Capnographic indices

Measurable capnographic waveform deformation using shape indices, allows asthma assessment.

Shape indices

In a previous study, several capnographic indices were established and validated focusing on main components of capnogram deformation :
     
  •  Modifications in E2 and E3 are evaluated by calculating their respective slopes ("slopes" S1, S2 et S3),
  • Opening of angle Q is evaluated by calculating S1/S2 ratio ("slope ratio" SR) ,
  • Blunting of Q is evaluated by :
    • calculation of A1/A2 ratio ("area ratio" AR),
    • calculation of mean second derivative (curvature indice) of the capnographic curve in this zone ("second derivative" SD1-2-3).

 

 

Main capnographic phasesCapnographic phases

Shape indices in a normal trace (above) and abnormal (below)Capnographic indices

Indice validation

In order to measure reproducibility and validity of these indices in asthma assessment, two successive measurements (10 minute interval) of indices then a forced expiratory spirometry were performed in 10 healthy subjects and 30 asthmatic patients.

Reproducibility of each indice was evaluated by calculating the noise/signal ratio Vi/VI (intra-individual /inter-individual variabilities). Good overall results were found (25-35%) except for later indices (SD3 and S3) which appear to be artefact dependent.

Correlations to FEV1 are very good (r=.73-.93) for all indices (p<.001 in all cases).

S1

S2

S3

SR

AR

SD

Vi/VI %

35.1

35.2

61.7

28.2

23.3

24.9

FEV1 % Pred

0.73

0.89

0.90

0.89

0.85

0.93

Reproduceability and correlations of capnographic indices with FEV1.


Mean values of capnographic indices in control subjects and in asthmatic patients
There are significant differences (p<.001) between capnographic indice values in healthy and asthmatic subjects. There are also differences between three subgroups of asthmatic patients classified according to spirometric criteria (FEV1%Pred >80, 40-80, <40).
This confirms the quantitative character of capnography in bronchospasm assessment.

Controls

Asthmatics

Total asthm.

FEV1>80%

FEV1 40-80

FEV1<40%

n=10

n=30

n=10

n=10

n=10

FEV1 %P

104 ±7.7

***

62 ±26

89 ±7.1

***

67 ±10

***

29 ±6.4

S1

15.0 ±1.8

***

11.4 ±2.3

11.4 ±2.3

*

11.5 ±1.2

**

9.3 ±1.7

S2

0.23 ±0.06

***

0.55 ±0.3

0.31 ±0.1

ns

0.45 ±0.2

***

0.90 ±0.2

S3

0.16 ±0.05

***

0.35 ±0.2

0.18 ±0.04

ns

0.28 ±0.1

**

0.59 ±0.2

SR

1.6 ±0.05

***

5.5 ±3.9

2.4 ±0.9

*

4.0 ±2

**

10.1 ±3

AR

88 ±3

***

78 ±9

86 ±6

*

79 ±6

**

20.0 ±5.6

SD

18.2 ±2

***

11.2 ±4.3

15.3 ±1.35

*

12.2 ±2.5

***

6.5 ±1.9

FEV1 and capnographic indices in control and asthmatic subjects


In this particular study, SD2 is the indice which best correlates with spirometric measurements.

Relationship SD2 vs FEV1

FEV1 / SD2 relationship

Conclusion

Measurement of capnographic indices allows reliable assessment of bronchospasm with specific advantages : non-invasiveness and effort-independency.