Exam 9 - Hypothermia & Bypass Flashcards
1
Q
Bigelow
A
- First to introduce idea of hypothermia
- 1950
- Used animals
2
Q
Lewis
A
- First application of hypothermia in cardiac surgery
- Closed ASD
- Used surface cooling
3
Q
Sealy
A
- Used hypothermia with CPB circuit
4
Q
Thermoregulation
A
- Thermoreceptors in skin
- Causes hypothalamus to trigger sympathetic
- Vasoconstrict skin vessels
- Vasodilate skeletal muscles
5
Q
Endocrine affect on thermoregulation
A
- Increase O2 consumption
- Increase HR
- Increase CO
- Increase BP
6
Q
Induced hypothermia
A
- can be global or localized
- buys time in case something fails on CPB
- decreases metabolic demand
7
Q
Why do we induce hypothermia
A
- reduce ALL metabolic rate
- especially enzyme reactions / clotting - reduce O2 consumption
- Preserve high energy phosphate stores
- Reduces excitatory neurotransmitter release
8
Q
Benefits of hypothermia on CPB
A
- Allows lower pump flows ~ lower the temp -> lower the flow (because lower O2 used) - better myocardial protection - less blood trauma - better organ protection
9
Q
Mild Hypothermia
A
- 32-35
- Most common level
10
Q
Tepid Hypothermia
A
- 35-37
11
Q
Moderate Hypothermia
A
- 28-31
- Bigger cases like CABG, DaVinci, AAA
- Peds
12
Q
Deep Hypothermia
A
- 18-27
- More complex cases
- Peds
13
Q
Profound Hypothermia
A
- <18
- Circulatory arrest
- Peds
14
Q
Affect of hypothermia on blood flow
A
- biggest decrease to skeletal muscles/extremities
- also decreases:
- kidneys
- splanchnic beds
- heart
- brain
15
Q
Hypothermia on the heart
A
- Decreased HR
- Same or Increased contractility (tries to maintain CO)
- Dysrhythmias common but cause unknown
~ electrolytes? Uneven cooling? ANS? - Coronary blood flow well preserved
~ this means dysrhythmias not due to ischemia/hypoxia
16
Q
Hypothermia on Lungs
A
- Decrease ventilation
- Gas exchange unaffected
- Doe snot matter if on CPB
17
Q
Hypothermia on kidneys
A
- Largest PROPORTIONAL decrease in blood flow
- Increases renal vascular resistance
- decrease blood flow to inner/outer cortex
- decrease O2 delivery
- Na/H2O/Cl transport decreased (slow active transport)
- Impaired ability to concentrate/reabsorb (slow active transport)
- Impaired glucose handling (shows in urine)
18
Q
Hemodilution and hypothermia after bypass
A
- Hemodilution/hypothermic bypass improves renal blood flow and protects renal tubules post-op
19
Q
Hypothermia on Liver
A
- Blood flow reduced in proportion to reduction in CO
- decrease in metabolic and excretory function
- changes drug actions/requirements
- need to adjust drug dosages
20
Q
Hyperglycemia
A
- Happens on hypothermic CPB
- Decreased insulin production
- Patient does not respond to insulin if given on cold CPB
- Also caused by giving dextrose in cardioplegia
21
Q
Blood viscosity and hypothermia
A
- Water moves from plasma into the cells
- increases Hct
- increase in viscosity
- need to hemodilute patient…especially if circ. arrest
22
Q
O2 carrying capacity
A
- reduced if on hypothermia
- BUT blood/oxygen is actually getting to microcirculation
- If temp decreases….gas is more soluble in blood
23
Q
SVR and hypothermia
A
- SVR/PVR increases
- Big effect if < 26
- increase in viscosity/ epi and NE/ hemoconcentration/ cell swell
- Thoroughfare channels form…decrease O2 to tissues
- Thrombocytopenia…less platelets
24
Q
Clinical use of Hypothermia
A
- Mild to moderate (> 25)
- Profound selective myocardial hypothermia
- used during aortic x-clamp
- topical ice
- cold cardioplegia
- get to 2-4 but NOT freezing….freezing kills cells
25
Methods of hypothermic induction
- Surface cooling....ice
- Core cooling...using CPB
- Deep hypothermic circ. arrest (drain all blood into VR)
- Low/Intermittent flow deep hypothermic circ. arrest
- circ arrest for so long need to flush out metabolic waste
26
Surface cooling uses
- Small infants less than 2.5 kg
| - Myocardial protection on adults and peds
27
Profound hypothermia with arrest uses
- Repair complex congenital heart defects (infants/children)
- Aortic arch operations
- venous drainage difficulties
- surgeon has trouble seeing
- If arrest > 60 min...use intermittent arrest...low flow...is safer
28
Biochemical reactions and hypothermia
- all rxns decrease in rate
- metabolic / humoral / coag cascades decrease
- cellular functions decrease
- communication / receptor mechanisms / membrane proteins
29
Q10 principle
- relates increase/decrease in rxn rates/metabolic processes to a temperature change of 10 degrees
- Example: Q10=2
- rxn will double with a 10 degree increase in temp.
- rxn will be cut in half for a 10 degree decrease in temp.
- Q10 range in humans: 1.9-4.2 (mean = 2)
- most rxns are 2-3
- Q10 bigger at lower temps
30
7 degree principle
- deals with O2 demand
- reduced by 50% for every 7 degree drop in temp
- @30: VO2 is 50% of normal
- @20: VO2 is 25% of normal
- Anesthesia and muscle relaxers will decrease further
31
Hypothermia and gas solubility
- Solubility increase if decrease in temp
- Partial pressure down if decrease in temp
- SO....total content does not change
- just the proportions of components change
32
Henry's law
I. Pressure increase -> solubility of gas in liquid increases
II. Temp increase -> solubility of gas in liquid decrease
- Content = partial pressure x solubility
- Temp is DIRECTLY related to partial pressure
- Temp is INVERSE to solubility
33
PO2 / PCO2 and Temp
- Increase temp -> decrease solubility -> increase partial press.
- decrease temp -> increase solubility -> decrease partial press
- NO CHANGE in total gas content w/ change in temp
34
Profound hypothermia concerns
- Homogeneity of cooling:
- if cool unevenly...hurt tissues
- rate of cooling / temp gradients
- Homogeneity of warming:
- if warm unevenly.... bubbles
- need to warm much slower than cool...bubbles
- exposure to hyperthermia...protein denature
35
Rate of cooling/warming
- Cool at 1 degree/min
- Warm at 1 degree/ 3-5 min
- If too fast:
- build temp gradients / body cooling after CPB (gradients) / hyperthermia (cook brain)
36
Rate of cooling/warming limitations
- Water temp in heat exchanger
- BP and SVR
- Flow rate
37
Temp gradients
- Keep at 6 degrees
- reduce GME generation / too fast cool or rewarm
- Keep pO2 less than 200 mmHg
- Gradient between:
- HE / venous blood
- patient / arterial blood
38
Hyperthermia causes
- Warm too fast
- Efficiency of HE (more/less than expected)
- High water temps
- Not paying attention
- Causes cerebral injury
39
DHCA
- Deep hypothermic circ arrest
- 18-20 degrees
- turn off pump for 30-60 min
- Brain at greatest risk
- Optimal depth of cooling?
- multiple temp site monitoring / EEG cessation
40
Normal way to DHCA
- On CPB
- Cool
- XC and CPG
- Off CPB (arrest)
- On CPB and warm
41
Alternative way to DHCA
- On CPB
- Cool
- Off CPB (arrest)
- XC and CPG
- On CPB and Warm
- sometimes on peds
- if cant give CPG right away
- have to turn on pump before giving CPG
42
DHCA benefits
- allows exposure
- reduces metabolic rate and molecular movement
- cessation of circulation
- excitatory neurotransmitter reduction
43
DHCA negatives
- neurological injury / morbidity
- brain is at risk
- >60 min is detrimental
- >40 min increases risk
44
HLFB / HILFB
- Safer than just DHCA
- lower rates of neural dysfunction
- getting more common
45
How to increase brain tolerance to ischemic insult
- Thiopental: short acting barbituate (slows brain metabolic)
- Solumedrol: anti-inflammatory / stabilize cell membranes
46
How to achieve homogenous temp
- rate of warming/cooling
- hemodilution
- acid/base management
- head on ice
47
Better recovery - reprofusion conditions
- Perfusate temp
- Perfusate composition: mannitol / bicarb / etc
- Rewarming tools:
- bear hugger
- warming blanket
48
Sequence of CPB
- Patient brought in
- ECG leads placed by anesthesia
- IVs placed
- Art line placed in radial/femoral
- Baseline ABG and ACT
- Swan cath placed / central line placed / CO recorded
- Patient put under
- Patient intubated
- Foley cath placed
- Temp probes placed (bladder/nasopharyngeal/rectal)
- Patient prepped
- Patient draped
- Lines to table
- Incision made
- Sternum split
- LIMA taken down
- Saph vein harvested
- Heparin given and post-Hep ACT from anesthesia
- Purse strings
- Arterial cannula inserted
- Pump sucker on
- Venous cannula inserted
- Check ACT and ON CPB
- Monitor Temp
- Fill CPG line
- Flow down for XC (1L)
- XC on and flow up (watch pressures)
- CPG (vent off for antegrade)
- Vent on
- Draw ABG/VBG/ACT
- More CPG
- Rewarm
- Hotshot
- Check ECG
- Flow down and XC
- Flow up (watch pressure)
- ABG/VBG/ACT draw
- Wean off CPB / Fill heart / flow down
- STOP CPB
- Calculate protamine dose
- Give protamine (suck and vent off)
- Post protamine ACT
- Wires in and stop bleeders
- Close chest and twist wires
- Close skin
- Lines from table
- Transport patient
