Blood Gase Dr. Ambrisko Flashcards
What are the buffering systems of the body
Chemical
Respiration
Renal
How fast do chemical buffers work
extracellular: in seconds (HCO3-)
intracellular: within hours (phosphate & proteins)
How fast does respiration buffer work
adjusting CO2 within minutes
How fast does the renal buffering system work
within hours to days by excreting H+ & retaining HCO3-
metabolic compensation
What is the simple way to understand how changes in HCO3 influence pH
pH = HCO3-/PaCO2
What are the measured blood gas variables
pH
PaCO2
PaO2
What are the calculated blood gas variables
HCO3
BE
Oxygen content (CaO2)
define -emia
applies to changes in blood
define -osis
applies to physiological processes (conditions)
What is assessed for respiratory component of pH
PaCO2
What is assessed for metabolic component
BE or HCO3
is there any metabolic compensation during anesthesia for pH changes
No! No time
Normal blood pH
7.35-7.45
define acidosis
a physiological process, that occuring alone, tends to cause acidemia
define alkalosis
a physiological process, that occurring alone, tends to cause alkalemia
what are mixed acid-base disorders
different kinds of acidosis a/o alkalosis occurring together
what is a primary acid-base disorder
defined by the initial change in HCO3 or PaCO2
What is compensation
secondary change in HCO3 or PaCO2 in order to attenuate the effect of the primary disorder on blood pH
in respiratory acidosis would PaCO2 elevated?
what would the compensation be?
yes
HCO3 would elevate to compensate (metabolic compensation)
in respiratory alkalosis would PaCO2 elevated?
what would the compensation be?
No!
Metabolic compensation would mean a decrease in HCO3
in metabolic acidosis would HCO3 be elevated?
what would the compensation be?
No
Respiratory compensation would be a decr in PaCO2
in metabolic alkalosis would HCO3 be elevated?
what would the compensation be?
Yes
the respiratory compensation should be an incr PaCO2
How to tell if an acid-base disorder is mixed
pH value is unexpected from a change in HCO3 or PaCO2
normal pH w/ abnormal HCO3 or PaCO2
normal HCO3
24 +/- 4 mEg/L
cats tend to be lower - think about diet, higher acid
herbivores tend to be higher - think about diet, higher base
normal PaCO2
35-45 mmHg
cats tend to be lower (just weird animals)
What is Base excess (BE)
refers to an excess or deficit in the amount of base present in blood
defines the metabolic component of acid-base disturbances
What does a positive BE signify
metabolic ALKALOSIS
(positive for excessive base in blood)
What does a negative BE signify
metabolic acidosis
Normal BE
0 +/- 4 mEq/L
How does CO2 exist in blood
as HCO3 & dissolved CO2
What is TCO2
Total CO2
almost the same as HCO3
can use interchangeably if HCO3 not available
What is PaCO2 equation
partial pressure (mmHG) of CO2 in the arterial blood
What is important about PaCO2
it defines alveolar ventilation
What are the factors that can reflect (make up) any PaCO2 value
Any combo of RR, TV or breathing effort
A PaCO2 (mmHg) of >45 would indicate which blood condition
which state of ventilation
hypercapnia
hypoventilation
A PaCO2 (mmHg) of 35-45 would indicate which blood condition
which state of ventilation
eucapnia
normal ventilation
A PaCO2 (mmHg) of < 35 would indicate which blood condition
which state of ventilation
hypocapnia
hyperventilation
What will an incr PaCO2 cause
(hypoventilation)
lower PAO2
lower pH
reflect respiratory acidosis
reflect respiratory compensation for metabolic alkalosis
*PAO2 = partial pressure (mmHg) of O2 in Alveolar space
what do you need to remember when interpreting PaO2
does not reflect O2 content
interpreted in light of
- FiO2
- ambient pressure
- PaCO2
what is FiO2
inspired O2 fraction
- at 100% O2 ⇒ FiO2 = 1*
- at air ⇒ FiO2 = .21*
what is PiO2
inspired O2 pressure (FiO2 x Pambient)
- at 100% O2 ⇒ PiO2 = 760 mmHg*
- at air ⇒ PiO2 = 160 mmHg*
What should the PaO2 be when breathing 100% O2
and if not what expected?
expect PaO2 > 500 mmHg
Lower than expected PaO2 values may indicate V/Q mismatch resulting from atelectasis
What is A-a difference
aka A-a gradient ⇒ is the real life difference between the Pressure of O2 in the Alveolar space vs the Pressure of O2 in arterial blood
What is most likely cause for a high A-a difference (or gradient)
V/Q mismatch ⇒ #1 cause under anesthesia
reasons for V/Q mismatch
Atelectasis (common under anesthesia)
What can A-a difference be used for
to distinguish lung diseases (V/A mismatch) from hypoventilation as a cause of hypoxemia
with hypoventilation: A-a difference normal
with V/Q mismatch: A-a difference abnormal
What is normal PaO2/FiO2 ratio
> 500 mmHg
Name 5 causes for hypoxemia (low PaO2)
Low FiO2
Hypoventilation
Diffusion impairment
V/Q mismatch
R-L shunt
define hypoxia
causes
insufficient oxygenation of the tissues
hypoxemia
insufficient perfusion
insufficient O2 uptake
where is most O2 carried in arterial blood
carried by Hb
small portion dissolved in plasma
in a clinical situation if lactate is >2 mmolL what is the significance
lactate >2mmol/L is high & may indicate incr anaerobic metabolism
what is abnormally high lactate called
most common causes
lactic acidosis
Systemic: shock, sepsis (e.g. GDV, colic)
Local: tournequet, under-perfusion of dependent muscles, intestinal torsion, etc.
what is the parameter that is a prognostic indicator when managing sick patients
lactate level
