46 terms

Pulmonary Diagnostic Testing

Alveolar Air Equation
calculates the partial pressure of oxygen (oxygen tension) in the alveoli (Alveolar PO2 or PAO2)
PAO2 =
(PB - PH2O)FIO2 - (PCO2/0.8)
shortcut for PAO2 equation
(7 x FI02 as a whole #) - (PaCO2 + 10)
normal value for PAO2
varies directly with pt's FIO2 and PB
A-aDO2 (A-a Gradient)
measures the difference(D) or gradient between alveolar and arterial PO2 (best done after pt on 100% for 20 minutes)
A-aDO2 equation
A-aDO2 = PAO2 (use Alveolar Air Equation)-PaO2 (use ABG)
normal value for A-a Gradient
25- 65 mmHg
V/Q mismatch shown in A-a Gradient
66-300 mmHg
Shunting (shunt refers to perfusion without ventilation) shown in A-a Gradient
> 300 mmHg
causes of pulmonary shunting
A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused
why is increasing the FIO2 unable to help increase the PaO2 when shunting is present?
Because shunt represents areas where gas exchange does not occur
how do you improve oxygenation when their is shunting
PEEP or positive pressure therapy
how do you improve oxygenation with a V/Q mismatch
increase the FIO2
CaO2= Arterial oxygen content
measures the oxygen carried by the hemoglobin as well as that dissolved in the plasma
where is the majority of the arterial oxygen content of the blood found
dissolved in the plasma
what is the best index of oxygen transport
CaO2 =
Hb x 1.34 x SaO2 (if SaO2 and Hb in normal range, then CaO2 will be in normal range)
normal value for CaO2
17-20 vol% (mL/dL)
CvO2 = mixed venous oxygen content
monitors tissue oxygenation
CvO2 =
Hb x 1.34 x SvO2
normal value for CvO2
12-16 vol%
what is CvO2's relation to Qt
direct (Qt decreases, CvO2 decreases)
C(a-v)O2 arterial-venous oxygen content difference
measures the oxygen consumption of the tissues
C(a-v)O2 =
normal value for C9a-v)O2
4-5 vol%
C(a-v)O2 relationship to CvO2 and Qt
inverse C(a-v)O2 increases as CvO2 decreases
PaO2/FIO2 ratio or P/F ratio
used in determination of acute lung injury or acute respiratory distress syndrome
normal P/F ratio
380 mmHg or more
P/F ratio that signifies acute lung injury
<300 mmHg
P/F ratio that signifies ARDS
< 200 mmHg
VO2 / C(a-v)O2 x 10
normal Qt
4-8 L/min
Qs/Qt (shunt equation)
the portion of cardiac output that is shunted (perfusion without ventilation)
Qs/Qt =
(shortcut) for every 100 T (mmHg) of my A-a graident I add %5 shunt after starting with 5% normal physiological shunt. (if A-a gradient is 300 mmHg then Qs/Qt would be (5% x 3) +5% = 20% shunt
normal Qs/Qt
the % of hemoglobin that is bound by O2
how can actual SaO2 be measured
oximeter or co-ox (ABG only calculates)
estimating SaO2 based on PaO2
if PaO2 between 40 and 60 add 30 to get SaO2 (if PaO2 50 then SaO2 is about 80)
at what temperature are ABGs typically reported
normal body temp of 37 degrees C
if pt has fever how will blood gas values be affected
lower PaCO2, PaO2 and higher pH (higher pH=patient Has fever)
VD/VT ratio
% of tidal volume doesn't participate in gas exchange. Dead-space to tidal volume ratio-ventilation without perfusion (the life is in the blood-so dead-space represents areas of the lung that are being ventilated but not perfused)
normal value for VD/VT ratio
20-40%, up to 60% for ventilator patients
what VD/VT ratio would suggest that a vent pt. could be weaned
< 60%
Increase in VD/VT ratio would indicate what
dead-space producing disease-i.e. pulmonary embolus
VD/VT ratio=
{(PaCO2 - PECO2) / PaCO2} x 100
desired VE =
(VE x PaCO2) = (VE x PaCO2)