Shock Syndromes I

Microcirculatory blood flow

The microcirculation consists of arterioles, capillaries, and venules and is where oxygen

release to the tissues occurs.

ii.

Traditional resuscitation strategies have focused on hemodynamic and Do2 end points (the

“macrocirculation”), but the microcirculation plays a key role in tissue oxygenation in shock

(particularly in septic shock) and has historically been overlooked.

iii.

Of importance, microcirculatory blood flow and Do2 cannot be predicted by global

(macrocirculatory) hemodynamics.

iv.

Microcirculatory blood flow can be visualized with orthogonal polarization spectral imaging

or sidestream darkfield imaging. These devices use green light to illuminate tissue, which

is absorbed by the hemoglobin of red blood cells. This allows the microcirculation to be

visualized because of its red blood cell content.

The sublingual microcirculation has been studied most often because of its accessibility.

vi.

Studies have shown that the microcirculation is often altered in patients with sepsis, persistent

microvascular alterations are associated with multisystem organ failure and death, alterations

are more severe in non-survivors than in survivors, and improvements in microcirculatory

blood flow correspond with improved patient outcomes.

vii.

Decreased vascular density, decreased capillary perfusion, and a decreased percentage of

perfused small vessels are the most common microcirculatory alterations. The proportion of

perfused small vessels seems to be the strongest microcirculatory blood flow predictor of

patient outcomes. In one study of patients with sepsis and septic shock, this was a stronger

predictor of mortality than global hemodynamic markers.

viii.

Heterogeneity has been observed in microcirculatory blood flow in the same tissue bed (with

as little as a few millimeters between observations) and between different tissue beds.

ix.

Evaluation of the microcirculation is not commonly used in clinical practice because it requires

extensive user experience to obtain proper measurements and time to analyze the results.

However, this is an attractive marker of tissue perfusion that, with technical advances, may be

used more often in the future.

Elevated lactate concentrations may also indicate regional tissue hypoperfusion (e.g., mesenteric

ischemia or critical limb ischemia).

Capillary refill time (measured as the time it takes to recolor a fingertip after pressure application)

can also be used as a marker of tissue hypoperfusion. Capillary refill times greater than 3 seconds

are often considered abnormal and an indicator of poor tissue perfusion.

mucosal perfusion and near-infrared spectroscopy, a noninvasive method of measuring tissue

oxygen saturation (Sto2) in a skeletal muscle.