Chapter 27 — Blood Gas and Oxygenation Monitoring

Third chapter of Part V. ORGANISING INSIGHT: on ECMO the two gases have been UNBUNDLED onto two knobs that do not interact — oxygen transfer is limited by haemoglobin presented to the membrane per minute (levers: blood flow and haemoglobin), carbon dioxide clearance by gas passing the other side (lever: sweep). Every common error is one gas treated with the other's knob. Corollary that inverts the intuition: the SWEEP is the on-off switch — ELSO Red Book Ch 25, 'irrespective of extracorporeal blood flow, once the sweep gas flow ceases, ECMO support effectively ceases'. SECOND STRUCTURAL CLAIM: a blood gas on ECMO is uninterpretable without its SITE — four distinct samples (pre-membrane, post-membrane, right radial, distal arterial) answering four different questions; the pre-membrane sample is a true mixed venous sample on VA and is NOT one on VV. Worked four-site case where three gases are reassuring and only the right radial describes the brain. KEY BEDSIDE RULES: the blender is not the oxygen knob (a working membrane saturates outlet blood on room air at 1-2 L/min sweep — keep FsO2 at 1.0 to preserve the post-membrane PO2 as a surveillance signal); pre-membrane saturation can only exceed arterial saturation in the setting of recirculation, no exceptions; correct hypercapnia over 4-8 h, not ten minutes; titrate the sweep to pH, not PaCO2; reduce the sweep, never the blood flow, when CO2 must rise; read the carboxyhaemoglobin line (the circuit manufactures CO by haem catabolism — over 3% points at haemolysis). Reproduces the published recirculation table showing a 59% loss of oxygen transfer with the console unchanged, and the 80 kg worked example in which Hb 11 to 14 g/dL removes 0.9 L/min of required flow. TWO CONTROVERSIES: transfuse versus raise flow (Bartlett's explicit recommendation, whose stated transfusion harms are a generation out of date — this book declines it and hands the threshold to Ch 46); and the arterial saturation target, where four primary sources give four non-identical figures, none traced to a primary study — VERIFICATION REQUIRED. SCOPE FINDING: a tracker query established that NO CHAPTER in the 92-chapter map owns extracorporeal carbon dioxide removal; Ch 3 or Ch 78 are the candidates and a decision is owed before Part XII. SEVEN PROBLEMS FLAGGED UNCORRECTED, including Ch 6's rated-flow inlet-saturation discrepancy now widened to three-way (65 / 70 / 75%), with the anchor physiological review disagreeing with itself inside one paper; the recurrence of Ch 6's Hufner-constant flag; and three delivery-to-consumption ratios now standing in this book (5:1 normal, 3:1 ECMO floor, 2:1 from Ch 6's verified source).