Multiple regression

Researchers investigated the association between sleep apnoea and hypertension. A total of 2677 adults, aged 20­85 years, referred to a sleep clinic in Toronto with suspected sleep apnoea syndrome were recruited. Severity of sleep apnoea was measured using the apnoea­hypopnoea index, defined as the total number of apnoeic events plus hypopnoeic events divided by the total number of hours of sleep. Blood pressure measurements were averaged across several readings, taken under standard conditions, with the patients awake and supine, just before getting out of bed in the morning.1

Multiple regression was used to examine the association between blood pressure and apnoea­hypopnoea index, age, sex, and neck circumference. Separate regression analyses were performed for systolic and diastolic blood pressure (table⇓). Analyses were restricted to 1865 patients not taking antihypertensive drugs.

Which of the following statements, if any, are true?

• a) Systolic and diastolic blood pressures were assumed to be linearly related to each of apnoea-hypopnoea index, age, and neck circumference

• b) It can be concluded that the effect of the apnoea-hypopnoea index was independently associated with systolic and diastolic blood pressure in patients with apnoea not taking antihypertensive drugs

• c) The results of the analyses can be extrapolated outside the observed range of values for apnoea­hypopnoea index, age, and neck circumference

• d) The results of the analyses can be generalised to all patients referred to the sleep clinic with suspected sleep apnoea