Microcirculatory perfusion as measured by sublingual Side Stream Darkfield (SDF) imaging and Near Infrared Spectroscopy (NIRS) in mild therapeutic hypothermia (TH) and non-TH patients

Treatment with mild therapeutic hypothermia (TH) is associated with an assumed
protective decrease of oxygen consumption under conditions of potentially
inadequate oxygen delivery.
In this context, during TH adequate oxygen delivery as measured by S(c)vO2 has
been reported, despite lower cardiac output, caused by relative bradycardia and
lower stroke volume (1).
However, such macrohemodynamic variables may not reflect potential distributive
microcirculatory alterations, both as a result of the inflammatory response
caused by ischemia-reperfusion and by TH as a therapeutic entity.
A recent retrospective observational study in our own ICU in 62 patients after
cardiac arrest and invasive hemodynamic monitoring (2), using pulmonary artery
catheter (Vigilance) or arterial pulse contour analysis (PiCCO), revealed an
increase in Cardiac index (CI) and S(c)vO2 during steady state hypothermia of
33*C. CI rose from 2.3 L/m2 (1.9-3.0) to 2.7 (2.3-3.8), p=0.002 and S(c)vO2
increased from 77% (70-84) to 80 (75-85), p= 0.12 respectively (1). At the same
time, arterial lactate levels at the beginning of steady state hypothermia were
2.1 (1.2-3.7) mmol/L and increased significantly to 3.4 (1.8-4.9) at the end of
hypothermia (p= 0.000).
This raised the question whether TH itself might induce distributive
microcirculatory failure. Alternative explanations for the observed combination
of increase of lactate, and S(c)vO2/CI may be the inability to prevent
microcirculatory failure despite adequate treatment with TH or direct effects
of treatment aimed at optimizing hemodynamics.

Over the last decades, attempts have been made to identify perfusion
abnormalities at the organ level, for example with tonometry, lactate levels,
near infrared spectroscopy (NIRS) and microdialysis.
Direct visualization of the microcirculation with orthogonal polarizing
spectral (OPS) imaging and its technical successor, side stream darkfield (SDF)
imaging (3) - with the ability to quantify abnormalities both at the bedside
and off-line has recently elucidated the distributive nature of
microcirculatory flow abnormalities and its correlation with prognosis in
sepsis and heart failure. Recent studies indicate that distributive alterations
in the microcirculation of patients not only can be observed, but are also
associated with outcome. The importance of direct visualization of the
microcirculation is further stressed by studies that do not observe a clear
association between systemic hemodynamic variables and microcirculatory
parameters, suggesting that indeed the microcirculatory compartment plays a
crucial role in the development of organ failure.
SDF has been used to reveal microcirculatory blood flow changes both
sublingually (human) and in the cerebral vascular compartment (animal studies)
during cardiac arrest and CPR; the observed abnormalities were predictive of
outcome. (4)
Changes in muscle tissue oxygenation (StO2) after an ischemic challenge using
near-infrared spectroscopy (NIRS) were found in patients with sepsis and in
particular in septic shock (5). NIRS may be a useful technique to monitor
microcirculatory changes in patients after cardiac arrest.

To investigate the prevalence of microcirculatory dysfunction, SDF measurements
of the sublingual microcirculation will be performed in all patients recruited
for the TTM trial in the Medical Center Leeuwarden. The technique consists of a
handheld microscope with a light guide and a disposable sterile lens at the
tip. This light guide is placed on tissue (e.g. sublingual mucosa), and light
with a wavelength within the absorption spectrum of hemoglobin is emitted. Side
illumination is used at the area under investigation: the lens is optically
isolated from the outer ring with LEDs, thereby preventing the influence of
surface reflections (fig) In this way the perfusion (e.g. flow) as well as the
density of the functional capillaries can be determined at the bed side.

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