Capnography

Question 1

VD/VT

Answer 1

VD/VT = (PaCO2 - PĒCO2 ) ÷ PaCO2
• Using the mean exhaled CO2 (PĒCO2 )
• Normal ranges for PĒCO2 are 24 to 32 mmHg
• Normal ranges for VD/VT are 20% - 40%
• Slightly higher with intubated patients on mechanical ventilators
• 60% or greater is a contraindication to weaning

Question 2

PaC02 PETC02 Gradient

Answer 2

Example:
PaC02 from arterial blood gas report 35 torr
PETC02 on capnograph ABG draw - 32 torr
PaC02 - PETC02 = 3 torr

Normal PaC02-PETC02 gradient is 3-5 torr

Question 3

Mechanical deadspace (VDmech)

Answer 3

(rebreathed gas in the ventilator circuit) - Tubing between the “Wye” and the patient. Tubing between the patient connector and T-piece etc. (about 50ml).

Question 4

Anatomic deadspace (VDanat)

Answer 4

(VDanat = IBW)
Females: IBW = 105 + (5 x Inches)
Males: IBW = 106 + (6 x Inches)

Question 5

Alveolar VD (VDalv)

Answer 5

Alveoli that are ventilated but not perfused

VD Alveolar = VD Total - (VDmecc+VDanat)

Question 6

Physiological (VDphys)

Answer 6

Is the sum of anatomic and alveolar VD.

VDanat+VDalveolar = Physiologic VD

Question 7

Total Deadspace

Answer 7

VD/VT ratio x Vt

Example: 0.35 VD/VT ratio x 600ml = 210 Total VD

Question 8

Causes of Increased VD/VT

Answer 8

• Pulmonary Emboli
• Emphysema
• Acute lung injury
• Pulmonary hypertension
• Decreased cardiac output (CHF)