Figure 20.1

The regulation of the cerebral arteriolar tone under different metabolic circumstances.

It is obvious that there are different systemic factors that may influence cerebral arteriolar tone by decreasing it through vasodilation (increases in PaCO₂ levels, decreases in PaO₂ levels) or vasoconstriction (hypocapnia or hyperoxia producing oxygen free radicals) that may frequently occur in critically ill patients. ADP may be an indicator of a decreased energy storage of the cells and therewith an important regulatory factor to increase local or systemic cerebral blood flow in critical conditions. Additionally this is the place where we have to mention the cerebral metabolic rate for oxygen (CMRO₂). In some circumstances of ICU treatment CMRO₂ may be increased, resulting in overproduction of CO₂ and increased ADP concentration in the cells resulting in vasodilation of the arterioles (e.g., this is the case in hypermetabolic states, high fever and epileptic status). In contrast, CMRO₂ may be decreased after administration of several sedato-hypnotic drugs or opiates used frequently during ICU treatment and also after the use of inhalational anesthetics. These changes in the cerebral arteriolar tone are most sensitively reflected by the pulsatility index that decreases during vasodilation and increases after vasoconstriction of the cerebral arterioles.

Most important indications and findings of transcranial Doppler monitoring in critical care settings

There are numerous critical care conditions in which TCD is regularly used as a monitoring tool. In some other critical care conditions, TCD is more an ancillary test or a method that helps in understanding the pathophysiological background of the systemic disease. Table 20.1 summarizes the typical alterations of mean blood flow velocities and pulsatility indices in different critical care conditions.

Table 20.1 Typical alterations of mean blood flow velocities (MBFV) and pulsatility indices (PI) in different critical care conditions

Critical care condition	MBFV	PI
TBI/elevated ICP	↓	↑
Vasospasm	↑	↓
Brainstem death	↓ until 0-flow	↑
Meningitis	↑	↑
Loss of cerebral autoregulation upper threshold	↑	↑
Loss of cerebral autoregulation lower threshold	↓	↓
Severe preeclampsia/eclampsia	↑	–
Severe sepsis, septic encephalopathy	↓	↑
Fulminant hepatic failure	↓	↑
Sickle cell anemia	↑	↓
Decreased cardiac output	↓	↑
Shock, above the threshold of autoregulation	↓	↑
PaCO₂↑	↑	↓
PaCO₂↓	↓	↑
Hypothermia	↓	↑
Rewarming after hypothermia	↑	↓
Hypermetabolism/fever	↑	↓
Anesthetic induction agents/sedato-hypnotics	↓	–
Volatile anesthetic agents (MAC-dependent)	→ or ↑	→ or ↓