32, 84, 10, 26
There is a ceiling on what the sweep can buy, and it arrives earliest at low blood flow — at 2 L/min of blood flow a gas-to-blood ratio of four is a sweep of 8 L/min, which many units reach during weaning or in a carbon dioxide removal configuration. Past that ceiling the knob still turns and the flowmeter still rises while nothing further is cleared, and the team discovers this at the worst possible moment. The ratio is what matters and it is displayed nowhere, so it has to be calculated from the sweep and the blood flow recorded on the same line. When the ratio approaches four, escalate the enquiry rather than the knob, and consider raising the blood flow instead where the patient allows it.
Within-circuit comparison across ratios
DOI 10.1177/0267659120906767; PMID 32397880
Systematic variation of the gas-flow-to-blood-flow ratio and of the pre-oxygenator carbon dioxide tension
ABSTRACT READ ONLY; full text not retrieved. Ex vivo, a single device, two oxygenators in series in a closed circuit — no patients and no clinical endpoint. The ratio of 4.0 is a property of this oxygenator in this configuration and should not be transferred to another device as a number; what transfers is the existence of a ceiling and the mechanism (rising dead space fraction) behind it. Published in a journal supplement.
Carbon dioxide transfer rose with gas flow and PLATEAUED above a gas-to-blood flow ratio of about 4.0. Exhaust carbon dioxide rose linearly with the carbon dioxide tension delivered to the device, with the slope of that line falling at high ratios. OXYGENATOR DEAD SPACE FRACTION WAS 0.3 TO 0.4 AT LOWER RATIOS AND ROSE TO 0.8 TO 0.9 AT RATIOS ABOVE 4.0. The authors conclude that carbon dioxide clearance within this oxygenator is limited at high ratios and that the limitation correlates with a rising dead space fraction, with implications for patients requiring high levels of support.
Ex vivo only — no patients. A closed blood-primed circuit with two adult Maquet Quadrox-iD oxygenators in series on a Maquet PLS Rotaflow pump; the first oxygenator used to set the carbon dioxide tension delivered to the second, sweep flows of 100 percent oxygen varied to give different ratios of gas flow to blood flow.