Blood Gas Analysis Flashcards
Dead space ventilation
usually a 5-7mmHg difference
causes for increased deadspace
PPV (increased airway pressure)
upright posture
PE
PA thrombosis,
hemorrhage
hypotension
CPOD/Emphysema (blebs, loss of alveolar septa and vasculature)
Age
Dead Space-to-Tidal Volume Ratio
(VD/VT)
ππ·/ππ =(πaπΆπ2βππΈππΆπ2)/πππΆπ2
Normal <0.3
Hyperventilation and deadspace
Hyperventilation can completely overcome increased dead space
We typically shoot for PETCO2 around 33 mmHg = PaCO2 of 40 mmHg
Hypoxemia caused by
low PO2
Hypoventilation
Venous Admixture (Shunt, V/Q mismatch)
Hypoxia
PO2 <60mmHg
Alveolar Gas Equation
PaO2 = (Pb-Ph20)F102 - (PaCO2/RQ)
A-a Gradient
= PAO2-PaO2
Normal 5-10mmgHg in young health person in RN
increased with age/4 +4
(ex 80yo, normal is 24)
P/F Ratio
= PaO2/FiO2
Constant regardless of inspired concentration of O2
< 300 Mild
< 200 Moderate (>20% shunt fraction)
< 100 Severe
Mixed Venous PO2
related to both cardia output & tissue oxygen consumption
normal value 40mmHg (70% sat)
low values suggest tissue hypoxemia
hypoxemia can occur despite elevated mixed venous PO2
Fick Equation
ππ2=πΆπ π₯ (πΆππ2βπΆπ£π2)
rearrange, leads to
SvO2 = SaO2 β [(VO2) / (Hb x 1.36 x CO)]
Oxyhemoglobin Dissociation Curve: Decreased P50 (increased affinity)
decrease Temp
decreased PCo2
decreased 2,3-DPG
increased pH
Oxyhemoglobin Dissociation Curve: Increased P50 (decreased affinity)
increase temperature
increase PCO2
increase 2,3-DPG
decrease pH
Normal P50
26 mmHg
Hypothermia impact on O2 and CO2 solubility and partial pressure
partial pressure decreases
O2 and CO2 solubility increases
Left shift HbO2 dissociation curve
hypothermia
alkalosis
