Week I Physiology Flashcards
Dalton’s Law for Inspired Air
PIO2 = (PB – 47 Torr) × 0.21
Gives you the partial pressure of O2 in inspired air when it has just entered the airways
If breathing 100% oxygen PIO2 = 713 Torr
you multiply (760-47) x 1.0
Alveolar Gas Equation
Simplest form PAO2 = PIO2 - PaCO2 (for R=1)
PAO2 = PIO2 - (PaCO2/R)
R= 0.8 for normal diet. Respiratory exchange ratio of 8/10 CO2:O2 produced
Alveolar Gas Equation
Simplest form PAO2 = PIO2 - PACO2 (for R=1)
PAO2 = PIO2 - (PACO2/R)
R= 0.8 for normal diet. Respiratory exchange ratio of 8/10 CO2:O2 produced
If patient breathing 100% O2, assume that R=1 b/c only gas available to replace any CO2 deficit is O2
Alveolar Ventilation Equation
PACO2 = VCO2/VA x k
- PACO2 is partial pressure of CO2 in alveoli. Rapidly equilibrates = equivalent to arterial CO2.
- VCO2 is quantity of CO2 produced in one minute.
- VA is ventilation rate
Alveolar Ventilation Equation
PACO2 = V’CO2/V’A x k
- PACO2 is partial pressure of CO2 in alveoli. Rapidly equilibrates = equivalent to arterial CO2.
- V’CO2 is quantity of CO2 produced in one minute.
- V’A is alveolar ventilation
Another Solution to the Alveolar Ventilation Equation
PaCO2 (new)/ = V’A (old)/
PaCO2 (old) V’A (new
Arterial Oxygen Content
Quantity of O2 delivery
CaO2 = (SaO2 x [Hb] x 1.39) + (0.003 x PaO2)
-1.39 is the maximum volume of O2 (in ml) that can combine with 1 gram of hemoglobin (with units of ml/gm)
-usually 20.7 ml O2/100 ml blood
D’O2 = Q’ x CaO2
-For a typical cardiac output Q ̇ = 5,000 ml/minute, the oxygen delivery will be (5,000 ml blood/min) x (20.7 ml O2/100 ml blood) = ~1,000 ml O2/min.
Oxygen Consumption
V’O2 = Q’ x (SaO2 -SvO2) x [Hb] x 1.39
Henderson-Hasselbalch Equation for Bicarbonate/CO2.
pH = pKa + log [HCO3-]/[CO2] pH = 6.1 + log[HCO3-]/[0.03xPCO2] -normal pH is 7.4 (7.38-7.43, perhaps a bit higher here in Denver -PaCO2 = 36±2 -[HCO3]- = 22±2 meq/L
Most Common Causes of Acid Gap Acidosis
MUDPILES (Metabolic acidosis: addt'l acids in the blood causing a larger-than-expected acid gap) Methanol Uremia, DKA (ketoacidosis in general like starving, alcoholism), Propylene glycol Isoniazid Lactate Ethylene Glycol Salicylates
Respiratory Acidosis/Alkalosis:
How do pH and bicarb change with an acute 10 Torr rise in PaCO2? How about bicarb per chronic 1 Torr change in PaCO2?
- 08 fall in pH. 1 mEQ rise in bicarb.
- 4 bicarb change in same direction of CO2 (chronic respiratory acidosis). 0.003pH and 4 mEQ bicarb rise per 10torr rise
What is Winter’s Formula and when does it apply?
Used to calculate the expected pCO2 in compensation of Metabolic Acidosis
pCO2 = 1.5[HCO3-] + 8 +/- 2
if pCO2 measured on ABG is close to expected, you consider it a “compensated acidosis.” Body’s done what it can.
What happens to PaCO2 in metabolic alkalosis?
[HCO3-] increase of 1mEq/L increases PaCO2 by 0.7 Torr
Tell me about the ion gap, including how to calculate it.
The ion gap represents the difference in charge between the major cation [Na+] and the two major anions [Cl- + HCO3-]
AG = Na+ - (Cl- + HCO3-) = 12 +/- 2 normally
Anion Gap acidosis –> think MUDPILES
Non-gap acidosis –> think loss of bicarb via gut or kidney
What is the only mediator of ventilation response to metabolic acid/base insults?
Carotid Peripheral Chemoreceptors! Bonus: they mediate fast responses to High arterial PCO2 and high arterial [H+]
