question about dead space and alveolar ventilation

[cox2ketamine]

Hey, I'm going through questions in Hall q book. 138. 70 kg woman. Lungs are mechanically ventilated with these parameters: VE 5000mL and RR 10 breaths/min. Assuming no change in VE how would VA change if the RR were increased from 10 to 20 breaths/min?

A. increase by 500mL
B. increase by 1000mL
C. decrease by 750mL
D. decrease by 1500mL

Answer is D. The explanation is VD 150mL and VA 350mL assuming normal VT 500mL will have VD with minute ventilation with 1500mL and VA minute ventilation 3500mL (VE 5000mL) at rate of 10 breaths/min. If RR doubled but VE stays the same then VD would also double to 3000mL for an increase of VD by 1500mL and decrease VA by 1500mL.
The first part I understand, but some how I'm missing how increasing the RR increases VD and decreases VA. Anyone have an explanation to help me understand? Thanks.

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[pgg]

Hey, I'm going through questions in Hall q book. 138. 70 kg woman. Lungs are mechanically ventilated with these parameters: VE 5000mL and RR 10 breaths/min. Assuming no change in VE how would VA change if the RR were increased from 10 to 20 breaths/min?

A. increase by 500mL
B. increase by 1000mL
C. decrease by 750mL
D. decrease by 1500mL

Answer is D. The explanation is VD 150mL and VA 350mL assuming normal VT 500mL will have VD with minute ventilation with 1500mL and VA minute ventilation 3500mL (VE 5000mL) at rate of 10 breaths/min. If RR doubled but VE stays the same then VD would also double to 3000mL for an increase of VD by 1500mL and decrease VA by 1500mL.
The first part I understand, but some how I'm missing how increasing the RR increases VD and decreases VA. Anyone have an explanation to help me understand? Thanks.

The reason is that dead space ventilation is constant, independent of tidal volume.

Before:
rate 10
minute ventilation = 5000 mL
tidal volume = 500 mL, of which 150 is dead space and 350 is alveolar ventilation
total alveolar ventilation = 10 x 350 = 3500 mL

After:
rate 20
minute ventilation = 5000 mL
tidal volume = 250 mL, of which 150 is STILL dead space and 100 is alveolar ventilation
total alveolar ventilation = 20 x 100 = 2000 mL

Decrease of 1500 mL

 

[cox2ketamine]

Thank you for the explanation!

 

[deleted171991]

This question also shows why it's so important to maintain calm and ventilate slowly during airway emergencies.