Topic 5 Flashcards
Human body is homeostatic in regulation and is controlled by
Hypothalmic and Endocrine Systems
Hypothalamus regulates
vascular regulation
skeletal muscle activity
Endocrine system regulates
metabolic regulation
stress response
Cold is sensed by _______ in the skin- Causes hypothalamus to trigger _____ nervous system response
thermoreceptors
sympathetic
Vasoconstriction of the skin vessels=
Decrease heat loss (convection)
Vasodilation of skeletal muscle vascular beds=
Increase muscular activity to produce heat by tensing/ shivering
Mild Hypothermia=
32-35°
Moderate Hypothermia=
28-31°
Deep Hypothermia=
18-27°
Profound Hypothermia=
< 18°
Induced hypothermia=
-not physiological
-clinical use – global or localized to an area
During bypass provides:
•degree of organ & organism protection
•margin of safety
Mechanisms of protection=
- reduction in metabolic rate and oxygen consumption
- preservation of high-energy phosphate stores
- reduction of excitatory neurotransmitter release due to ischemia
reduction in metabolic rate and oxygen consumption=
reduction in reaction rate of all biochemical processes especially enzymatic reactions
preservation of high-energy phosphate stores=
less energy usage = more availability
reduction of excitatory neurotransmitter release due to ischemia=
- important central nervous system protection
- glutamate accumulation opens calcium channels and activates multiple destructive enzymatic systems
The safe period of hypothermic cardiopulmonary bypass (CPB) is …
longer than the period predicted on the basis of reduced metabolic activity alone
Hypothermia general effects
- Allows lower pump flows (better surgeon visualization)
- Better myocardial protection
- Less blood trauma (low flow = low damage)
- Better overall organ protection
Flow= 2.4 L/min/m2 Temp= ?
34-37˚C
Flow= 2.0 L/min/m2 Temp= ?
30-34˚C
Flow= 1.8 L/min/m2 Temp= ?
25-30˚C
Flow= 1.5 L/min/m2 Temp= ?
18-20˚C
Flow= 1.0 L/min/m2 Temp= ?
<18˚C
Methods of Hypothermia Induction=
- Surface cooling (ice)
- Surface cooling with supplementary partial bypass
- Core cooling – Total extracorporeal circulation
•Deep hypothermic total circulatory arrest (DHCA)
•Low-flow, deep/profoundly hypothermic bypass (HLFB)
•Intermittent, low flow deep/profoundly hypothermic bypass (HILFB)
Surface cooling=
- Inverse effect related to size: small infants less than 5 kg
- Myocardial cooling used on adults and peds
Profound hypothermia with arrest (DHCA)=
- repair complex congenital heart defects – small infants & children
- operations involving the aortic arch
- operations where maintenance of venous drainage 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
Important items: hot and cold
- Warming/cooling blanket
- Blanketrol water warmer
- Warm air blower
- Blanket warmer cabinet
Physiology of Hypothermia=
- Biochemical= metabolic regulation
2. Chemical= acid-base regulation
Biochemical Reactions=
- all reactions decrease in rate as temperature decreases
•Metabolic – energy producing/utilizing
•Humoral and coagulation cascades
•Cellular – maintenance of cellular function
•communication pathways
•receptor mechanisms
•membrane proteins (enzymatic & environmental)
Cold=
- decreased energy
- more bleeding
- diminished neuro transmission, receptor function, and protein activity
THIS EFFECT IS GLOBAL WITH CPB
Q10 Principle=
relates an increase or decrease in reaction rates to a change in 10° C
Most physiological rates Q10 are
2-3 in the body
Oxygen consumption is a
reaction
Q10 is a useful way to express the
temperature dependence of a process
If your Q10 = 2 then a decrease in temperature 10° C will result in a
50% reduction in reaction rates.
Reduction in Oxygen Consumption (VO2) (7°C Principle)=
Every 7°C drop in temperature will result in a 50% decrease in oxygen consumption/demand
Gas solubility is ______ related to temperature
inversely
gas content in solution =
partial pressure of gas x solubility
if you decrease the temperature, then solubility
increases which increases the amount dissolved
Temperature does not change
O2 or CO2 content – just the proportion of the components (dissolved vs. partial pressure)
pH responds to changes in
CO2 content, not changes in the CO2 components
Henry’s Law part 1=
- As pressure increases, solubility of gasses in liquids increases
Part one in English: More pressure means more gas can be dissolved in a liquid. Decreasing pressure causes that gas to come out of the liquid. (Δ P = Δ solubility - direct relation)
Henry’s Law part 2=
- As temperature increases, solubility of gasses in liquids decreases
Part two in English: Colder liquids hold more gas than warmer liquids. As a liquid warms up, the gas starts to come out of solution. (Δ T = Δ solubility – inverse relation)
Henry’s Law formula
Content = Partial pressure x Solubility
At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is
directly proportional to the partial pressure of that gas in equilibrium with that liquid
Temperature is DIRECTLY related to
partial pressure
Temperature is INVERSELY related to
solubility
Increased temperature=
Decreased solubility
Increased partial pressure
Decreased temperature=
Increased solubility
Decreased partial pressure
Rate of Cooling
cool at 1C per minute
Rate of Warming
warm at 1C per 3 to 5 minutes
Consequences of cooling/warming too fast
- development of temperature gradients within tissues
- body cooling after bypass
- exposure to hyperthermia
Cooling/warming limitations
- water temperature of heat exchanger
- blood pressure and systemic vascular resistance
- flow rate
Importance of Temperature Gradients=
- Gaseous micro emboli (CME) formation (gas solubility and temperature)
- Temperature gradients (between heat exchanger and venous blood/ between arterial blood and patient)
- Reduce probability of GME generation
Reduce probability of GME generation=
- limit temperature gradient between arterial and venous blood (adults: <200 mmHg
How does Exposure to Hyperthermia happen
time pressure of rewarming
- efficiency of heat exchanger (more or less than expected)
- high water temperatures
- perfusionist inattention
Consequence of hyperthermia
risk of cerebral injury
Deep Hypothermic Circulatory Arrest (DHCA)=
- Cool patient to 18 to 20C. Turn off pump for limited period of time (30 to 60 minutes)
- Organ at greatest risk – brain
- Optimal depth of cooling-temperature monitoring – multiple sites. EEG cessation
DHCA Safe Circ Arrest Times- 4 – 5 min
Temp?
O2 consumption?
37C
100%
DHCA Safe Circ Arrest Times- 8 – 10 min (Moderate)
Temp?
O2 consumption?
29C
50%
DHCA Safe Circ Arrest Times- 16 – 20 min (Deep)
Temp?
O2 consumption?
22C
25%
DHCA Safe Circ Arrest Times- 32- 40 min (Profound)
Temp?
O2 consumption?
16C
12%
DHCA Safe Circ Arrest Times- 64 – 80 min
Temp?
O2 consumption?
10C
6%
DHCA Benefits
- Allows exposure
- Reduces metabolic rate and molecular movement
- Allows cessation of circulation
- Excitatory neurotransmitter reduction
DHCA disadvantages
- Neurologic injury & morbidity
- Brain is at the most risk
• >40 min increases risk
• >60 min arrest is detrimental
Increase tolerance of brain to ischemic insult by using
- thiopental – short acting barbituate
2. solumedrol – anti-inflammatory, stabilize cell membranes
Homogenous temperature=
- rate of cooling / warming
- hemodilution
- acid-base management
- head in ice
Recovery – reperfusion conditions=
- perfusate temperature
2. perfusate composition: mannitol, bicarbonate, others
