Topic 7 Flashcards
Buffer systems designed to maintain constant ratio of
[OH-] to [H+] (16:1) despite changes in temperature
Bicarbonate
protein
phosphate
Primary blood buffering system responsible for this is the
imidazole moiety of the amino acid histidine
-Found on most blood borne proteins
Imidazole
HIm+(Protonated) H+ + Im (Nonprotonated)
In blood, this dissociation accounts for most of the changes in pH due to
changes in temperature
Alpha stat and pH stat management ONLY affect control of the
arterial pCO2 and pH
Arterial pO2 MUST be
temperature corrected
Allowing CO2 and pH to change “naturally” based on solubility changes and changes in Imidazole dissociation=
Total CO2 contend DOES NOT CHANGE!!!!
•This represents alpha stat management of CO2 & pH
pH of the blood will vary _____ with changes in temperature
inversely
-pH will change from organ to organ
Forcing CO2 and pH to remain at “normal” values=
Total CO2 contend HAS TO INCREASE!!!
•CO2 has to be added to the system
•This represent pH stat management of pCO2 & pH
pH-stat=
- maintain constant pH with change in temperature
- CO2 content has to increase as temperature decreases and has to decrease as temperature increases
- ratio of unprotonated histidine imidazole group to H+ (alpha value) changes as CO2 content increases
Alpha-stat=
- maintain constant CO2 content with change in temperature
- pH will increase as temperature decreases and will decrease as temperature increased
- ratio of unprotonated histidine imidazole group to H+ (alpha value) remains constant
pH-stat:
as temperature decreases
CO2 is added to maintain “normal” pH
pH-stat:
as temperature increases
CO2 must be removed
pH-stat consequences during hypothermia
- blood more acidic
- hydrogen ion gradient across the cell membrane changes
- acidity alters metabolic activity
- cells do not maintain electroneutrality
- added CO2 influences blood flow regulation***
alpha-stat :
as temperature decreases
pH will rise
alpha-stat :
as temperature increases
pH will fall
alpha-stat consequences during hypothermia
- maintain constant charge on alpha-imidazole side chain
- hydrogen ion gradient across cell does not change
- cellular enzyme functions are maintained
- cells maintain electroneutrality
- auto-regulation of blood flow maintained***
Alpha-Stat: allow an increase in pH (decrease in pCO2) with a
decrease in temperature
Alpha-stat maintain constant
total CO2 content in the blood
pH-Stat: keep pH normal at all
temperatures (when compared to alpha stat at each temperature, allowed a decrease in pH with a decrease in temperature)
pH-stat: total CO2 content in the blood must
increase
- active addition of CO2
- alteration of ventilation
Affects on Tissue Oxygenation:
pH-stat shifts oxy-hemoglobin dissociation curve to the
right
-which offsets leftward shift due to hypothermia
Cerebral blood flow normally autoregulated=
flow tightly coupled to cerebral metabolism
flow independent of blood pressure
flow regulated by PCO2
Hypothermia: Affect on Cerebral Blood Flow=
Loses autoregulation when temperature falls below 20C
- flow becomes pressure dependent
- luxury flow (flow»_space; metabolic rate)
Affect on Cerebral Blood Flow:
pH-stat uncouples flow and metabolism=
- greater blood flow – pressure dependent
- improved cooling – more homogeneous
- luxury flow – predispose to risk of embolic injury, high intracranial pressure, edema
Affect on Cerebral Blood Flow:
Alpha-stat maintains autoregulation=
- lower blood flow – based on metabolism
2. predispose to adverse distribution of blood flow – risk of ischemic injury
What is BEST Strategy for REGULAR BYPASS ADULTS
- With little or no hypothermia might not make any difference.
- With mild to moderate alpha-stat might be better
What is BEST Strategy for PROFOUND WITH ARREST ADULTS
Combination (cross-over) best approach – use pH stat while cooling to ensure maximal cerebral metabolic suppression – switch to alpha-stat to remove acidosis
What is BEST Strategy for INFANTS
Appears pH-stat may offer better outcomes (major cause of brain injury may relate more to hypoperfusion rather than emboli
To manage pH-stat:
- Keep temperature corrected pCO2 at 40 mmHg
- Keep temperature corrected pH at 7.40
- uncorrected pH will decrease and the uncorrected PCO2 will increase
To manage alpha-stat:
- keep uncorrected pH and pCO2 values at normal levels (7.4 & 40 mmHg)
- OR keep corrected PCO2 equal to the patient’s core temperature (i.e. PCO2 will decrease) and corrected pH will increase
Which value to use for pO2
Alpha?
pH?
Alpha= Corrected ph= Corrected
Which value to use for pCO2
Alpha?
pH?
Alpha= Non corrected ph= Corrected
Which value to use for pH
Alpha?
pH?
Alpha= Non corrected ph= Corrected
Increased Temp= pp? h+? Solubility? pH?
pp= increase
h+= increase
Solubility=decrease
pH= decrease
Decreased Temp= pp? h+? Solubility? pH?
pp= decrease
h+= decrease
Solubility=increase
pH= increase
