Resuscitation in the ICU: Small Vessels, Big Targets
By Christian S. Guay, MD, from the IARS, AUA and SOCCA 2019 Annual Meetings*
The advent of bedside tools to measure microcirculatory variables and their association with clinical outcomes has inspired intensivists to reconsider fundamental principles and assumptions of resuscitation. Dr. Ashish Khanna and Dr. Kunal Karamchandani explore the rapidly evolving field of macro and microcirculation in resuscitation, followed by an update on sepsis delivered by Dr. Mark Nunnally.
Dr. Khanna started with an overview of clinical trials targeting different mean arterial pressures (MAP) in critically ill patients. A recurring theme of his presentation was the difficulty of precisely conducting a blood pressure target trial in the dynamic context of critical illness. Nonetheless, he presented persuasive data suggesting that the traditional MAP goal of 65 mmHg may be too low for some patients, who would require MAP goals as high as 85 mmHg. From a personalized medicine perspective, he noted that optimal blood pressure targets most likely exhibit significant variance between patients, depending on their physiological and pathological contexts. For example, an otherwise healthy 30 year old may have a lower optimal MAP goal in critical illness compared to a chronically hypertensive 80 year old. Dr. Khanna also noted the importance of integrating measures of pressure (e.g., MAP) with measure of flow (e.g., microcirculatory perfusion), providing a seamless transition to Dr. Karamchandani’s discussion of microcirculation and shock.
The microcirculation operates at the scale of micrometers, consisting of arterioles, capillaries and venules. From a physiological perspective, microcirculatory flow is the endgame of cardiac output, providing the substrates for cellular metabolism. Using sophisticated bedside video microscopy techniques such as orthogonal polarization spectral imaging (OPI) and side-stream dark field imaging (SDF), clinicians can visualize the microcirculation and measure the proportion of perfused vessels at this level. Impaired microcirculatory perfusion has been observed in both sepsis and cardiogenic shock, and the proportional perfusion of these small vessels has been used to successfully discriminate between survivors and non-survivors of sepsis. Despite the biological plausibility and outcome data, meaningful interventions targeting the microcirculation are lacking. Furthermore, the relationship between traditional measures of the macrocirculation such as MAP and emerging measures of microcirculatory perfusion is still a topic of active investigation and debate. Future resuscitation strategies may include both macro and microcirculatory monitoring to personalize care in an era of precision medicine.
Dr. Nunnally concluded the education session with an update on sepsis. The overarching theme of his presentation was that our traditional principles of sepsis still apply today; namely cultures, fluids, antibiotics and source control. He further emphasized that the updated 2016 guidelines include a one-hour bundle of lactate, blood cultures, broad-spectrum antibiotics, fluid resuscitation and vasopressors. This one-hour bundle effectively combines the previous three- and six-hour bundles, consistent with the notion that sepsis is considered a medical emergency. This notion is also reflected in the updated 2016 definitions of sepsis, which center on organ dysfunction in the setting of infection. Dr. Nunnally concluded by acknowledging the challenges of implementing sepsis guidelines at the bedside: “aspirations and operations do not always match up.” Nonetheless, incrementally improving outcomes in sepsis are a testament to the research enterprise and quality of care provided to these critically ill patients.
*Coverage of SOCCA: Education Session II: Beyond the Standard: New Resuscitation Paradigms in Critical Care, presented by Ashish Khanna, MD, Kunal Karamchandani, MD, and Mark Nunnally, MD during the SOCCA 2019 Annual Meeting