Is Timing Everything?

Thank you Ben and AmSECT for organizing this conference. And thank you all for showing up on Saturday morning. My objectives for this presentation are to review the current knowledge of time spent below critical thresholds and more specifically the relationship between low oxygen delivery and organ function following cardiopulmonary bypass (CPB). Additionally, we will investigate the time–dose relationship between goal-directed perfusion (GDP) parameters and acute kidney injury (AKI). Here are my disclosures. [Slide: I have received speaker honorariums from LivaNova. I have no conflicts of interest for this presentation]. I did not conspire with Bob (Groom) to bring up Galletti, but it’s important for us to remember our history, otherwise, as they say, we’re doomed to repeat it. Galletti, was the first textbook on CPB published in 1962. In that book, there is an entire chapter on perfusion adequacy. In the summation of that chapter, he says that “the ultimate aim of perfusion is to provide satisfactory tissue gas exchange” (1). What we’re measuring with GDP is tissue gas exchange. The other quote from Galletti that I didn’t show, from that same chapter, is that each organism is in their own control. Each organism, if we measure respiratory tissue gas exchange on bypass, will tell us exactly what they need or define their own unique critical threshold for oxygen delivery. How do we describe adequate perfusion? In conventional terms, many of us were taught and practice to include multiple facets of not only delivery of blood flow but also the patient’s response to perfusion. That may include elements such as venous saturation monitoring or venous pO2 monitoring. It certainly includes maintaining an adequate pH during CPB, arterial pCO2 measurement, and monitoring blood flow or cardiac index, typically in the range of 2.2–2.6 during normothermia. Some adjustment may be based on temperature. As we saw in the audience response survey, some of us even include lactate monitoring in that measure of perfusion adequacy. When we discuss GDP, many perfusionists are initially worried that they have to give up everything that they have practiced as an adequate measure of perfusion, and that’s not what this concept is about. GDP is about enhancing our current knowledge of adequate perfusion and using respiratory-based parameters to give us a whole new level of detail of what’s happening to the patient on a tissue level. In GDP, we take conventional terms of perfusion adequacy and we add to it concepts of cellular respiration, including oxygen delivery index (DO2i), oxygen consumption index (VO2i), and carbon dioxide production index (VCO2i). Instead of isolated elements like blood flow, hematocrit, and global measures of oxygen utilization like SvO2 and PvO2, we now describe blood flow as DO2i, the product of blood flow and hemoglobin. We can monitor how that patient is using the delivered oxygen as the oxygen consumption index (VO2i). And more importantly, the end product of that oxygen utilization the carbon dioxide production index (VCO2i), which includes elements of aerobic and anaerobic respiration (Table 1). Equally importantly in our management is the relationship of these different parameters to each other. Important relationships in GDP include the oxygen delivery index to carbon dioxide production index ratio (DO2i/VCO2i). The DO2i/VCO2i ratio tells us if the oxygen delivery is adequate for the current metabolic rate of the patient. We also look at the oxygen consumption to oxygen delivery index or the oxygen extraction ratio (VO2i/DO2i). How much of that delivered oxygen is the patient removing and using. A new concept for many of us, but one that Dr. Rivers pointed out earlier in his work on resuscitation of patients, is the respiratory quotient. The respiratory quotient is the ratio of CO2 production index to the oxygen consumption index (VCO2i/VO2i). And as we’re learning more about Address correspondence to: George Justison, CCP, University of Colorado Hospital, Perfusion Services, 12605 E. 16th Avenue, Aurora, CO 80205. E-mail: [email protected]