Hemodynamic coherence and the rationale for monitoring the microcirculation

Abstract

This article presents a personal viewpoint of the shortcoming of conventional hemodynamic resuscitation procedures in achieving organ perfusion and tissue oxygenation following conditions of shock and cardiovascular compromise, and why it is important to monitor the microcirculation in such conditions. The article emphasizes that if resuscitation procedures are based on the correction of systemic variables, there must be coherence between the macrocirculation and microcirculation if systemic hemodynamic-driven resuscitation procedures are to be effective in correcting organ perfusion and oxygenation. However, in conditions of inflammation and infection, which often accompany states of shock, vascular regulation and compensatory mechanisms needed to sustain hemodynamic coherence are lost, and the regional circulation and microcirculation remain in shock. We identify four types of microcirculatory alterations underlying the loss of hemodynamic coherence: type 1, heterogeneous microcirculatory flow; type 2, reduced capillary density induced by hemodilution and anemia; type 3, microcirculatory flow reduction caused by vasoconstriction or tamponade; and type 4, tissue edema. These microcirculatory alterations can be observed at the bedside using direct visualization of the sublingual microcirculation with hand-held vital microscopes. Each of these alterations results in oxygen delivery limitation to the tissue cells despite the presence of normalized systemic hemodynamic variables. Based on these concepts, we propose how to optimize the volume of fluid to maximize the oxygen-carrying capacity of the microcirculation to transport oxygen to the tissues.

Figures

Figure 1

Microcirculatory alterations associated with loss of hemodynamic coherence. Microcirculatory alterations underlying the loss of hemodynamic coherence between the macrocirculation and the microcirculation resulting in tissue hypoxiaType 1: heterogeneous perfusion of the microcirculation as seen in septic patients with obstructed capillaries next to perfused capillaries resulting in a heterogeneous oxygenation of the tissue cells. Type 2: hemodilution with the dilution of microcirculatory blood resulting in the loss of RBC-filled capillaries and increasing diffusion distance between RBCs in the capillaries and the tissue cells. Type 3: stasis of microcirculatory RBC flow induced by alterd systemic variables (e.g. increased arterial vascular resistance(R) and or increased venous pressures causing tamponade 4 alterations involve edema caused by capillary leak syndrome and which results in increased diffusive distance and reduced ability of the oxygen to reach the tissue cells. Red, well-oxygenated RBC and tissue cells; purple, RBC with reduced oxygenation; blue, reduced tissue cell oxygenation

Figure 2

Typical sublingual microcirculatory images taken with a Cytocam IDF hand-held microscope during cardiac surgery showing how the administration of colloid causes volume expansion while a crystalloid solution does not. a Sublingual microcirculation during cardiac surgery with crystalloid 0.9% NaCl as priming solution during cardiopulmonary bypass. b Sublingual microcirculation during cardiac surgery with HES as priming solution during cardiopulmonary bypass. Images show that the use of HES results in more volume expansion as indicated by the increased distance between the RBCs in the capillaries as would be expected from a colloid, in comparison with a in which crystalloid was used in the pump where it is expected that the crystalloid solution equilibrates more rapidly with the tissues less affecting intravascular volume status.

Figure 3

Microcirculatory-guided fluid therapy. To optimize the oxygen-carrying capacity of the microcirculation, optimization is required of the convective (sufficient flow) and diffusive capacity (optimal FCD to have short diffusion distances between the oxygen-carrying RBCs and the tissue cells). Observation of sublingual microcirculation using hand-held microscopy in states of hypovolemia identifies low convective flow (left), indicating the need for fluid administration. Microcirculatory fluid responsiveness indicates the success of fluid therapy by showing enhanced convective RBC flow. A reduction in FCD signals the occurrence of a type 2 microcirculatory alteration (right) and this indicates that too much fluid has been administered, causing increased diffusion distance between the RBCs and tissue cells reducing the oxygen transport capacity of the microcirculation. This approach provides a personalized physiological-based patient-centered fluid resuscitation strategy to optimize the oxygen-carrying capacity of the microcirculation. Adapted from [89].