Analysis of FEV₁ Decline in Relatively Healthy Heavy Smokers: Implications of Expressing Changes in FEV₁ in Relative Terms

Figures & data

Figure 1.  Flowchart of dropouts.

Table 1.  Characteristics of the study population

		Smokers		Former smokers
Characteristic		No AFL	AFL	No AFL	AFL	All
Number		1,209	1,219	495	295	3,218
Males		657	662	274	171	1,772
Age at baseline	years	57.0	58.5	58.0	58.9	57.9
Height	cm	173.0	174.0	173.2	174.0	173.5
BMI	kg/m^2	25.5	24.4	26.8	25.9	25.3
Smoking history	pack-years	34.4	38.0	34.6	37.9	36.1
FEV1	litres	2.86	2.48	2.87	2.44	2.68
	% pred.	94.5	81.5	95.9	80.8	88.5
FEV1/FVC	%	75.1	63.4	75.2	63.9	69.7
Annual loss (∆FEV1) ml	104.7	121.4	94.1	86.5	107.7
	%	3.71	5.04	3.30	3.70	4.15
*Correlation	∆FEV1 in ml	0.26	0.25	0.24	0.22	0.15
coefficient	p-value	<0.001	<0.001	<0.001	<0.001	<0.001
	∆FEV1 in%	−0.10	−0.19	−0.10	−0.14	−0.12
	p-value	0.010	<0.001	0.071	0.046	<0.001

*Correlation coefficient between mean FEV₁ and ∆FEV₁ (slope) for each subject.

Figure 2.  The influences of various factors on decline in lung function. Figures in upper row (A-C) are results from the absolute model and figures (D-F) are based on the relative model.

Table 2.  Results of the mixed effects models showing the effect of explanatory variables on level of FEV₁ at age 60 and the annual rate of change (slope) in relation to a reference group of former smoking men with no air flow obstruction at entry to the trial

				Absolute model				Relative (or logarithmic) model
				Level of FEV1 (ml)at age 60		Annual rate of change in FEV1 (ml/year)		Relative level of FEV1 (%) at age 60		Relative annual rate of change in FEV1 (%/year)
			Number of subjects at baseline	Estimate	p-value	Estimate	p-value	Estimate	p-value	Estimate	p-value
Reference group			*254	3,212 ml	−	−73.32 ml	p < 0.001	#100%	–	−2.12%	p < 0.001
Explanatory variables		Male	1,772	Reference		Reference		Reference		Reference
	Sex	Female	1,446	−579 ml	p < 0.001	14.46 ml	p < 0.001	−19.52%	p < 0.001	−0.071%	p = 0.535
	Height	per cm	3,218	29 ml	p < 0.001	−0.67 ml	p < 0.001	1.14%	p < 0.001	0.0001%	p = 0.312
	BMI	per kg/m^2	3,218	−18 ml	p < 0.001	−0.15 ml	p = 0.490	−0.69%	p < 0.001	−0.026%	p = 0.002
	Smoking history	per 10 pack-yr	3,218	−11 ml	p = 0.046	−0.66 ml	p = 0.379	−0.48%	p = 0.025	−0.044%	p = 0.150
	Smoking	Former	790	Reference		Reference		Reference		Reference
		Current	2,428	−77 ml	p < 0.001	−2.34 ml	p = 0.294	−2.87%	p < 0.001	−0.20%	p = 0.027
	GOLD grade	No AFL	1,704	Reference		Reference		Reference		Reference
		I	836	−46 ml	p = 0.008	−5.68 ml	p = 0.016	−1.81%	p = 0.007	−0.22%	p = 0.018
		II + III	678	−741 ml	p < 0.001	−1.12 ml	p = 0.660	−26.63%	p < 0.001	−0.94%	p < 0.001
Age^2	per year^2		3,218	0.886 ml	p < 0.001	–	–	−0.019%	p < 0.001	–	−

*The reference person was defined as a man of age 60 years, height 1.75 m, body mass index (BMI) 25 kg/m², 35 pack-years, former smoker, and no airflow limitation (AFL), and the number refers to the reference group of former smoking men who had no AFL.

^#corresponds to 3,193 ml.

Figure 3.  Effect of smoking on decline in lung function. A subject who began smoking at age 25 and continued smoking the same amount until age 70 (solid line). The dashed line from age 50 to 70 shows the estimated change in lung function assuming the subject quitted smoking at age 50 that is no further smoking-related increase in slope after age 50. In grey the Fletcher-Peto diagram (2).

Figure 4.  Backward and forward extrapolation beyond the observed data to an age range from 0 to 120 years. Assuming a constant height.