A female patient who is 5'7 tall and weighs 68 kg is being ...

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A female patient who is 5'7 tall and weighs 68 kg is being mechanically ventilated with vol-ume-controlled continuous mandatory ventilation (VC-CMV), set rate 12,
 
  patient trigger rate 25 breaths/min, tidal volume (VT) 500 mL, set flow rate 60 L/min, fractional inspired oxygen (FIO2) 40, positive end-expiratory pressure (PEEP) 5 cm H2O. The patient is currently in distress using accessory muscles of inspiration. The respiratory therapist performs a patient-ventilator system check. The flow-time waveform shows a failure of the expiratory flow to return to zero before the next breath is triggered. The most appropriate action for the respira-tory therapist to take includes which of the following?
  a. Sedate the patient.
  b. Switch to pressure-controlled continuous mandatory ventilation (PC-CMV).
  c. Decrease set rate to 8 breaths/min.
  d. Switch to volume-controlled intermittent mandatory ventilation (VC-IMV).

Question 2

A 6'2 male patient is being ventilated in the volume-controlled continuous mandatory ventilation (VC-CMV) mode with a set tidal volume (VT) of 650 mL. There is 40 mL of mechanical dead space. Calculate the final alveolar ventilation.
 
  a. 432 mL
  b. 445 mL
  c. 510 mL
  d. 535 mL

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Answer to Question 1

ANS: D
There are two clues to the fact that this patient is having problems because of unintended positive end-expiratory pressure (auto-PEEP): the patient's trigger rate of 25 breaths/min and the flow-time curve not returning to zero before the next breath is triggered. Sedating the patient is not the most appropriate action to take in this situation. Sedating and paralyzing the patient are reserved as last resorts for respiratory distress from patient-ventilator asynchrony. Switching to pressure-controlled continuous mandatory ventilation (PC-CMV) will most likely not change the situation too much, unless the pressure is markedly reduced to reduce the tidal volume. Decreas-ing the set rate to 8 breaths/min will not affect the patient's trigger rate and therefore will not change the situation. Switching to a mode where there is more spontaneous breathing is an ac-ceptable strategy for dealing with auto-PEEP.

Answer to Question 2

ANS: A
Volume of anatomical dead space (VDanat) = 1 mL/lb ideal body weight (IBW)
IBW = 106 + 6 (ht inches  60); tidal volume (VT)  volume of mechanical dead space (VDmech)  VDanat.

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