PaCO2 equation
Alveolar Ventilation
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1
The Key to Blood Gas Interpretation:
Four Equations, Three Physiologic Processes
Equation
1)
2)
3)
4)
PaCO2 equation
Alveolar gas equation
Oxygen content equation
Henderson-Hasselbalch equation
Physiologic Process
Alveolar ventilation
Oxygenation
Oxygenation
Acid-base balance
These four equations, crucial to understanding and interpreting arterial blood
gas data.
PaCO2 Equation: PaCO2 reflects ratio of metabolic
CO2 production to alveolar ventilation
VCO2 x 0.863
PaCO2= ---------------------------VA = VE – VD
VCO2 = CO2 production
VE = minute (total) ventilation = resp. rate x tidal volume
VD = dead space ventilation = resp. rate x dead space volume
0.863 converts VCO2 and VA units to mm Hg
PaCO2
Condition in Blood
State of Alveolar Ventilation
> 45 mm Hg
Hypercapnia
Hypoventilation
35 - 45 mm Hg
Eucapnia
Normal ventilation
< 35 mm Hg
Hypocapnia
Hyperventilation
Dead Space
VDA
ETT
VDequip
Airways
VDanat
PaCO3 Equation
Low Production
High Production
•
•
•
•
•
•
•
•
•
•
Hypothermia
Hyporthyroidism
Underfeeding
Neuromuscular blockade
High fatty acid substrate
PaCO2=
Cell
Metabolism
.
VCO2
VE * (1- VD/VT)
Respiratory
Rate
Tidal
Volume
Sepsis/inflammation
Hyperthermia
Hyperthyroidism
High carbohydrates
Seizure and agitation
VDequip
HME
VDanat
VDA
PEEP
Low BP
Hypercapnia
↑PaCO2
↑VCO2
= ----------------------↔VA = VE – VD
Increased CO2 production but not able to hyperventilate:
Fever
Sepsis
Hyperthyroidism
Overfeeding with carbohydrates
Agitation
Hypercapnia
↑PaCO2
↔VCO2
= -----------------------
↓VA = ↓VE – VD
Decreased Alveolar Ventilation due to Decreased
Minute Ventilation (VE= ↓VT X ↓RR)
Sedative drug overdose
Respiratory muscle paralysis
Central hypoventilation
Hypercapnia
↑PaCO2
↔VCO2
= -----------------------
↓VA = VE – ↑VD
Decreased Alveolar Ventilation due to Increased Dead
Space Ventilation (VD= Dead Space Volume X RR)
Pulmonary embolism
High PEEP
Pulmonary hypertension
Chronic obstructive pulmonary disease
Hypocapnia
↓PaCO2
↓VCO2
= ----------------------↔VA = VE – VD
Decreased CO2 production but same minute ventilation:
Hypothermia
Paralysis
Hypothyroidism
Underfeeding with carbohydrates
Sedation
Hypocapnia
↓PaCO2
↔VCO2
= -----------------------
↑VA = ↑VE – VD
Increased Alveolar Ventilation due to Increased
Minute Ventilation (VE= ↑ VT X ↑ RR)
CNS stimulants
Agitation
Central hyperventilation
Eucapnia
↑VCO2
↔PaCO2 = ----------------------↑VA = ↑VE – VD
Increased CO2 production and Increased
Alveolar Ventilation:
Fever and sepsis
Hyperthyroidism
Agitation
Eucapnia
↓VCO2
↔PaCO2 = ----------------------↓VA = ↓VE – VD
Decreased CO2 production and decreased Alveolar
Ventilation
Hypothermia
Hypothyroidism
PCO2 vs. Alveolar Ventilation
The relationship is shown for
metabolic carbon dioxide
production rates of 200 ml/min
and 300 ml/min (curved lines). A
fixed decrease in alveolar
ventilation (x-axis) in the
hypercapnic patient will result in a
greater rise in PaCO2 (y-axis)
than the same VA change when
PaCO2 is low or normal.
This graph also shows that if
alveolar ventilation is fixed, an
increase in carbon dioxide
production will result in an
increase in PaCO2.
PaCO2 and Alveolar Ventilation:
Test Your Understanding
What is the PaCO2 of a patient with respiratory rate
24/min, tidal volume 300 ml, dead space volume 150
ml, CO2 production 300 ml/min? The patient shows
some evidence of respiratory distress.
PaCO
2=71.9
PaCO
2
VCO
VCO
=259
X .863
VCO
X
0.863
2=300
2
2
= -----------------------
VA = VE – VD
VE
(7.2)
– VD
VD(150
(3.6)X 24)
VA
= –3.6
VA = VA
VE =(300X24)
PaCO2 and Alveolar Ventilation:
Test Your Understanding
What is the PaCO2 of a patient with respiratory rate
10/min, tidal volume 600 ml, dead space volume 150
ml, CO2 production 200 ml/min? The patient shows
some evidence of respiratory distress
VCO2 X 0.863
PaCO2 = ----------------------VA = VE – VD
PaCO2 and Alveolar Ventilation:
Test Your Understanding
A man with severe chronic obstructive pulmonary disease
exercises on a treadmill at 3 miles/hr. His rate of CO2
production increases by 50% but he is unable to augment
alveolar ventilation. If his resting PaCO2 is 40 mm Hg and
resting VCO2 is 200 ml/min, what will be his exercise PaCO2?
↑300
200 2XX
0.863
VCO
X0.863
0.863
=
----------------------PaCO
PaCO
PaCO
=59.9
=40
2
22
VA
VA==4.32
VE –L/min
VD
Effective Ventilation
VDA
ETT
VDequip
Airways
VDanat
VT= 500
RR= 10
VDequip= 50
VDanat= 125
VDA= 25
VTe= 300
VT= 250
RR= 20
VDequip= 50
VDanat= 125
VDA= 25
VTe= 50
VE= 5 L/min
Ventilator Course in Sudan: December 15-16, 2011