Comparison of End-Tidal PCO2 and Average Alveolar Expired PCO2 During Positive End-Expiratory Pressure

Field: Medicine (Health Sciences)

I am grateful for the careful reading of two of our papers [1,2] by Dr. Fletcher. He raises several important issues, which I address in order. In his review of the literature, he found an average anatomical dead space (VDANA) for dogs of approximately 4.0 +/- 0.6 mL/kg (mean +/- SD). He questions the higher values for VDANA that we reported in our previous articles (8.3 and 6.7 mL/kg, respectively). We have reexamined scale diagrams drawn from the actual experimental setup. Placement of the Fleisch pneumotachometer between the proximal end of the endotracheal tube (ETT) and the capnometer T-piece sampling site added 68 mL of instrumental dead space volume between the CO2 measuring site and the ETT. Therefore, the effective VDANA calculated at the proximal end of the ETT for those articles are 5.3 and 3.6 mL/kg, values similar to the average value (4.0 mL/kg) quoted by Dr. Fletcher. In other words, the modified Fowler analysis of the CO2 expirogram to determine VDANA depends on the capnometer sampling site. Figure 1b displays the CO2 spirogram [3] (plot of PCO2 versus volume) during inspiration (dashed data line) and expiration (solid data line). Volume is measured as the time integral of flow. The area under the dashed line (inspiration) is the inspired CO (2) volume, representing CO2 from the previous expiration in the tubing between the capnometer sampling site and the connection of the inspiratory limb to the Y connector.Figure 1: Determination of the CO2 volume eliminated per breath (VCO2,BR) and the Fowler anatomical dead space (VDANA). a, Airway flow (thin line) and PCO2 (dashed line, corrected for transport delay and dynamic response) were plotted against time for one expiration and inspiration. The multiplication and integration of flow and PCO2 during expiration generated the volume of exhaled CO2 (VCO2,E heavy line). VCO2,I is the inspired CO2 volume during the first portion of inspiration, representing the CO2 remaining from the previous expiration (see arrow) in the instrumental dead space. The difference between VCO (2),E and VCO2,I was the overall pulmonary CO2 elimination for the respiratory cycle (VCO2,BR). Expiration was positive airway flow, and inspiration was negative flow. b, The same data were displayed as a CO2 spirogram, in which airway PCO (2) was plotted against tidal volume change (time integral of airway flow). VCO (2),E and VCO2,I are the areas under the curve during expiration (Exp, solid line) and inspiration (Insp, dashed line), respectively, and the difference in areas was overall VCO2,BR (stippled area). The modified Fowler anatomical dead space (VDANA) was line FE, where line BE was positioned so that triangle DBC equaled triangle DEF. Modified from [3] with kind permission from Kluwer Academic Publishers.As noted by Dr. Fletcher, to generate an accurate CO2 spirogram, the flow and PCO2 waveforms must be temporally synchronized (Figure 1a). During sidestream capnography, the raw measured PCO2 waveform must be advanced in time by transport delay (TD), the time required to suction gas through the sampling tube [4]. We measure TD in a bench apparatus [5]. Because TD can change due to variation in capnometer suction rate or changes in connection volume, we also determine in vivo an adjusted TD by forcing VCO2,BR/VT (VCO2,BR-digital measurement of CO2 volume exhaled per breath, VT-tidal volume) equal to a simultaneous measurement of PCO2 in mixed expired gas [4,6]. In the modified Fowler determination of VDANA (Figure 1b) [1,2,7,8], line BE is positioned so that the areas of triangles DBC and DEF are equal; VDANA is given by segment FE. Note that if the capnometer site is moved closer to the lung, the entire VCO2,BR (stippled area) will move to the left, with an apparent decrease in VDANA. At the same time, however, inspired VCO2 increases (area under the dotted data curve) due to CO2 rebreathing relative to the capnometer sampling site from the unchanged instrumental dead space. In contrast, moving the capnometer sampling site closer to the Y connector (and inspiratory limb) includes more of the instrumental dead space in the measurement of VDANA. However, regardless of the capnometer sampling site, all instrumental dead space contributes to wasted ventilation. Because the capnometer sampling site is usually positioned as close as possible to the lungs to collect the last and "best" alveolar PCO2, the smallest VDANA will be measured. Only the CO2 inspirogram (which is traditionally not included in the modified Fowler determination of VDANA) detects the CO2 rebreathing that occurs from the instrumental dead space and allow proper estimation of overall CO2 elimination (VCO2,BR) [3]. Finally, the careful measurement of VDANA provides insight into the effects of positive end-expiratory pressure (PEEP; 7.6 cm H2 O) to blunt the expiratory upstroke-alveolar plateau angle and increase the slope of the alveolar plateau of the CO2 expirogram. These effects on the CO2 expirogram were even more dramatic in another study [7] that applied larger values of PEEP (11 cm H2 O). In that study, we proposed that PEEP caused an increase in VDANA, which resulted in less penetration of tidal volume into the alveolar gas [9], with a smaller interface and consequent less diffusive mixing, generating steeper and less defined alveolar plateaus. Peter H. Breen, MD, FRCPC Department of Anesthesiology; University of California-Irvine; University of California-Irvine Medical Center; Orange, CA 92868