. American Journal of Medicine 2005; 118: 948-956.