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Comment
. 2016 Jul;117(1):109-17.
doi: 10.1093/bja/aew127.

Capnogram Slope and Ventilation Dead Space Parameters: Comparison of Mainstream and Sidestream Techniques

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Free PMC article
Comment

Capnogram Slope and Ventilation Dead Space Parameters: Comparison of Mainstream and Sidestream Techniques

A L Balogh et al. Br J Anaesth. .
Free PMC article

Abstract

Background: Capnography may provide useful non-invasive bedside information concerning heterogeneity in lung ventilation, ventilation-perfusion mismatching and metabolic status. Although the capnogram may be recorded by mainstream and sidestream techniques, the capnogram indices furnished by these approaches have not previously been compared systematically.

Methods: Simultaneous mainstream and sidestream time and volumetric capnography was performed in anaesthetized, mechanically ventilated patients undergoing elective heart surgery. Time capnography was used to assess the phase II (SII,T) and III slopes (SIII,T). The volumetric method was applied to estimate phase II (SII,V) and III slopes (SIII,V), together with the dead space values according to the Fowler (VDF), Bohr (VDB), and Enghoff (VDE) methods and the volume of CO2 eliminated per breath ([Formula: see text]). The partial pressure of end-tidal CO2 ([Formula: see text]) was registered.

Results: Excellent correlation and good agreement were observed in SIII,T measured by the mainstream and sidestream techniques [ratio=1.05 (sem 0.16), R(2)=0.92, P<0.0001]. Although the sidestream technique significantly underestimated [Formula: see text] and overestimated SIII,V [1.32 (0.28), R(2)=0.93, P<0.0001], VDF, VDB, and VDE, the agreement between the mainstream and sidestream techniques in the difference between VDE and VDB, reflecting the intrapulmonary shunt, was excellent [0.97 (0.004), R(2)=0.92, P<0.0001]. The [Formula: see text] exhibited good correlation and mild differences between the mainstream and sidestream approaches [0.025 (0.005) kPa].

Conclusions: Sidestream capnography provides adequate quantitative bedside information about uneven alveolar emptying and ventilation-perfusion mismatching, because it allows reliable assessments of the phase III slope, [Formula: see text] and intrapulmonary shunt. Reliable measurement of volumetric parameters (phase II slope, dead spaces, and eliminated CO2 volumes) requires the application of a mainstream device.

Keywords: capnography; carbon dioxide; intraoperative monitoring; mechanical ventilation; ventilation-perfusion ratio.

Figures

Fig 1
Fig 1
Representative time (top) and volumetric (bottom) mainstream (continuous traces) and sidestream (dashed traces) capnograms.
Fig 2
Fig 2
Sidestream capnogram curve (continuous line, left axis) together with the flow in the sampling tube (dashed line, right axis) in a representative patient.
Fig 3
Fig 3
Correlations between the phase III slopes in the time (a, SIII,T,MS vs SIII,T,SS) and volumetric domains (c, SIII,V,MS vs SIII,V,SS) obtained by mainstream (horizontal axis) and sidestream capnography (vertical axis), with the regression lines (dashed) and the lines of identity (continuous). Regression equations: SIII,T,SS=0.142+0.996·SIII,T,MS and SIII,V,SS=5.09+0.93·SIII,V,MS. The corresponding Bland–Altman plots are demonstrated on the right for the time (b, SIII,T,MS vs SIII,T,SS) and volumetric (d, SIII,V,MS vs SIII,V,SS) capnograms. The means of differences are −0.019 kPa s−1 and −0.55 kPa litre−1 (continuous), and the limits of agreement are 0.06 kPa s−1 and 0.63 kPa litre−1 (dashed) for the time and volumetric capnograms, respectively. Each data point represents one expiration. SIII,t,ms, phase III slope of time capnogram; SIII,v,ms, phase III slope of volumetric capnogram; SII,t,ms, phase II slope of time capnogram; SII,v,ms, phase II slope of volumetric capnogram; Vdf,ms/Vt, normalized Fowler dead space; Vdb,ms/Vt, normalized Bohr dead space; Vde,ms/Vt, normalized Enghoff dead space; Vs,ms/Vt, normalized difference between the Enghoff and Bohr dead spaces; Vt, tidal volume.
Fig 4
Fig 4
Correlations between phase II slopes in the time (a, SII,T,MS vs SII,T,SS) and volume domain (b, SII,V,MS vs SII,V,SS), angles α (c, αMS vs αSS) and Fowler's dead space indices (d, VDF,MS/VT vs VDF,SS/VT) obtained by mainstream (horizontal) and sidestream (vertical) capnography, with the regression lines (dashed) and the lines of identity (continuous). Regression equations: SII,T,SS=39+0.298·SII,T,MS, SII,V,SS=140.2+0.1·SII,V,MS, αSS =−0.84+1.05·αMS, and VDF,SS/VT=−22.3+1.52·VDF,MS/VT. Each data point represents one expiration.
Fig 5
Fig 5
Correlation between normalized dead space indices calculated according to Bohr (a, VDB,MS/VT vs VDB,SS/VT) and Enghoff (b, VDE,MS/VT vs VDE,SS/VT), and their difference [c, Vs,MS/VT=(VDE,MSVDB,MS/VT) vs Vs,SS/VT=(VDE,SSVDB,SS)/VT] obtained by mainstream (horizontal) and sidestream (vertical) capnography, with the regression line (dashed) and the line of identity (continuous). Regression equations: VDB,SS/VT=0.11+0.82·VDB,MS/VT, VDE,SS/VT=0.12+0.86·VDE,MS/VT, and Vs,SS/VT=0.034+0.774·Vs,MS/VT. (d) The corresponding Bland–Altman plot is demonstrated for Vs,MS/VT and Vs,SS/VT. The mean of differences is 6.17 (continuous), and the limit of agreement is 24.2 (dashed). Each data point represents one expiration.
Fig 6
Fig 6
Relative differences in dead space and pulmonary shunt parameters obtained with the two methods in patients with the lower (L, compliance <37 ml cm H2O−1) and higher (H, compliance >53 ml cm H2O−1) quartiles and medium (M) interquartile range. *P<0.05 vs H; #P<0.05 vs M.

Comment on

  • Respiratory dead space.
    FOWLER WS. FOWLER WS. Fed Proc. 1948 Mar;7(1 Pt 1):35. Fed Proc. 1948. PMID: 18932700 No abstract available.

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