Tang, Jin-Ling; (1991) Giving up smoking and the risk of ischaemic heart disease. Doctoral thesis (Ph.D), University of London, Royal Free Hospital School of Medicine.

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Abstract

Cigarette smoking is a major risk factor for ischaemic heart disease (IHD). However, there is controversy about the time required for the excess risk of IHD to disappear after stopping smoking. It is generally accepted that the risk reverts very rapidly to that seen in never smokers after stopping smoking. This thesis examines the issue; (i) by reviewing published literature and (ii) by examining data from the British Regional Heart Study (BRHS). The BRHS is a prospective study of cardiovascular disease among 7,735 men aged 40-59 years randomly drawn from one general practice in each of 24 British towns. The smoking-IHD relationship was examined based on 8-year follow-up, focusing on the risk with years-since-quitting. By the end of eight years of follow-up, there were 487 new major IHD events. Sources of bias and confounding effects were extensively examined. Particular emphasis has been laid on who is likely to give up smoking, on the principles for the adjustment of baseline characteristics and on the highly inverse correlation of smoking-years with the years-since-quitting smoking. After excluding men with a doctor diagnosis of IHD at entry, and adjusting for age, systolic blood pressure and serum total cholesterol, the risk of IHD relative to never-smokers was 2.4 among current cigarette smokers and 2.1 among secondary pipe/cigar (P/C) smokers. Primary P/C smokers seemed to carry no excess risk. Although ex-smokers, who on average had given up smoking for 13 years at entry, exhibited marginal (26%) excess risk compared to never-smokers, a significant trend of declining risk with the years-since-quitting was observed. Importantly, it appears that there was a substantial fall in risk of IHD within 5 years of quitting, ex-smokers remained at a raised risk of IHD compared to never-smokers for an extended period thereafter. There is also some evidence that the risk took longer to disappear in former heavy smokers than in former light smokers. It is concluded that the benefit of giving up cigarette smoking is more gradual than is usually accepted. This may cast light on mechanisms for the role of cigarette smoking in the development of IHD. It also carries important implications for prevention: that emphasise should be placed on preventing the young from starting to smoke and encouraging young smokers to give up smoking altogether and as early as possible.

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