Cerebral circulation II: pathophysiology and monitoring Flashcards
Key points
- Adverse postoperative neurological outcomes are common. In patients aged 65 yrs and older undergoing major non-cardiac surgery, the risk of a clinically silent stroke is 7%.
- There is a 20% incidence of postoperative cognitive decline in patients undergoing elective hip surgery.
- Adverse neurological outcomes correlate with the depth and duration of intraoperative cerebral desaturation events.
These events can be detected by monitors such as near-infrared spectroscopy (NIRS).
- The safe conduct of anaesthesia relies on prompt recognition and correction of cerebral hypoperfusion, intracranial hypertension and metabolic disturbances such as hypoxia or
hypoglycaemia. - Processed EEG and NIRS intraoperative monitoring may reduce the risk of accidental awareness, improve early recovery times and reduce the incidence of postoperative delirium and
postoperative cognitive dysfunction.
Cerebral circulation, adverse neurological
outcomes and safe anaesthesia
major surgery and general anaesthesia
such as unstable haemodynamics, systemic desaturation,
anaemia, hypocapnia, cardiac dysfunction and increased
cerebral oxygen consumption
The depth and duration of cerebral
desaturation events correlates with the incidence of severity of adverse neurological outcomes such as perioperative
stroke, postoperative cognitive dysfunction (POCD) and
postoperative delirium (POD
The timing of urgent procedures should be carefully evaluated at the light of
this, and large-scale population analysis suggests that elective
surgeries should be deferred until at least 9months after a prior
stroke
incidence of POCD and POD
Although the precise incidence of POCD and POD is difficult
to assess because of methodological heterogeneity in published
studies, the reported prevalence of POCD is approximately
20% in patients undergoing elective hip surgery and as
high as 60% in patients undergoing cardiac surgery
neuroprotection
The term neuroprotection refers to the sum of therapeutic
strategies intended to prevent or minimise injury to the nervous
system. In the operating theatre, neuroprotection relies
on three interlinked aspects:
- Prevention or rapid correction of cerebral hypoperfusion
- Optimisation of physiology and rapid correction of neurotoxic
states, such as hypoxia, extreme hypocapnia and hypoglycaemia - Prevention and treatment of intracranial hypertension in patients
with acute encephalopathy undergoing non-deferrable
surgical interventions
Cerebral hypoperfusion
Cerebral hypoperfusion is defined as a mismatch between
cerebral blood flow (CBF) and cerebral metabolic demand.
Hypoperfusion can therefore be caused by a critical reduction
in CBF, an abnormally increased cerebral metabolic rate of
oxygen consumption (CMRO2) and/or hypoxia, or a combination
of the above
A critical reduction in oxygen delivery to the
CNS rapidly results in cellular energy failure, neuronal
dysfunction and apoptosis (i.e. cytotoxic oedema).
restoration of cerebral perfusion pressure, correction
of vascular obstruction and/or reduction of CMRO2 by titrating
the depth of anaesthesia, controlling seizures and managing
the patients’ temperature.
The indications for perioperative and intraoperative CBF
monitoring can be summarised in the following subsections
Patients with acute encephalopathy
Patients at risk of compromised CBF secondary to
selective vascular clamping, thromboembolic events,
or both
Patients expected to experience a compromised
cardiac output during surgery with arterial
hypotension close or below the lower limit of
autoregulation
Patients with acute encephalopathy
traumatic brain injury,
intracranial space-occupying lesions
with associated injuries
requiring emergency extracranial surgical intervention, or both
requiring emergency extracranial surgical intervention, or
both
to secondary intraoperative neurological
injury from a combination of intracranial hypertension crises,
haemodynamic instability, or both. The accepted gold standard
in this clinical scenario is invasive ICP monitoring
Concomitant monitoring of ICP and MAP allows continuous
assessment of cerebral perfusion pressure (CPP¼MAPeICP),
which should be maintained above the lower limit of cerebral
autoregulation, typically CPP >50e60 mmHg.
the use of noninvasive
modalities such as transcranial Doppler (TCD),
near-infrared spectroscopy (NIRS) and processed EEG (pEEG) is
Patients at risk of compromised CBF secondary to
selective vascular clamping, thromboembolic events,
or both
These patients (e.g. undergoing carotid endarterectomy or
aortic arch repair) are at risk of suffering intraoperative cerebral
hypoperfusion despite normal cardiac output and normal arterial blood pressure
Safe anaesthesia during surgical clamping of arteries supplying the
CNS relies on prompt recognition of critical cerebral hypoperfusion and modulation
of MAP by means of vasopressors to improve collateral CBF
and/or shunting and/or reduction of CMRO2 by up-titration of
anaesthetic agents and controlled temperature management
Patients at risk of POD, POCD and delayed emergence
from anaesthesia
The BALANCED trial identified a broad range of the depth of
anaesthesia that be delivered safely when titrating volatile
anaesthetic concentrations using pEEG (BIS target, 35e50).
Patients expected to experience a compromised
cardiac output during surgery with arterial
hypotension close or below the lower limit of
autoregulation
These include patients undergoing cardiac surgery, patients
with expected intraoperative haemorrhage and/or managed
with controlled or deliberate hypotension. In this context,
non-invasive CBF monitoring with pEEG, NIRS or TCD (or a
combination of these modalities) may increase patient safety.
Consider developing specialist skills required to interpret data
from these devices.
Monitoring modalities
oxygenation and
oxygen extraction (NIRS), cortical electrical activity (pEEG) or
flow velocities in the arteries of the circle of Willis (TCD).
Intracranial pressure monitoring allows continuous assessment
of CPP. The selection of CBF monitoring modality depends
on the patient’s characteristics, type of surgery, local
availability and expertise.
Optimising the patient’s physiology and preventing cerebral
hypoperfusion relies on one hand on modulation of CBF
(i.e. maintaining CPP above the lower limit of autoregulation,
avoiding mechanical obstruction to venous outflow and
attention to the timing of arterial clamping) and on the other
hand on modulating cerebral metabolic demand (i.e. sedation,
temperature management and avoidance of seizures). The
most relevant neuromonitoring modalities are described in
the following paragraphs
Near infrared
spectroscopy (NIRS)
Any procedure at risk of POD, POCD and perioperative stroke
(older patients, history of cerebrovascular disease or cognitive decline, major surgery
Differential absorption of near-infrared light by
oxyhaemoglobin and deoxyhaemoglobin (rSO2).
Safe, easy to use, low cost.
Provides continuous information on adequacy
of cerebral oxygen delivery.
Determines oxygenation under the optodes (i.e. regional).
Requires further validation.
A reduction of rSO2 of >80% from baseline or
an absolute value below 50% indicates cerebral hypoperfusion
Two studies with 126 participants showed that active
cerebral NIRS monitoring may reduce the incidence of POCD
Processed
electroenecephalography
(pEEG)
Any procedure at risk of POD,
POCD and perioperative stroke.
Proprietary algorithmic analysis of EEG (e.g. PSI,
BIS). A reduction in indices during steady anaesthetic
state correlates with critical reduction in CBF.
Safe, easy to use, low cost.
Detects patients at risk of oversedation, delayed
emergence or awareness.
Maps frontal cortical activity. Rapidly evolving technology.
Requires further validation.
A reduction of PSI/BIS >30% indicates cerebral
hypoperfusion. SR >50% indicates close to- maximum
suppression of CMRO2.
Transcranial
Doppler (TCD)
Aortic arch and carotid surgery. Patients at risk of intracranial
hypertension in whom invasive ICP monitoring is not indicated.
Pulsed Doppler probes assess flow velocities in the circle of Willis.
+
Allows continuous assessment of cerebral autoregulation,
detection of emboli and noninvasive ICP monitoring.
- Technically challenging. Normal or high flow velocities do
not necessarily indicate adequate perfusion
(i.e. vasospasm).
No information on oxygen extraction and adequacy of perfusion.
A reduction MCAfv >50% from baseline or
a value below 25 cm 1 indicates high risk of
postoperative stroke.
Reduced or absent diastolic flow indicates ICP close or exceeding diastolic bp
Intracranial pressure (ICP)
Extracranial surgery in patients at risk of intracranial hypertension
(i.e. damage-control or orthopaedic surgery after major trauma
Intraparenchymal or intraventricular pressure monitoring allows prompt
identification of CBF compromise caused by intracranial hypertension
+
Well validated in traumatic brain injury.
Continuous assessment of CPP. EVDs allow correction of ICP by means of CSF drainage
-
Invasive. Potentially significant complications
(bleeding, infection).
Does not assess adequacy of oxygen delivery.
ICP >20 mmHg indicates a risk of herniation. CPP <50
mmHg indicates risk of cerebral hypoperfusion.
