Topic 5 - Hypothermia: physiology /clinical use Flashcards
Hypothalamus regulates -
vascular regulation
skeletal muscle activity
Hypothermia activates the Endocrine system’s – (2)
metabolic regulation
stress response
Hypothalmus senses cold where and causes?
sensed by thermoreceptors in the skin
- causes hypothalamus to trigger sympathetic nervous system
- vasoconstriction of skin vessels (to decrease heat loss)
- vasodialtion of skeletal muscular beds (to produce heat when tensing shivering)
Can you shiver when you are paralyzed?
NO
Endocrine System increases what? (4)
Increases: Oxygen consumption, HR, CO, and BP
Temp range
- Mild hypothermia
32-35 C
Temp range
- Moderate hypothermia
28-31 C
Temp range
- Deep hypothermia
18-27 C
Temp range
- Profound hypothermia
<18 C
Hypothermia mechanism of protection (3)
- reduction in metabolic rate and oxygen consumption (esp enzymatic reactions)
- preservation of high-energy phosphate stores(use less=more availability)
- reduction of excitatory neurotransmitter release due to ischemia(CNS protection)
The safe period of hypothermic CPB is ____ then the period predicted on the basis of __ ___ activity alone.
longer
reduced metabolic activity
reduction of excitatory neurotransmitter release due to ischemia - how?
- glutamate accumulation opens Ca channels and activated multiple destructive enzymatic systems
- important in CNS protection
general effects of hypothermia (4)
- allow lower pump flows
- better myocardial protection
- less blood trauma (bc low flow/low damage)
- better overall organ protection
% of platelets are toast when you go on bypass?
70%
Cardiac Index on:
- 34-37 C
2.4 L/min/m2
Cardiac Index on:
- 30-34 C
2.0 L/min/m2
Cardiac Index on:
- 25-30 C
1.8 L/min/m2
Cardiac Index on:
- 20-18 C
1.5 L/min/m2
Cardiac Index on:
- <18 C
1.0 L/min/m2
DHCA - deep(profound) hypothermia with circ arrest
- repair complex congenital heart defects - small infants and children
- operations involving aortic arch
- operations where maintenance of venous drainage is difficult
If arrest period longer than 60 minutes
- use intermittent arrest with brief periods of hypothermic flow
- low-flow hypothermic perfusion is safer than total arrest
physiology of hypothermia
- biochemical
metabolic regulation
physiology of hypothermia
- chemical
acid-base regulation
As temperature decreases —- biochemical reactions do what?
ALL reaction decrease in rate
decrease: Metabolic (producing & utilizing)
Humoral and coagulation cascades
Cellular maintenance of cell funct
Cold = diminished …..
diminished neurotransmission, receptor function ad protein activity
the cold effect is ____ with CPB
GLOBAL
Q10 principle
relates an increase or decrease in reaction rates to a change in 10C
a unitless quantity
ex Q10=2 a decrease in temp 10C will result in a 50% in reaction rates
Normal Q10 range?
are 1.9-4.2
oxygen consumption is a —
reaction
Q10 - temperature
in C
the factor by which a rate changes and is a useful way to express the temp dependence of a process
7C Principle
Reduction in Oxygen Consumption (VO2)
Every 7C drop in temperature will result in a 50% decease in oxygen consumption
from 37 to 30C what % decrease in Oxygen consumption?
50% decrease in oxygen consumption
“moderate”
37 to 23 C what % decrease in Oxygen consumption?
75% decrease in Oxygen consumption
“deep”
2nd Principle (Q7) - Reduction in metabolic rate
Oxygen demand is reduced 50% for every 7 C drop in temperature
ex @30C VO2=50% normal
2nd Principle (Q7) - Reduction in metabolic rate adjuncts ?
anesthesia (give cerebral vascular protection drugs, relaxants)
Gas solubility is ____ related to temperature
inversely
gas content in solution =
partial pressure of gas x solubility
decrease temp what happens to:
solubility?
partial pressure?
total content?
- solubility increases which increase the amount dissolved
- Partial pressure decreases which decreases the amount dissolved
- total content does not change
pH responds to changes in?
CO2 content
NOT changes in the CO@ components
How does temp affect O2/CO2 content?
Temp does not change O2 or CO2 content – just the prop of the components (dissolved vs partial)
Henry’s Law
- As pressure increases, solubility of gasses in liquids increase (more pressure = more gas can be dissolved in a liquid)
- As temperature increases, solubility of gasses in liquids decreases (warm liquid gas starts to come out of solution)
Henrys Law equation?
Content = Partial pressure x Solubility
Temperature is ____ related to partial pressure
DIRECTLY
Temperature is ___ related to solubility
INVERSELY
At a constant temp, the amount of a given gas that dissolves in a given type and volume of liquid is directly prop to the _____ of that gas in equilibrium with that liquid
partial pressure
Increase temp:
Solubility?
pO2 & pCO2?
Decrease solubility
increase pO2 & pCO2 (no change in content)
decreased temp:
Solubility?
pO2 & pCO2?
Increase solubility
Decreased pO2 & pCO2
Concerns with Profound Hypothermia? (2)
Homogeneity of cooling and warming
Homogeneity of cooling
rate of cooling
temperature gradients
– concern with profound hypothermia
recommended rate of cooling?
1 C per min
recommended rate of warming
1 C per 3 to 5 minutes
Consequence of cooling/warming too fast? (3)
- development of temperature gradients w/in tissues
- body cooling after bypass
- exposure to hypothermia
Limitation of rate of cooling/warming? (3)
- water temp of heat exchanger
- bld pressure and SVR
- flow rate
Temp gradients - max for adults and peds?
reduces probability of GME generation
limit temp gradient btwn arterial and venous blood
- adults <8 C
Reduces probability of GME generation
- arterial PO2 mmHg <?
keep arterial pO2 <200mmhg
when coming off bypass have a smaller gradient
HYPERTHERMIA - how does it happen?
Time pressure of rewarming
efficiency of HE
high water temps
perfusion inattention
Risk cerebral injury
Techniques of hypothermic bypass (3)
DHCA
HLFB
HILFB (hypothermic intermittent low flow bypass)
DHCA - cool to?
to 18-20 C
can turn off pump for 30-60 minutes
During DHCA what organ is at greatest risk?
Brain function, developmental capacity and IQ
Safe Circ Arrest times @ 37 C:
O2 consumption?
safe arrest time?
100%
4-5 minutes
Safe Circ Arrest times @ 29 C:
O2 consumption?
safe arrest time?
50%
8-10 min
Safe Circ Arrest times @ 22 C:
O2 consumption?
safe arrest time?
25%
16-20 min “deep”
Safe Circ Arrest times @ 16 C:
O2 consumption?
safe arrest time?
12%
32-40min “profound”
Safe Circ Arrest times @ 10 C:
O2 consumption?
safe arrest time?
6%
64-80min
Standard safe circ arrest temperature ?
18 C
Disadvantages of DHCA (4)
Neurologic injury and morbidity
Brain is at the most risk
>60 min arrest is detrimental
>40 min increases risk
Advantages to DHCA (4)
- exposure
- reduces metabolic rate and molecular movement
- allows cessation of circulation
- excitatory neurotransmitter reduction
DHCA >40 minutes
increases risk
DHCA <60min
arrest is detrimental
HLFB and HILFB (intermittant low flow bypass) patients demonstrated __ __ of neural dsyfunction in patient
lowered rate compared to DHCA
can combine HILFB and DHCA for __ mins every __ min
1-2 mins every 15-20 mins
Increase tolerance of brain to ischemic insult - meds(2)
Thiopental
Solumedral
Thiopental
short acting barbituate
Increase tolerance of brain to ischemic insult
Solumedral
anti-inflammatory, stabilize cell membranes
Increase tolerance of brain to ischemic insult
Homogenous Temp (4)
rate of cooling/warming
hemodilution
acid-base management
head in ice
Recovery - reperfusion conditions
perfusate temp
perfusate composition: mannitol, bicarbonate, others…
