some good answers.
but it aint if it is less than 7.4 as that is out of context. If all values are within 'normal limits', we dont ask that question--that question is used primarily to determine patial/complete compensation issues.
'normal' values (it is relative, as this may not be normal for a given disease process, in which case Hx is needed for proper context):
ph: 7.35 -7.45 (some books use 7.37)
p02 80-100 (not 60)
sa02 95-100 (not 90%, not 92%. something physiological/environmental is in place if it is less than 95)
Base excess (BE) -2 to 2
anion gap, without potassium, 12-16 (unlike the poster, acidosis does not have to be lactate, so we use this gap to look for poisoning, ie ethylene glycol, etc, and then check direct osmolality, should be less than 295-300mOsm)
first first: if all values are wnl, dont procede with the analysis below
ph less than 7.35 = acidosis
ph more than 7.45 = alkalosis
write answer down on paper
pco2 less than 35 = alkalosis
pco2 greater than 45 = acidosis
explanation: co2 in the blood forms with hco3 to form h2co3, or carbonic acid. If we can increase the respiratory rate or volume (alveolar minute ventilation--said this for a reason, not just minute ventilation as it does not compensate for dead space), we can remove this co2 so it does not form carbonic acid and thus increase acidosis. this system kicks in within minutes-hours (carbonic system and protein system actually in seconds)
write answer down on paper, labeled respiratory ______, as i just explained it is tied to the resp. system
hc03 less than 22, acidosis
hc03 greater than 26, alkalosis
explanation: kidneys can excrete Hydrogen (thus it can not form with co2) and retain K or Na accordingly. Therefore this is 'metabolic' control, kicks in within several hours to few days
label it, metabolic ________
note: you may have a mixed metabolic and respiratory component if they are in the both direction, ie both acidotic or both alkalotic
This next step is where we use 7.4 as the ABSOLUTE value. 7.4 is used to determine partial compensation or complete compensation
If there is an acidosis or alkalosis, and the ph is exactly 7.4, we have complete compensation. Rare.
If it is respiratory acidosis, but the hco3 is above 26, we have partial compensation (the metabolic system is trying to make the blood alkalotic to make the ph 'normal', ie 7.35 to 7.39)
if it is respiratory alkalosis, but the hco3 is below 22, we have partial compensation (the metabolic system is trying to make the blook more acidotic to make the ph 'normal')
if it is metabolic acidosis, but the pco2 is below 35, we have partial compensation (the respiratory system is hyperventilating trying to blow off co2 so it can not form carbonic acid, thus resp. alkalosis)
if it is metabolic alkalosis, but the pc02 is above 45, we have partial compensation (lower rate or volume of breathing, trying to retain co2 to make more acid to make more normal ph). metabolic alkalosis is almost always BAD NEWS--be forewarned and on guard.
It gets more complicated than this, but i dont want to type more and cause more confusion
I believe my answer is the best thus far.
I could be wrong.
it should get u 95% accuracy.
if the sa02 as measured via abgs is below 90, as far as the texts are concerned, we have hypoxia, as is the case with p02 60 or below, we correlates with the beforementioned on the oxyhemoglobin dissassociation curve. but it is more than this. use the other poster's formula with the fio2 multiply-er to determine what the true po2 should be.
personally, i feel u should be wary if it is below 95% and 80mmhg on the po2 assuming room air.
to determine the underlaying cause of the hypoxia, consider ketones, lactate, anion gap to assist in determining said underlaying cause, in addition to ventilation/perfusion mismatch or shunt and poisonings (carbon monoxide, cyanide, drug od like tricyclics, ethanols/ethylenes, etc)
furthermore, as the other poster stated, we need to determine:
adequate heart rate (HR)
adequate cardiac output
adequate hemoglobin (not hematocrit) (ie 12-18, but rarely transfuse until below 8mg/dl)
adequate 2,3 diphosphoglycerate (banked blood is depleted, thus massive transfusions with be deficit and the blood will not want to release the oxygen off the hgb, a shift in the oxy-hgb curve, increasing affinity of hgb and o2)
all the beforementioned is best evaluated as the poster said, not via oxygen delivery:
cardiac output X HGB X 1.34 (or 1.37 depending on book)
but by oxygen consumption
this is because THIS patient may NEED more oxygen.
the delivery equation assumes normalcy. would you want a 'normal' oxygen delivery if you were on a treadmill running as fast as you could for 1 hour?
No. you would need a much HIGHER delivery, and thus the correct measure is the extraction ratio, usually quickly calculated by considering the abg sao2 and the svo2 subtraction
However, the equation estimation is thus, based on FICK:
(cardiac output X sao2) - (cardiac output X mixed venous blood o2 (obtained via pulminary capillary wedge port, wedged)
as u can see, that roughly correlated with the beforementioned sa02 - sv02
that is all i can think of at the moment during my work break.