Equations & Physiology

Question 1

Normal PaO2, PaCO2, Vt, Vd

Answer 1

PaO2 > 80mmHg
PaCO2 = 40mmHg
Vt = 500 mL
Vd = 150 mL

Question 2

RQ Equation

Answer 2

RQ = VCO2/VO2
Based on diet/metabolism
Normal 0.8

Question 3

Alveolar Gas Equation (Oxygen)

Answer 3

PAO2 = PiO2 - (PaCO2/RQ)

where PiO2 = (PB - PH2O)*FiO2

Question 4

Alveolar Ventilation Equation (CO2)

Answer 4

PACO2 = 863mmHg*(VCO2/VA)

where VA = (RRVt)(1-(Vd/Vt))
and PACO2 = PaCO2 due to blood saturation curve not plateaued

Question 5

Oxygen Content Equation

Answer 5

C(O2) = (%O2,sat * 1.34 (mL O2/gm Hb) * [Hb]) + 0.003*PaO2

Normal [Hb] = 15gm/dL

Question 6

List the 3 key points on the O2-Hb Dissociation Curve

Answer 6

Normal: PO2 = 95mmHg, Hb-sat = 97%
Knee: PO2 = 60mmHg, Hb-sat = 90%
Mixed Venous: PO2 = 40mmHg, Hb-sat = 75%

Question 7

Normal A-a Difference and P-F ratio

Answer 7

A-a: PAO2:PaO2 - normal <10mmHg

P-F: PaO2/FiO2 > 300 (for when on supplemental O2/ventilation) bad if lower

Question 8

Criteria for ARDS (Timing, Imaging, Origin, Oxygenation, Pathophys, Histopath)

Answer 8

Timing: <1 week
Imaging: bilateral alveolar opacities
Origin: Non-cardiogenic 
Oxy: PaO2/FiO2 < 300
Pathophys: Injury to alveolar-capillary barrier, influx of protein-rich, low-pressure edema
Histopath: Diffuse Alveolar Damage (DAD)

Question 9

8 Causes of Hypoxemia

Answer 9

Low PAO2
1. Low PB (elevation)
2. Low FiO2 (smoke)
3. High PaCO2 (hypoventilation)
4. Low RQ
High A-a Difference
1. Shunt (V/Q = 0) (R->L Heart, BPWA)
2. V/Q mismatch (COPD, PE)
3. Diffusion Limitation (ILD, Emphysema)
4. Low mixed venous sat (low CO, Anemia, Hypoxemia, high VO2 - metabolism/exercise)

Question 10

Equation for mixed venous sat

Answer 10

MvO2 or SvO2 = delivered - lost
(CO* C(O2)) - VO2

Where C(O2) = (%O2,sat * 1.34 * [Hb] + 0.003 * PaO2); and [Hb] = 15

Question 11

Determining Primary Respiratory and Metabolic Acid Disturbances

Answer 11

Respiratory: pH and PCO2 move in opposite directions
R. Acid: low pH and high PCO2
R. Alka: high pH and low PCO2

Metabolic: pH and PCO2 move in same direction
M. Acid: low pH and low PCO2
M. Alka: high pH and high PCO2

Question 12

In Metabolic Disturbances, does bicarb change more or less than pH?

In Respiratory Disturbances, does bicarb change more or less than pH?

Answer 12

Bicarb changes less due to isohydric buffering - pH change reduced by other buffers

Bicarb changes more than pH because extra H+ are taken up by other buffers

Question 13

What is the buffer line? How is it determined?

Answer 13

Slope of where along PCO2 isobar the patient’s pH and HCO3- is
Based on the amount of non-bicarb buffer! More non-bicarb buffer = steeper slope (greater change in bicarb with pH)
No non-bicarb buffer - buffer line is flat - change in bicarb is 1:1 with H+ ions - negligible amount in blood

Question 14

How to calculate anion gap?

What does acid accumulation do to AG?

Answer 14

Na+ - (Cl- + HCO3-)
Normal: 10 +/- 2 mEq/L

Acid accumulation = increase in unmeasured anions (decrease in bicarb) = increases anion gap

Question 15

3 big causes of respiratory acidosis

Answer 15

1. CNS Drugs (opioids, sedatives)
2. SEVERE respiratory conditions
3. Acute neuromuscular problems
   ALL CAUSE HYPOVENTILATION

Question 16

Causes of Respiratory Alkalosis

Answer 16

ALL CAUSE HYPERVENTILATION
fear, anxiety, pain, fear
pregnancy
early sepsis
hyperthyroidism
drugs (stimulants) - cocaine, PCP, aspirin OD
MILD respiratory conditions (early stages)

Question 17

How does the body respond to respiratory acid-base disturbances? How long does this take

Answer 17

Kidney
Alkalosis: Secrete more bicarb to lower pH
Acidosis: Resorb more bicarb to increase pH
takes three days for full compensation

Question 18

How does the body respond to metabolic acid-base disturbances? How long does this take? How do you calculate this?

Answer 18

Lungs
Occurs immediately through ventilation changes
Winter's Formula:
1.5x[HCO3-] + 8 = PaCO2 (~+/-2)
OR decimals of pH ~ PaCO2

Question 19

Types of metabolic acidosis

Answer 19

AG elevated (MUDPILES)
non AG Elevated (USEDCARS)

Question 20

Types of AG elevated metabolic acidosis

Answer 20

Acid Accumulation
M - methanol (formic acid)
U - uremia (kidney)
D - diabetic ketoacidosis (or other ketoacids)
P - propylene glycol (antifreeze)
I - isoniazid (TB)
L - lactic acidosis
E - ethylene glycol (antifreeze)
S - salicyclic acid (asprin OD)

Question 21

Types of non-AG elevated Metabolic Acidosis

Answer 21

Bicarbonate Loss
U - uretero-enterostomy
S - saline (hyperchlorinemia)
E - endocrinopathies, adrenal insufficiency
D - diarrhea
C - carbonic anhydrase inhibitor
A - ammonium chloride (old cough syrup)
R - renal tubular acidosis
S - spironolactone

Question 22

Delta-Delta Analysis: how to do it, and what the results show

Answer 22

Determine if change in AG = change in bicarb

If you get a higher than normal bicarb –> concurrent metabolic alkalosis

If you get a lower than normal bicarb –> concurrent non-elevated metabolic acidosis (bicarb wasting)

Question 23

Types of Metabolic Alkalosis (and subtypes of those)

Answer 23

Need urinalysis

Chloride Responsive: Vomiting, NG Tube Suction, Over Diuresis (low Na/Cl in urine, low urine output)

Chloride Unresponsive: Hypokalemia, hyperaldosteronism

Question 24

What happens to the following in obstructive lung disease?
FEV1, FVC, FEV1/FVC
TLC, FRC, RV, PEF,
DLCO, Isovolume Flow

Answer 24

OBSTRUCTIVE (scoop shaped Q-V diagram)
FEV1 = down
FVC = normal
FEV1/FVC = down (defined by less than 0.7)
TLC/FRC/RV = up
PEF = down
DLCO = low (emphysema), normal/high (asthma with airway remodeling)
Isovolume Flow = low (lungs more compliant)

Question 25

What happens to the following in restrictive lung disease?
FEV1, FVC, FEV1/FVC
TLC, FRC, RV, PEF,
DLCO, Isovolume Flow

Answer 25

RESTRICTIVE (witch hat shaped Q-V diagram)
FEV1 = down
FVC = down
FEV1/FVC = up
TLC/FRC/RV = down
PEF = down
DLCO = low
Isovolume Flow = high (lungs less compliant)