Maximal cardiopulmonary exercise test in patients with chronic low back pain: feasibility, tolerance and relation with central sensitization. An observational study

Figures & data

Table 1. Hypotheses of feasibility of and pain-related tolerance after a maximal CPET.

	Supported when:	Definition
Feasibility	≥80% maximal performance	Maximal performance during CPET, either: Peak HR: ≥85% predicted maximal HR RER: ≥1.10
Pain-related tolerance	≤20% relevant pain increase	Relevant pain increase: MIC (≥2 absolute points) pain increase in NRS, PRQ part 1
	≤20% body reactions	PRQ part 2
	≤20% pain medication use	PRQ part 2

CPET: Cardiopulmonary Exercise Test; HR: heart rate; RER: respiratory exchange ratio; MIC: minimal important change; NRS: Numeric Rating Scale; PRQ: Pain Response Questionnaire.

Table 2. Description of participating patients (n = 74).

	n	Mean ± SD or %
Demographic characteristics
Age (years)	74	40.4 ± 12.4
Sex	74
Men	30	40.5%
Women	44	59.5%
Body mass index (kg/m^2)†	74	27.0 [24.1–30.6]
Pain duration (years)†	74	2.1 [1.3–4.2]
Use of medication	72
Use pain medication	53	73.6%
No use of pain medication	19	24.6%
Educational level	70
Primary	2	2.9%
Secondary	40	57.1%
Bachelor or higher	28	40.0%
Physical work demands (DOT)	74
Sedentary	18	24.3%
Light	32	43.2%
Medium	20	27.0%
Heavy	4	5.4%
Working status	74
Working	27	36.5%
Reduced/Adapted work	17	23.0%
Sick-leave	9	12.2%
Disability pension	7	9.5%
Not working	10	13.5%
Other	4	5.4%
Clinical characteristics
Pain intensity (VAS pain, 0–10)	73	4.7 ± 2.2
Disability (PDI, 0–70)	72	36.7 ± 11.9
Physical functioning (Rand36-PF, 0–100)	73	51.5 ± 19.5
Catastrophyzing (PCS, 0–52)	65	18.9 ± 10.2
Injustice (IEQ, 0–48)†	70	16.0 [9.8–23.0]
Distress (BSI-GSIT, 0–100)†	65	38.1 [33.0–45.5]
Aerobic capacity (CPET)
Maximal oxygen uptake per kilogram (ml/kg/min)†	74	23.9 [18.2–29.4]
Maximal performance	68	91.9%
Submaximal performance	6	8.1%
Oxygen uptake at anaerobic threshold (l/min)†	74	1.2 [1.0–1.4]
Workload (watts)†	74	162.0 [134.8–192.5]
Energy expenditure (MET)†	74	6.8 [5.2–8.4]
Patient's reported exertion (RPE, 6–20)	74	16.1 ± 2.3
Assessor's observed exertion (RPE, 6–20)	68	17.2 ± 2.0
Central sensitization (CS)
CSI
CS-related symptoms (CSI-A, 0–100)†	70	40.5 [31.0–50.0]
QST
Sharp cutaneous pain sensitivity (MPT, mN)†	70	73.3 [31.7–190.7]
Blunt pressure pain sensitivity (PPT, N)†	70	53.9 [41.7–72.6]
Allodynia (DMA)	69	0.0%
Touch sensation (MDT, mN)†	70	3.3 [1.6–8.2]
Temporal summation (WUR)†	59	2.4 [1.8–3.2]
Descending pain modulation (CPM, %)†	69	6.9 [-4.5–23.2]
HRV
Parasympathetic/vagal tone (RMSSD, ms)†	69	41.4 [31.5–54.6]
Parasympathetic/vagal tone (HF, ms^2/Hz)†	69	102.3 [63.0–196.0]
Mean inter-beat time interval (R-R interval, ms)†	69	816.7 [746.8–917.6]
Standard deviation of R-R intervals (SDNN, ms)†	69	56.9 [41.7–72.8]

†Variables not normally distributed, median and interquartile range [IQR 25–75] is given.

DOT: dictionary of occupational titles; VAS: Visual Analogue Scale; PDI: Pain Disability Index; PF: physical functioning; PCS: Pain Catastrophizing Scale; IEQ: Injustice Experience Questionnaire; BSI: Brief Symptom Inventory; GSIT: Global Severity Index T-score; CPET: Cardiopulmonary Exercise Test; MET: metabolic equivalent of tasks; RPE: rating of perceived exertion; CSI: Central Sensitization Inventory; QST: Quantitative Sensory Testing; MPT: mechanical pain threshold; PPT: pressure pain threshold; DMA: dynamic mechanical allodynia; MDT: mechanical detection threshold; WUR: wind-up ratio; CPM: conditioned pain modulation; HRV: heart rate variability; RMSSD: root mean square of successive differences; HF: high frequency; SDNN: standard deviation of normal-to-normal range.

Table 3. Description of pain response to CPET per the body locations (n = 74).

	Before CPET			Immediately after CPET				24 h after CPET
	n	Mean ± SD		n	Mean ± SD		Participants with MIC pain increase (%)	n	Mean ± SD		Participants with MIC pain increase (%)
Shoulders	63	2.4	±2.7	56	2.2	±2.6	5.4	59	2.2	±2.7	10.2
Upper back	59	2.0	±2.4	57	2.1	±2.4	12.5	60	2.1	±2.4	12.7
Low back	73	5.5	±2.2	73	5.2	±2.6	11.1	71	5.4	±2.4	8.6
Upper legs	57	2.6	±3.0	60	4.1	±3.0	39.6	63	3.4	±2.9	27.8

CPET: Cardiopulmonary Exercise Test; IQR: Interquartile range 25–75; MIC: minimal important change.

Table 4. Results of Spearman partial correlation analyses of the association of maximal aerobic capacity (VO₂max per kg) with CS measures, main secondary measures, and corrected for age and sex.

	Aerobic capacity
Aerobic capacity
Maximal performance (yes/no)	−0.34**
Oxygen uptake at anaerobic threshold	0.47***
Workload	0.73***
Patient's reported exertion (RPE)	−0.00
Assessor's observed exertion (RPE)	0.12
Central sensitization
CSI
CS-related symptoms (CSI-A)	−0.03
QST
Sharp cutaneous pain sensitivity (MPT)	−0.03
Blunt pressure pain sensitivity (PPT)	−0.02
Touch sensation (MDT)	−0.18
Temporal summation (WUR)	−0.01
Descending pain modulation (CPM)	0.07
HRV
Parasympathetic/vagal tone (RMSSD)	0.17
Parasympathetic/vagal tone (HF)	0.16
Mean inter-beat time interval (R-R interval)	0.38**
Standard deviation of R-R intervals (SDNN)	0.28*
Participants’ characteristics
Pain medication	−0.09
Pain intensity (VAS pain)	−0.25*
Disability (PDI)	−0.31*
Physical functioning (Rand36-PF)	0.35**
Catastrophyzing (PCS)	−0.06
Injustice (IEQ)	−0.09
Distress (BSI-GSIT)	0.04

Significance level: *p < 0.05; **p < 0.01; ***p < 0.001.

RPE: rating of perceived exertion; CSI: Central Sensitization Inventory; QST: Quantitative Sensory Testing; MPT: Mechanical pain threshold; PPT: pressure pain threshold; MDT: Mechanical detection threshold; WUR: wind-up ratio; CPM: conditioned pain modulation; HRV: heart rate variability; RMSSD: root mean square of successive differences; HF: high frequency; SDNN: standard deviation of normal-to-normal range; VAS: Visual Analogue Scale; PDI: Pain Disability Index; PF: physical functioning; PCS: Pain Catastrophizing Scale; IEQ: Injustice Experience Questionnaire; BSI: Brief Symptom Inventory; GSIT: Global Severity Index T-score.

Table 5. Multiple regression model of maximal aerobic capacity (VO₂max per kg) with CS (CSI, MDT, and RMSSD), corrected for age, sex and relevant confounders.

	B	[95% CI]	p	Beta	rpartial
(Constant)	3.22	[−8.52, 14.96]	0.585
Central sensitization
CS-related symptoms (CSI-A)	0.02	[−0.07, 0.11]	0.716	0.03	0.05
Touch sensitivity (MDT)	−0.14	[−0.30, 0.03]	0.099	−0.13	−0.21
Parasympathetic/vagal tone (RMSSD)	0.01	[−0.05, 0.07]	0.695	0.03	0.05
Confounders
Age	−0.17	[−0.28, −0.06]	0.003	−0.27	−0.38
Sex	−0.39	[−3.06, 2.27]	0.769	0.03	0.04
Workload	0.09	[0.06, 0.12]	<0.001	0.61	0.64
Mean inter-beat time interval (R-R interval)	0.02	[0.00, 0.03]	0.007	0.22	0.34

CSI: Central Sensitization Inventory; MDT: Mechanical detection threshold; RMSSD: root mean square of successive differences.