Macro- and Microcirculation Crosstalk in Brain-damaged Patients Under Anesthesia or in Intensive Care

In patients with severe brain injury, maintenance and control of blood pressure is at the primary focus of the management strategy in both anesthesia and intensive care. In order to restore cerebral perfusion pressure (CPP) to appropriate levels (60-70 mmHg) while ensuring optimal perfusion of other vital organs, intravenous administration of vasodilator, ino-dilator or vasoconstrictor agents is widely used.

These vasoactive agents widely used to correct hypotension or hypertension have their own effects on left ventricular loading conditions, arterial tree tone, but also effects on microcirculation.

Currently, the vast majority of anaesthesias and organ resuscitations are carried out from a macrocirculatory view of the system. Hemodynamic optimization is then based on the monitoring of macrocirculatory parameters such as arterial pressure, and sometimes cardiac output coupled with parameters of adequacy of tissue oxygenation (arterial and venous O2 saturation).

The microcirculatory status of a tissue cannot be reliably predicted by considering only the usually measured macrocirculatory parameters. Peripheral capillary perfusion pressure is the result of the coupling between peripheral microcirculatory tone and resistance and macrocirculatory impedance.

The response of microvessels to the various vasoactive agents widely used in the clinic has been little studied.

However, it should be noted that excessive use of vasoconstrictor amines, particularly in the presence of hypovolemia, may, paradoxically to the macrocirculatory impression, induce closure of the capillary network and aggravate tissue ischemia. This capillary de-recruitment is due in particular to the dissipation of pressure in the systemic arterial system, due to the excessively high peripheral resistances and fatally leads to a rarefaction of the open capillaries.

In addition, the increased macrocirculatory impedance imposes a myocardial workload that may cause a decrease in cardiac output, amplifying the tissue damage. This leads to a new situation of uncoupling, characterized by decreased macrocirculatory flow and elevated microcirculatory resistance.

This is why, in situations where organ perfusion is inadequate or weakened, patient management that includes the integration of the impact of vasoactive agents on the microcirculation seems essential for global hemodynamic treatment.

Impaired microcirculation is a key pathophysiological factor in organ failure and death in patients. The impact of organ failure (renal, respiratory, cardiac, etc.) on the quality of cerebral oxygenation is major and conditions the brain's recovery potential. Organ failure by its repercussions on the conditions of brain recovery severely worsens the prognosis of brain damaged patients (secondary brain insults of systemic origin (SBISOs)

In order to recognize and avoid the pitfalls associated with these aortocapillary uncoupling mechanisms, specific monitoring of microcirculatory flow and tissue perfusion during the administration of vasoactive agents is largely relevant. The major obstacle is the difficulty of studying microcirculatory behavior at the bedside in a non or minimally invasive manner.

Direct visualization of the capillary circulation in vivo is now possible using a minimally invasive technique of orthogonal spectral polarization (OSP) videomicroscopy and, more recently, Sidestream Dark Field imaging (SDF).

This last technique, validated in humans mainly in septic shock situations, uses a polarized light source whose absorbance by hemoglobin and reflection by other tissues allows the construction of a contrasted image. This technique allows the visualization of capillary networks on the surface of solid organs covered with a thin epithelial layer to a depth of about 300 µm. An analysis of the recorded image assisted by calculation software allows the semi-quantitative calculation of capillary density (proportion of perfused vessels; PPVSL), capillary flow (Microcirculatory Flow Index; MFISL) and heterogeneity (heterogeneity index; HETSL) of these.

To date, several non-invasive techniques are available, each measuring different parameters specific to this microcirculation:

Capillary density, capillary or tissue flow estimation, hemoglobin saturation, oxygenation and tissue CO2 pressure measurements.

The non-invasive study of microcirculatory perfusion and its interactions with the macrocirculatory network, thanks to a minimally invasive method such as videomicroscopy, should allow a better use of the treatments used.

Overall, the trend in anaesthesia and intensive care is increasingly towards individualized management, and the use of monitoring (macro- and microcirculatory) associated with a thorough knowledge of the treatments used (vasoconstrictors, vasodilators and vascular filling in particular) seems to be an essential basis for these considerations.

The empirical use of vasoactive agents, without specific monitoring of these effects, and the unpredictable impact of changes in one of the two compartments on the final state of the system during adequate macrocirculatory resuscitation could even be deleterious at the microcirculatory level.

These pathophysiological concepts specific to macro/microcirculation cross-talk demonstrate the imperative of developing monitoring specific to the microvascular network in critical patients.

The techniques have evolved and the tools are now available and can be used safely in patients.

For brain-damaged patients, routine management already includes a very complete monitoring including all systemic cardio-pulmonary and cerebral parameters. Participants receive interventions as part of routine medical care to restore Cerebral Perfusion pressure (CPP) to appropriate levels (60-70 mmHg)

The availability of minimally invasive monitoring tools of the microcirculation such as videomicroscopy allows the study of the functional capillary situation according to the vasoactive agent used.

The study of microcirculatory perfusion and its interactions with the macrocirculatory network, using a minimally invasive method such as videomicroscopy, should allow a better use of the treatments used.