Inaccuracy of Estimating Peak Work Rate from Six-Minute Walk Distance in Patients with COPD

Figures & data

Table 1.  Regression equations predicting peak work rate including 6-min walk distance in patients with COPD

Authors	Wpeak regression equation
Hill et al. (#1) (14)	(0.122 × 6MWD) + (72.683 X height) –117.109
Hill et al. (#2) (14)	17.393 + (1.442 x6MWW [km·kg–1])
Luxton et al. (16)	103.217 + (30.50 × gender) + (–1.613 × age) + (0.002 × 6MWW [m·kg–1])
Cavalheri et al. (13)	-27.9717 + [3.7792*(6MWD*FFM)]
Kozu et al. (#1) (15)	(0.168 × 6MWD) -4.085
Kozu et al. (#2) (15)	(2.310 × 6MWW [km·kg–1]) + 8.820

FEV1 = forced expiratory volume in 1 second; FFM = fat free mass; 6MWD = 6-min walk distance; 6MWW = 6-min walk-work (6MWD in meters multiplied by body weight in kilograms). Height in meters; 6MWD in meters; gender is given a value of 1 for male and 0 for female.

Table 2.  Characteristics

Variable	Value	Mean ± SD
Sex	M/F	1529/1377
Age	years	62.9 ± 9.4
Height	centimetres	168 ± 9.4
Body weight	kg	73.3 ± 17.4
BMI	kg/m2	25.8 ± 5.5
FFM	kg	47.2 ± 9.6
FFMI	kg/m2	16.6 ± 2.5
FEV1	litres	1.19 ± 0.54
FEV1	% pred	44.0 ± 17.6
FEV1/VC	%	39.4 ± 14.1
Wpeakactual	watts	60.0 ± 33.2
6MWD	meters	400 ± 120

M = male; F = female; kg = kilogram; BMI = body mass index; FFM = fat-free mass; FFMI = fat-free mass index; FEV1 = forced expiratory volume in 1 second; VC = vital capacity; Wpeakactual = actual peak work rate; % pred = percentage predicted value; CPET = cardiopulmonary exercise test; 6MWD = distance achieved by the 6-minute walk test.

Figure 1.  Bland and Altman plots and distribution of COPD patients of the existing Wpeak regression equations.

Table 3.  Percentage differences between Wpeakestimated and Wpeakactual

Regression equation	Whole group	< -6 Watts	-5 to +5 Watts	> +6 Watts
Hill et al. (#1) (14)	11 (-88, +2150)	-29 (-88, -6)	0 (-33, +40)	72 (7, 2150)
Hill et al. (#2) (14)	25 (-68, +2400)	-25 (-68, -6)	1 (-13, +21)	78 (5, 2400)
Luxton et al. (16)	50 (-121, +2400)	-29 (-121, -3)	0 (-100, +40)	86 (5, 2400)
Cavalheri et al. (13)	-24 (-380, +1200)	-44 (-380, -5)	-2 (-100, +125)	60 (5, 1200)
Kozu et al. (#1) (15)	29 (-83, +2500)	-27 (-83, -6)	1 (-27, +80)	73 (6, 2500)
Kozu et al. (#2) (15)	55 (-65, +2950)	-20 (-65, -4)	1 (-17, +33)	80 (3, 2950))
New regression equation	13 (-280, +1550)	-27 (-280, -5)	1 (-31, +42)	60 (5, 1550)

Data are presented as mean (range).

Figure 2.  Bland and Altman plots and distribution of COPD patients of the new Wpeak regression equations.

Table 4.  Characteristics of the study subjects after stratification for Wpeakestimated-Wpeakactual

Variable	Value	Wpeakestimated - Wpeakactual
		< -6 Watts	-5 to +5 Watts	> +6 Watts
Patients	Numbers (%)	1029 (35)	761 (26)	1116 (38)
Sex	M/F	568/461	356/405	605/511
GOLD, I/II//III/IV	numbers	40/301/353/335	23/192/290/256	58/339/432/287
Age	years	63.0 ± 9.6	63.3 ± 9.4	62.7 ± 9.3
Height	centimetres	169 ± 9.3*	167 ± 8.9	169 ± 9.7*
Body weight	kg	74.2 ± 16.7*	70.7 ± 16.0	74.3 ± 18.8*
BMI	kg/m2	26.0 ± 5.1*	25.3 ± 5.1	26.0 ± 6.0*
FFM	kg	47.7 ± 9.6*	45.5 ± 8.8	47.9 ± 10.1*
FFMI	kg/m2	16.7 ± 2.4*	16.3 ± 2.3	16.7 ± 2.7*
FEV1	litres	1.21 ± 0.58*	1.09 ± 0.47	1.24 ± 0.54*
FEV1	% pred	43.9 ± 17.4	42.4 ± 17.1	45.4 ± 18.1*
FEV1/VC	%	40.7 ± 14.6*	37.8 ± 12.7	39.4 ± 14.5
Wpeakactual	watts	79.4 ± 37.2*	56.2 ± 24.5	44.7 ± 24.4*‡
Wpeakestimated	watts	60.5 ± 31.2	56.1 ± 24.5	62.5 ± 24.7*
6 MWD	meters	396 ± 133	397 ± 115	405 ± 111

* p < 0.05 versus -5 to 5 Watts.

‡ p < 0.05 versus < -6 Watts.

M = male; F = female; BMI = body mass index; FFM = fat-free mass; FFMI = fat-free mass index; FEV1 = forced expiratory volume in 1 second; VC = vital capacity; CPET = cardiopulmonary exercise test; Wpeakactual = actual peak work rate; Wpeakestimated = estimated peak work rate; 6MWD = distance achieved by the 6-minute walk test; % pred = percentage predicted value.

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