test 5 part 3 Flashcards
Neurologic Injury
- Over 1.4 million cardiac surgical procedures are carried out worldwide
- Neurologic deficits continue to be a major source of morbidity and mortality
- Overt stroke occurs in 1-5% of all patients
- Neurologic dysfunction may be present in 25-80% of patients
- Annual cost for treating these patients exceeds $2 billion/ year
Neurologic Deficits Include
- Decreased psychomotor speed
- Lack of attention
- Lack of concentration
- New learning ability loss
- Short term memory loss
- Pediatrics: seizures, movement disorders, developmental delays
Classification: Neurologic Deficit: Transient Ischemic Attack (TIA)
- Localized event
- Rapid onset and recovery (minutes)
- Usually produces no permanent damage
Classification: Neurologic Deficit: Reversible Ischemic Neurologic Deficit (RIND)
- Similar to TIA but lasts longer (24-72hrs)
* Usually complete recovery
What are the causes of TIA?
- Atherosclerosis
- A-fib
- LV thrombi
- Cardiogenic emboli
- Carotid plaque
- Aortic arch plaque
Classification: Neurologic Deficit: Lacunar Brain Infarct (stroke)
- Specific focal deficit from cerebral artery occlusion
- Deep in brain
- Much more severe, often does not resolve
Classification: Neurologic Deficit: Global Ischemia
- Due to long periods of hypoperfusion or massive embolic load
- Poor recovery: >50% of patients are brain dead and never wake
Preoperative Risk Factors
- Many cardiac pts have pre-existing risk factors for stroke and cognitive impairment
* Without added risk of cardiac surgery and bypass - Cardiac surgical patients experience more serious neurologic morbidity than age- and health-matched controls undergoing non-cardiac surgery
Examples of Preoperative Risk Factors
- Advanced age
- History of prior neurologic events
- Aortic atherosclerosis
- Carotid stenosis
- Atrial arrhythmias
- Hypertension
- Diabetes
- Genetic
Preoperative Risk Factors: Age
- Age
* < 60 years old: 1% incidence of stroke
* >70 years old: 4 - 8.0% incidence of stroke - Continued trend of older patients coming to surgery
- Cardiac surgical patients over age 80 will grow faster than any other group
Preoperative Risk Factors: Atherosclerosis
• Embolic events related to aortic plaques
• Often associated with specific surgical events
- Age and atherosclerosis are the two main patient risk factors that largely determine post-surgical neurologic outcome
Preoperative Risk Factors: HTN and DM
- HTN: 55% of cardiac surgical patients
- DM: 25% of cardiac surgical patients
- HTN and DM both cause:
* Change in cerebral autoregulation
* Narrow arteries penetrating the brain
* Decrease collateral blood flow
* Decrease ischemic tolerance
Preoperative Risk Factors: History of previous neurologic incident
- 13% of cardiac patients have a history of TIA/stroke
* 3X greater risk of new deficit or exacerbation of previous deficit
Preoperative Risk Factors: Carotid stenosis
• 15% of cardiac surgery patients have greater than 50% carotid stenosis
Surgical Stress
- Brain ischemia during surgery may result from
* Hypoperfusion
* Inadequate tissue perfusion
* Embolization
* Obstruction of vessels
* Inflammation
* Endothelial damage compromising oxygen delivery
Embolization
- On pump embolization
* Primary cause of serious brain injury during cardiac surgery - Embolic events related to:
* Aortic plaques
* Platelet-fibrin and leukocytes aggregates
* GME from CPB circuit - Embolic events associated with specific surgical events
Hypoperfusion
- Regional hypoperfusion may occur as a result of:
* Vascular disease
* Inability to compensate for regional ischemia
* Due to microembolization - MAP may play a role in regional hypoperfusion
Inflammation
- Many pathophysiologic mechanisms that may impact the vascular lining
- Endothelium regulates:
* Vasomotor tone
* Thrombosis
* Fluid transport
* Inflammatory response - Alterations in these endothelial functions may play a role in post-CBP neurologic dysfunction
- CPB is associated with ischemia and reperfusion injury to heart and lungs
* Potent triggers for activation of leukocytes and leukocyte-platelet binding - Vascular integrity may be impaired by:
* Capillary plugging
* Free radicals
* Proteolytic enzymes - Exposure of blood to bypass circuit results in activation of platelets, monocytes, and neutrophils
- Inflammatory process has been associated with increased incidence of postoperative delirium
* 26 – 50% of patient may develop post-op delirium
Cerebral Metabolic Requirement for Oxygen (CMRO2)
• 3.3 ml/100g of brain tissue/min
Cerebral Blood Flow (CBF)
• 55 – 60 ml/100g/min
Utilization of total resting oxygen: (%)
20%
Average brain weighs
about 1400g
CBF is influenced by
- Cerebral Metabolic Requirement for Oxygen, PaCO2, Hct, MAP
* All may increase or decrease cerebral blood flow
Cerebral Physiology Without bypass
• Cerebral oxygen delivery (CDO2) normally exceeds the oxygen demand
• When delivery decreases, CMRO2
is maintained by increasing oxygen extraction
• Further decrease in delivery will result in ischemia
Brain regulates its flow in response to
• its oxygen demand
• Increases or decreases in CMRO2
are associated with proportional changes in CBF (FLOW-METABOLISM COUPLING)
Determinants of CPB
- Temperature
- Mean arterial pressure (MAP)
- Carbon Dioxide (PaCO2)
- Hematocrit (Hct)
Determinants of CPB - Temperature
- Primary determinant of cerebral blood flow (CBF)
- 10oC temperature reduction decreases metabolic rate more than 50%
- Hypothermic temperatures cause a change in blood flow characteristics
* Changes responsiveness of the cerebral vasculature - Below 22oC – 23oC have flow-metabolism uncoupling
* Changes in CBF does not track changes CMRO2
Determinants of CPB - Mean Arterial Pressure (MAP)
- CBF autoregulation occurs at MAPs from 50 – 150 mmHg
* Awake patients - Anesthetized patients at moderate hypothermia
* May have preserved autoregulation down to 28mmHg
* Deeper hypothermia – down to 20mmHg - Recommended to keep MAP at least 50 – 55 mmHg to allow some margin of safety
* Most patient presumed to have cerebral vascular disease
Determinants of CPB - Carbon Dioxide
• Major player in determining CBF
• Changes in PaCO2
alter CBF independent of CMRO2
• Autoregulation is preserved with α-stat management
• MAPs of 55 – 95 mmHg
• CBF becomes largely pressure passive when PaCO2 is elevated
Determinants of CPB - Hematocrit
• Hemodilution reduces blood viscosity and vascular resistance which increases CBF
• CBF supports cerebral oxygen delivery (CDO2) as Hct is reduced
• Progressive hemodilution compromises CDO2
and O2 consumption
• CBF can no longer compensate for the reduced oxygen content
• Oxygen extraction capacity has been exhausted
Interventions to Reduce Neurologic Morbidity in Cardiac Surgery
- Attention to Aorta
* Use epiaortic ultrasound for cannulation, cross clamp, and proximal anastomosis sites
* Devices to deflect / trap emboli - Pre-op carotid studies in older patients and those with a history of TIA, stroke, or carotid disease
- Minimize aortic manipulations
* Single clamp technique vs. double clamp (cross-clamp plus partial clamp) - Flood chest cavity with CO2
- Use care during decannulation
- Utilize TEE to ensure proper de-airing of heart
- Pharmacologic agents that reduce CMRO2
- Ensure air removed from IV’s and arterial lines
- Apply manual compression on carotid arteries with cross clamp removal
- Use of arterial line and cardiotomy filters
- Ensure proper de-airing of circuit (CO2 flush)
- Maintain adequate anticoagulation
- Monitor warming/cooling gradients
* Slow rewarm is better - Avoid hyperthermia
- Communicate with surgeon and understand surgical sequence of events
- Alpha-stat acid-base management (preserve autoregulation)
Near-Infrared Spectroscopy (NIRS)
- Noninvasive assessment of regional brain oxygenation
- Sensitive to temperature, PaCO2, Hct, CPB flow
- Monitoring of cerebral oxygen saturation can decrease the incidence of postoperative neurocognitive dysfunction
Transcranial Doppler
- Measures blood velocity in middle cerebral artery
* Correlation to CBF - Sensitive to temperature, MAP, pump flow, PaCO2, Hct
- Reliable flow velocity requires a constant vessel diameter
* Not always true on bypass
* Better trending device - Better at emboli detection than indicator of CBF
Take Home Message
- Despite the best surgical technique, patient selection, use of modern and appropriate perfusion equipment
* Some patients will develop acute stroke in the post-op period - A structured stroke team is crucial for successful patient management
