PaCO2 equationAlveolar Ventilation

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The Key to Blood Gas Interpretation:Four Equations, Three Physiologic Processes

Equation Physiologic Process1) PaCO2 equation Alveolar ventilation2) Alveolar gas equation Oxygenation3) Oxygen content equation Oxygenation4) Henderson-Hasselbalch equation 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

PaCO2---------------------------- =VCO2 x 0.863VA = VE – VD

VCO2 = CO2 productionVE = minute (total) ventilation = resp. rate x tidal volumeVD = dead space ventilation = resp. rate x dead space volume0.863 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

AirwaysAlveoli

Alveolus

ETT

Alveolus

VDequip VDanat

VDA

High PEEP

High PEEP

PaCO3 Equation

• Hypothermia• Hyporthyroidism• Underfeeding• Neuromuscular blockade• High fatty acid substrate

• Sepsis/inflammation• Hyperthermia• Hyperthyroidism• High carbohydrates• Seizure and agitation

Low Production High Production

PaCO2=VCO2

.

VE * (1- VD/VT)Respiratory Rate

Tidal Volume

VDequip

VDanat

VDA

Cell Metabolis

m

HME

PEEPLow BP

Hypercapnia

↑PaCO2↑VCO2

----------------------- =↔VA = VE – VD

Increased CO2 production but not able to hyperventilate:FeverSepsisHyperthyroidismOverfeeding with carbohydratesAgitation

Hypercapnia

↑PaCO2↔VCO2

----------------------- =↓VA = ↓VE – VD

Decreased Alveolar Ventilation due to Decreased Minute Ventilation (VE= ↓VT X ↓RR)

Sedative drug overdoseRespiratory muscle paralysisCentral hypoventilation

Hypercapnia

↑PaCO2↔VCO2

----------------------- =↓VA = VE – ↑VD

Decreased Alveolar Ventilation due to Increased Dead Space Ventilation (VD= Dead Space Volume X RR)

Pulmonary embolismHigh PEEPPulmonary hypertensionChronic obstructive pulmonary disease

Hypocapnia

↓PaCO2↓VCO2

----------------------- =↔VA = VE – VD

Decreased CO2 production but same minute ventilation:HypothermiaParalysisHypothyroidismUnderfeeding with carbohydratesSedation

Hypocapnia

↓PaCO2↔VCO2

----------------------- =↑VA = ↑VE – VD

Increased Alveolar Ventilation due to Increased Minute Ventilation (VE= ↑ VT X ↑ RR)

CNS stimulantsAgitationCentral hyperventilation

Eucapnia

↔PaCO2↑VCO2

----------------------- =

Increased CO2 production and Increased Alveolar Ventilation:

Fever and sepsisHyperthyroidismAgitation

↑VA = ↑VE – VD

Eucapnia

↔PaCO2↓VCO2

----------------------- =

Decreased CO2 production and decreased Alveolar Ventilation

HypothermiaHypothyroidism

↓VA = ↓VE – VD

PCO2 vs. Alveolar VentilationThe 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.

VCO2 X 0.863

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.

PaCO2 ----------------------- =VA = VE – VD

VCO2=300 X .863

VA = VE (300X24) – VD (150 X 24)

VCO2=259

VA = VE (7.2) – VD (3.6)VA = 3.6PaCO2=71.9

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?

VCO2 X 0.863 PaCO2 ----------------------- =

VA = VE – VDPaCO2=40

200 X 0.863 VA = 4.32 L/min

↑300 X 0.863 PaCO2=59.9

Effective Ventilation

AirwaysAlveoli

Alveolus

ETT

Alveolus

VDequip VDanat

VDA

VT= 500 VDequip= 50 VDanat= 125

VDA= 25 VTe= 300RR= 10

VT= 250 VDequip= 50 VDanat= 125

VDA= 25 VTe= 50RR= 20

VE= 5 L/min

Ventilator Course in Sudan: December 15-16, 2011