Volumetric capnography: lessons from the past and current clinical applications

Abstract

Dead space is an important component of ventilation-perfusion abnormalities. Measurement of dead space has diagnostic, prognostic and therapeutic applications. In the intensive care unit (ICU) dead space measurement can be used to guide therapy for patients with acute respiratory distress syndrome (ARDS); in the emergency department it can guide thrombolytic therapy for pulmonary embolism; in peri-operative patients it can indicate the success of recruitment maneuvers. A newly available technique called volumetric capnography (Vcap) allows measurement of physiological and alveolar dead space on a regular basis at the bedside. We discuss the components of dead space, explain important differences between the Bohr and Enghoff approaches, discuss the clinical significance of arterial to end-tidal CO2 gradient and finally summarize potential clinical indications for Vcap measurements in the emergency room, operating room and ICU.

Fig. 1

Riley three compartment model. Compartment A: shunt = perfused but not ventilated alveolae (V/Q = 0). Compartment B: ideal condition. Compartment C: dead space = ventilated but not perfused alveolae (V/Q = ∞). VDaw airway dead space, VDalv alveolar dead space, VDphys the sum of airway and alveolar dead space

Fig. 2

Concentration of CO₂ during a tidal expiration. Phase I: beginning of expiration; expired gas represents contents of the conduction compartment of the respiratory system. Phase II: transition between anatomic and alveolar dead space. Phase III: alveolar gas. Expired FCO ₂ (%) fraction of expired CO₂, SIII slope of phase III, VTCO ₂ ,br CO₂ elimination per breath

Fig. 3

Fletcher approach for evaluating expired gases. The shaded area is the total dead space for the breath. Area z (area to the left of the solid line) is the airway dead space (VDaw), area y (area above the slope of phase III) is the alveolar dead space (VDalv, in this case as per Enghoff). As per the Fowler approach [2], area q is equal to area p. Area x (area under capnogram curve) is the volume of CO₂ expired per breath (VTCO2,br)

Fig. 4

Volumes identified with volumetric capnography (based on Tang et al. [19]). The line a–b defines equal area q and p as in Fig. 3. The line c–d is created so that area A equals area B. The distance from b to d defines alveolar dead space (VDalv). Tang et al. did their analysis with the Enghoff approach which uses PaCO₂ instead of PACO₂ as in the Bohr approach. If PACO₂ were used instead the line c–d would be more to the left and the value of VDalv smaller. VDaw is the anatomical dead space, VDalv is the alveolar dead space, VDphys is the physiological dead space, VTalv-eff is the efficient alveolar tidal volume, VTalv is the alveolar tidal volume, VT is the tidal volume

Fig. 5

Difference between the Bohr approach and Enghoff approach. VDaw is the anatomical dead space, VDalv is the alveolar dead space, PACO ₂ is the alveolar partial pressure of CO₂, PaCO ₂ is the arterial partial pressure of CO₂, PĒCO ₂ is the mixed expired partial pressure of CO₂

Fig. 6

Schematic representation of three-compartment lung model, showing specific indices of capillary, alveolar and global efficiency of gas exchange. See text for abbreviations