Dynamic and Static Cardiac Function Measurements During Sepsis

What is the difference between dynamic and static cardiac function measurements during sepsis, and how should these measurements be used?A Alexander P. Johnson, MSN, RN, ACNP-BC, CCNS, CCRN, Tara Fallon, BSN, RN, and Deborah Thorgesen, MSN, RN, NEA-BC, reply:The move to dynamic rather than static measures of cardiac function represents a paradigm shift in hemodynamics and fluid resuscitation. Best practices for monitoring are transitioning from pressure-based parameters to flow-based parameters, and from static to dynamic variables. Many of us were trained to use static variables, such as blood pressure (BP), heart rate, and urine output. These parameters indirectly correlate with cardiac output, but because they are slow to change owing to compensatory mechanisms, they can be misleading or can be late signs of patient status. On the other hand, dynamic variables, such as stroke volume (SV) and SV variation, are more closely associated with cardiac output, are more sensitive to changes in flow, and are much earlier and more accurate signs of changes in cardiac output and fluid responsiveness.Have you ever noticed that a patient’s BP may remain within a normal range until moments before a code? This phenomenon is due to compensatory mechanisms that occur when BP drops. In hypovolemia, our physiological capacity for increased heart rate, extraction of more oxygen from hemoglobin, and vasoconstriction is so advanced that BP can remain normal until the latest stages of compensation. This ability makes BP, a static parameter, a poor predictor of fluid responsiveness.Stroke volume, however, is a sensitive and evidence-based dynamic parameter that can indicate whether cardiac output will improve with a fluid challenge. When blood volume decreases, the first physiological consequence is a drop in left ventricle filling, evidenced by a decrease in SV. Changes in other parameters associated with this decrease in SV are secondary and compensatory. Conversely, an SV (dynamic parameter) increase of 10% or more after a fluid challenge suggests that the patient is fluid responsive, even if the BP (a static parameter) changes little or not at all.In 10 randomized controlled trials,1–10 an improvement in SV by 10% or more after a fluid challenge was associated with a significant improvement on patient outcomes, independent of the size of the bolus or the type of fluid used. Outcomes with SV optimization included shorter hospital stay, decreased use of vasopressors, lower lactate levels, and fewer complications. The impact of SV optimization is now being observed in cases of sepsis. In 2020, Douglas et al11 reported on the FRESH (Fluid Responsiveness Evaluation in Sepsis-associated Hypotension) trial, a sepsis resuscitation study that also showed improved patient outcomes when fluid resuscitation was guided by evaluation of SV. In this study, investigators used the passive leg raise (PLR) maneuver to guide SV optimization and predict fluid response (Figure 1). This study also demonstrated reductions in ventilator use (17.7% vs 34.1% in the usual care group), renal replacement therapy (5.1% vs 17.5%), and diuretic use in the intervention group.11 Interestingly, the intervention group, in which SV response was measured by the PLR result, also achieved a lower mean fluid balance compared with the usual care group. This combination of improved outcomes and lower positive fluid balance was achieved because patients in the SV-PLR protocol group received the right amount of fluid at the right times, based on their unique physiological needs, which was made possible through the use of dynamic variables.Dynamic variables are an important consideration in sepsis, because sepsis is a time-sensitive condition and decreases in preload can increase the risk of cellular dysfunction, tissue hypoxia, and end-organ damage. The Surviving Sepsis Campaign recently published new evidencebased guidelines for the management of severe sepsis and septic shock and now recommends the use of dynamic variables to guide fluid resuscitation.15 The 30-mL/kg fluid challenge is still suggested for sepsis-induced hypoperfusion or septic shock; however, it is not intended to be a one-size-fits-all approach. Many patients will require more or less fluid, and SV and dynamic parameters should be used to determine how much more or less is required (Figure 2). Without SV monitoring, we will not know whether to stop or continue fluid challenges or how to time fluid boluses appropriately.Monitoring stroke volume can demystify fluid resuscitation and help us avoid common errors such as administering less fluid to patients with a history of heart failure (HF) or acute renal failure (ARF). In fact, studies are now showing that sepsis patients with HF or ARF who receive 30 mL/kg of fluid may actually have higher survival rates than patients without these conditions and that fluid boluses are not associated with an increased risk of intubation.17,18 Preexisting conditions such as HF and ARF do not reliably predict a patient’s pressure-volume curve, which is based on their individual ventricular function and affected by compensatory mechanisms.Stroke volume optimization customizes the amount of fluid administered according to that patient’s individual cardiac performance at a specific point in time. The PLR mimics a fluid bolus, so if this maneuver results in a significant change in cardiac output, the patient is likely to respond to further fluid resuscitation. In Figure 2, the steep portion of the volume-flow curve on the left indicates a fluid-responsive state; administration of fluid will result in an increase in SV based on current cardiac function. As the patient undergoes resuscitation, their status moves toward the right side of the curve, which is more flat than steep, indicating that additional fluid will not affect the SV. It is crucial to reevaluate patients to determine their position on the curve over time. If the PLR maneuver or a fluid bolus does not result in an increase in SV, administration of inotropes or titration of vasopressors may be the best alternative. However, these decisions should be based on the procedures’ ability to improve SV response.All fluid challenges we give are ultimately designed to improve the flow of blood, oxygen, and nutrients to the tissues by optimizing SV. Using dynamic measures of cardiac function empowers clinicians to take a more proactive approach to fluid resuscitation to ensure adequate SV and optimize cardiac output. Ultimately, this approach is more likely to result in optimal patient outcomes than waiting to act until the appearance of late signs of patient changes, such as a drop in BP.