Embed Size (px)
Citation preview
PaCO2 equation
Alveolar Ventilation
www.mecriticalcare.net
1
The Key to Blood Gas Interpretation:Four Equations, Three Physiologic Processes
Equation Physiologic Process1) PaCO2 equation Alveolar ventilation
2) Alveolar gas equation Oxygenation
3) Oxygen content equation Oxygenation
4) 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 0.863
VA = VE – VDVCO2 = 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
Airways
Alveoli
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
PEEP
Low BP
Hypercapnia
↑PaCO2
↑VCO2 ----------------------- =
↔VA = VE – VDIncreased 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 – ↑VDDecreased 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 – VDDecreased 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 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.
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
Airways
Alveoli
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
