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# Normal Capnogram The capnographic waveform displays plots of $CO_2$ concentration against time (time-based capnography) or against exhaled volume (volumetric capnography), providing information of greater clinical utility. ## Time-Based Capnography A typical single-breath time-based capnogram h...

# Normal Capnogram The capnographic waveform displays plots of $CO_2$ concentration against time (time-based capnography) or against exhaled volume (volumetric capnography), providing information of greater clinical utility. ## Time-Based Capnography A typical single-breath time-based capnogram has four phases. * **Phase I:** Inspiration occurs, $PCO_2$ is zero. * **Phase II:** After dead space is cleared, $PCO_2$ progressively rises. This phase represents a mixture of alveolar gas and air. * **Phase III:** The alveolar plateau is associated with a slight rise in $CO_2$ due to variations in ventilation/perfusion ($V/Q$) ratios in the lung. Alveoli with lower $V/Q$ ratios have a longer emptying time, leading to a continuous upward trend. End-tidal $PCO_2$ ($PETCO_2$) is reached at the peak of this phase. * **Phase IV:** $PCO_2$ declines sharply towards zero as fresh gas is inhaled. Two angles are important indicators: * **Alpha Angle:** The angle between phases II and III is related to variations in time constants within the lung, indirectly indicating V/Q status. * **Beta Angle:** The angle between phases III and IV helps determine the presence and extent of rebreathing. The horizontal baseline of phase 0 and phase I can be elevated above normal (Figure 7-1). ## Volume-Based Capnography (Volumetric Capnography) The waveform is similar to time-based capnography. * **Phase I:** Gas from the dead space and tubes. * **Phase II:** A mixture of alveolar gas and air. * **Phase III:** Primarily contains $CO_2$ from alveoli, creating an alveolar plateau. * **Phase IV:** Similar to time-based phase IV. **Figure 7-1** shows $CO_2$ concentration plotted against time (time-based capnography). $PCO_2$ is the partial pressure of $CO_2$ in the expired gas, while $PETCO_2$ is the partial pressure of end-tidal $CO_2$. **Figure 7-1** also includes a graph depicting the different phases (I-IV) and relevant parameters against time. A horizontal axis labelled "Time" is present as well. The graph shows "PCO₂" or partial pressure of $CO_2$ being plotted against the "Time" with relevant markings indicating the phases, expiratory upstroke, alveolar plateau and inspiratory down stroke. Importantly, a respiratory baseline, specifically shown as "Respiratory baseline (should be 0 mm Hg)" is marked on the graph to provide context.