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
