Physiological Cost Index
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Physiological Cost Index

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Purpose

The purpose of the Physiological Cost Index (PCI) is to measure the energy expenditure index of gait in different populations.

Acronym PCI

Area of Assessment

Aerobic Capacity
Functional Mobility
Gait

Assessment Type

Physiological

Administration Mode

Paper & Pencil

Cost

Free

Actual Cost

$0.00

Cost Description

Cost of materials

Populations

Stroke
Pediatric Disorders
Non-Specific Patient Population
Limb Loss and Amputation

Key Descriptions

  • The Physiological Cost Index (PCI) uses heart rate to indicate the energy cost of walking. The measurement relies on the assumption that heart rate is linearly related to oxygen expenditure (vo2)
  • 3 items: resting heart rate, working heart rate, and walking speed
  • These 3 measurements are obtained during the walking test and used to calculate PCI based on this equation:
    PCI = (Working HR - Rest HR) / Walking speed
  • Resting heart rate is measured first in beats per minute. Participants sit quietly for a few minutes until heart rate reaches a steady state.
    Walking speed is measured in meters per minute as the participant walks on a level, indoor surface of known distance at their “preferred pace.”
    Working heart rate, also measured in beats per minute, when walking heart rate reaches a steady state. Participants should walk until they reach a steady heart rate (at least 4 minutes).
  • Subtract the HR measurements and divide by the walking speed to obtain PCI in beats/meter.
  • Lower PCI indicates more efficient energy expenditure. Higher PCI indicates lower energy efficiency.

Number of Items

3

Equipment Required

  • Heart rate monitor
  • Stopwatch and a measured distance (or treadmill that calculates walking speed)
  • Calculator (as needed)

Time to Administer

 minutes

Time to administer is dependent on the time it takes for patients to reach a steady-state (plateau) in heart rate and oxygen consumption

Required Training

No Training

Required Training Description

No specific required training. The administration of this tool should be performed by a clinician who knows how to measure heart rate and walking speed.

Age Ranges

Children

5 - 17

years

Adult

18 - 64

years

Elderly Adult

65 - 99

years

Instrument Reviewers

Lance Bennett, Katie Buck, Lauren Cordova, Jessie McLaughlin, Tyler Mullen (Duke Doctor of Physical Therapy Students); Derek Clewley, PT, DPT, PhD (faculty)

ICF Domain

Body Function

Measurement Domain

General Health

Non-Specific Patient Population

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Test/Retest Reliability

Healthy Adults (Graham, 2005; n = 40; Mean Age = 34.5 (12.6) years; British sample) 

  • Excellent test-retest reliability (ICC = .92)

Interrater/Intrarater Reliability

Intrarater reliability

  • Healthy Adults (Graham, 2005)
    • Adequate intrarater reliability for 20m track (r = .73)
    • Excellent intrarater reliability for 12m track (r = .79)

Interrater reliability

  • Healthy Adults (Graham, 2005)
    • Adequate interrater reliability for 20m track (r = .62)  
    • Adequate interrater reliability for 12m track (r = .66)

Criterion Validity (Predictive/Concurrent)

Healthy adults (Graham, 2005)

  • No significant concurrent validity between PCI and oxygen cost for either 20m or 12m track
    • The absence of a correlation between PCI and Eo2 and the presence of only a weak correlation between V̇o2 and change in heart rate would suggest that the PCI is not a valid measure of oxygen cost in healthy, moderately active subjects walking on either track

Face Validity

The PCI appears to measure what it is purported to measure and therefore demonstrates adequate face validity.

Stroke

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Standard Error of Measurement (SEM)

Stroke (Danielsson et al., 2007; n = 20; Median Age = 54 years; Median Time Post-Stroke = 19 months)

  • SEM = 0.187 (Calculated using standard deviation and intraclass correlation values reported by Danielsson et al.)

Minimal Detectable Change (MDC)

Stroke (Danielsson et al., 2007)  

  • MDC95 = 0.52
  • *Calculated using calculated SEM values from data reported by Danielsson et al.

Normative Data

Stroke (Danielsson et al.)

  • PCI = 0.76 ± 0.50 beats/m

Stroke (Fredrickson et al., 2007; n = 17; Mean Age = 58.6 years; Mean Time Post-Stroke = 44 months)

  • PCI= 1.34 ± 0.90 beats/m

Stroke with Hemiplegia (Delussu et al., 2014; n = 6; Mean Age = 66 years; Mean Time Post-Stroke = 8 weeks)

  • PCI= 0.35 ± 0.06 beats/m

Test/Retest Reliability

Stroke with Hemiparesis (Danielsson, et al; Sample with Hemiparesis >6 months Post-Stroke):

  • Excellent test-retest reliability: (ICC=.86)
  • “A person could be expected (with 95% probability) to have a retest difference between the limits of agreement.”
    • 95% LOA= 0.55 to -0.57

Interrater/Intrarater Reliability

Intrarater Reliability

Stroke with Hemiparesis (Danielsson, et al; Sample with Hemiparesis >6 months Post-Stroke)

  • Excellent test-retest reliability: (ICC=.86)

Criterion Validity (Predictive/Concurrent)

Concurrent Validity:

Stroke (Fredrickson et al., 2007)

  • Excellent concurrent validity of the PCI with oxygen cost for stroke survivors (r =0.831)  
  • Poor concurrent validity of the PCI with age (r =0.072)

Stroke with Hemiplegia (Delussu, 2014)

  • PCI has a high correlation with ECW (they both detect differences between control (healthy) and patient (stroke) groups, which indicates concurrent validity
    Stroke

Construct Validity

 Construct Validity:

Stroke (Delussu, 2014)

  • Excellent construct validity between stroke patients (r = 0.9191, p < 0.001) and healthy individuals (r = 0.852, p < 0.001)

Discriminant Validity:

Stroke (Delussu, 2014)

  • Significant discriminant validity between stroke patients (1.45±0.87 beats/m) and healthy subjects (0.35±0.06 beats/m) (p=0.012) 

Pediatric Disorders

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Standard Error of Measurement (SEM)

Healthy Children (Bratterby et al., 2011; n = 20; Mean Age (SD) = 10.4 ± 3.3 years)

  • SEM for non-steady state = 0.0222 beats/meter
  • SEM for steady state = 0.0323 beats/meter

Cystic Fibrosis (Bratterby et al., 2011; n = 11; Mean Age (SD) = 9.9 ± 2.5 years)

  • SEM for non-steady state = 0.0244 beats/meter
  • SEM for steady state = 0.0465 beats/ meter

Cerebral Palsy (Bratterby et al., 2011; n = 8; Mean Age (SD) = 8.9 ± .08 years)

  • SEM for non-steady state = 0.0133 beats/ meter
  • SEM for steady state = 0.0489 beats/ meter

Standard Error of Measurement was calculated using standard deviation and intraclass correlation values reported by Bratterby et al.

Minimal Detectable Change (MDC)

Healthy Children (Bratterby et al., 2011; n = 20; Mean Age (SD) = 10.4 ± 3.3 years)

  • MDC for non-steady state = 0.0615 beats/meter
  • MDC for steady state = 0.0895 beats/ meter

Cystic Fibrosis  (Bratterby et al., 2011; n = 11; Mean Age (SD) = 9.9 ± 2.5)

  • MDC for non-steady state = 0.0676 beats/ meter
  • MDC for steady state = 0.129 beats/ meter

Cerebral Palsy (Bratterby et al., 2011; n = 8; Mean Age (SD) = 8.9 ± .08 years)

  • MDC for non-steady state = 0.0369 beats/meter
  • MDC for steady state = 0.136 beats/meter

 Minimal detectable change was calculated using calculated SEM values from data reported by Bratterby et al.

Normative Data

Healthy Children (Bratterby et al., 2011; n = 20; Mean Age = 10.4 ± 3.3)

  • PCI=0.44 +/- 0.11 beats/meter

Cystic Fibrosis (Bratterby et al., 2011; n = 11; Mean Age = 9.9 ± 2.5)

  • PCI = 0.47 +/- 0.13 beats/ meter

Cerebral Palsy (Bratterby et al., 2011; n = 8; Mean Age (SD) = 8.9 ± .08 years)

  • PCI = 1.21 ± 0.96 beats/ meter

Test/Retest Reliability

Healthy Children (Bratterby et al., 2011; n = 20; Mean Age = 10.4 ± 3.3)

  • Non-steady state: Excellent Test-retest reliability (ICC = 0.837)
  • Steady state: Adequate Test-retest reliability (ICC = 0.460)

Cystic Fibrosis (Bratterby et al., 2011; n = 11; Mean Age = 9.9 ± 2.5)

  • Non-steady state: Excellent Test-retest reliability (ICC = 0.795)
  • Steady state: Adequate Test-retest reliability (ICC = 0.686)

Cerebral Palsy (Bratterby et al., 2011; n = 8; Mean Age (SD) = 8.9 ± .08 years)

  • Non-steady state: Excellent Test-retest reliability (ICC = 0.986)
  • Steady state: Excellent Test-retest reliability (ICC = 0.985)

Criterion Validity (Predictive/Concurrent)

Concurrent Validity:

Spastic Cerebral Palsy (Bowen et al., 1998; n = 10 [n = 5 children with spastic CP, n = 5 healthy controls])

  • Poor concurrent validity between PCI and oxygen consumption during exercise (r = 0.407 ± 0.232) and at rest (r = 0.503 ± 0.196)

Limb Loss and Amputation

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Standard Error of Measurement (SEM)

Unilateral Lower Limb Amputees: (Hagberg et al., 2011; n = 28 [Amputation Level: transtibial n=8, knee disarticulation n=5, transfemoral n=13, hip disarticulation n=2]; Mean Age = 49 years)

  • SEM* = 0.039

*Standard Error of Measurement was calculated using standard deviation and intraclass correlation values reported by Hagberg et al.

Minimal Detectable Change (MDC)

Unilateral Lower Limb Amputees: (Hagberg et al., 2011)

  • MDC95* = 0.11

*Minimal detectable change was calculated using calculated SEM values from data reported by Hagberg et al. 2011

Normative Data

Unilateral Lower Limb Amputees: (Hagberg et al., 2011)

  • PCI = 0.555 +/- 0.214 beats/meter for initial test
  • PCI = 0.581 +/- 0.236 beats/meter for second test (next day)

Transfemoral Amputees: (Vllasolli et al., 2015; n = 22; Mean Age= 40.6 +/-12.5 years; Mean Period of Prosthetic Use = 17.1 (10.5) years; Kosovan sample)

  • PCI= 0.57 +/-0.085 beats/meter
  • PCI = 0.62 +/- 0.05 for participants who use walking aids (n = 11)
  • PCI = 0.53 +/- 0.09 for participants who do not use walking aids (n = 11)

Transtibial Amputees: (Vllasolli et al., 2015; n = 61; Mean Age = 39.7 +/-13.1 years; Mean Period of Prosthetic Use = 14.5 (7.5) years; Kosovan sample)

  • PCI= 0.43 +/-0.087 beats/meter
  • PCI= 0.55 +/-0.09 beats/meter for participants who use walking aids (n = 10)
  • PCI= 0.40 +/-0.06 beats/meter for participants who do not use walking aids (n = 51)

Test/Retest Reliability

Unilateral Lower Limb Amputees: (Hagberg et al., 2011)

  • Excellent test-retest reliability (ICC = 0.966)

Bibliography

  1. Bailey, M. J. and C. M. Ratcliffe (1995). "Reliability of Physiological Cost Index Measurements in Walking Normal Subjects Using Steady-state, Non-steady-state and Post-exercise Heart Rate Recording." Physiotherapy 81(10): 618-623.
  2. Bowen, T.R., Lennon, N., Castagno, M.S., Miller, F., and Richard, J. Variability of energy-consumption measures in children with cerebral palsy. J Pediatr Orthop. 1998; 18: 738–742.                                      
  3. Bratteby Tollerz, Linda U (12/2011). "Reliability of energy cost calculations in children with cerebral palsy, cystic fibrosis and healthy controls.". Acta paediatrica (Oslo, Norway : 1992) , 100 (12), p. 1616.
  4. Danielsson, Anna, et al. “Measurement of Energy Cost by the Physiological Cost Index in Walking After Stroke.” Archives of Physical Medicine and Rehabilitation, vol. 88, no. 10, 2007, pp. 1298–1303.
  5. Delussu, Anna Sofia, et al. “Concurrent Validity of Physiological Cost Index in Walking over Ground and during Robotic Training in Subacute Stroke Patients.” BioMed Research International, vol. 2014, 2014, pp. 1–6.
  6. Fredrickson, E., et al. (2007). Physiological Cost Index as a Proxy Measure for the Oxygen Cost of Gait in Stroke Patients.
  7. Graham, R. C., et al. (2005). "The Reliability and Validity of the Physiological Cost Index in Healthy Subjects While Walking on 2 Different Tracks." Archives of Physical Medicine and Rehabilitation 86(10): 2041-2046.
  8. Hagberg, K., Tranberg, R., Zügner, R., & Danielsson, A. (2011). Reproducibility of the Physiological Cost Index among Individuals with a Lower-Limb Amputation and Healthy Adults. Physiotherapy Research International, 16(2), 92–100.
  9. Liao, H.-F., et al. (2007). "Effectiveness of Loaded Sit-to-Stand Resistance Exercise for Children With Mild Spastic Diplegia: A Randomized Clinical Trial." Archives of Physical Medicine and Rehabilitation 88(1): 25-31.
  10. Vllasolli, T. O., Orovcanec, N., Zafirova, B., Krasniqi, B., Murtezani, A., & Rama, B. (2015). Physiological cost index and comfort walking speed in two level lower limb amputees having no vascular disease. Acta Informatica Medica, 23(1), 12-17.
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