A patient has a VT of 625 ml and a physiological dead space of ...

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A patient has a VT of 625 ml and a physiological dead space of 275 ml and is breathing at a fre-quency of 16/min. What is the alveolar ventilation (VA)?
 
  a. 3000 ml/min
  b. 4400 ml/min
  c. 5600 ml/min
  d. 7000 ml/min

Question 2

A normal 150-lb man is breathing at a rate of 17 with a tidal volume of 450 ml. By estimation, what is his approximate alveolar ventilation?
 
  a. 7.65 L/min
  b. 5.10 L/min
  c. 3.85 L/min
  d. 2.60 L/min

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Answer to Question 1

ANS: C
Alveolar ventilation depends on tidal volume, dead space, and breathing rate. For this patient the respiratory rate is 16, VT of 625 ml, and dead space (VD) of 275 ml. The alveolar ventilation is calculated as follows:
VA = 16  (625 ml  275 ml) = 16  350 = 5600 ml/min or 5.6 L/min

Answer to Question 2

ANS: B
VDanat averages approximately 1 ml/lb of ideal body weight (2.2 ml/kg). For a subject who weighs 150 lb (68 kg), VDanat is approximately 150 ml. For our patient the VT is 450, the RR is 17, and we will say his actual weight is his ideal body weight so 150-lb = 150 ml VD.
VA = f(VT  VD)
VA = 17(450 ml  150 ml)
VA = 17(300 ml) = 5100 ml or 5.1 L

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