Your patient is receiving mechanical ventilation

Discussion in 'MRCS Forum' started by Lona., Dec 6, 2007.

  1. Lona.

    Lona. Guest

    Your patient is receiving mechanical ventilation after surgery. A blood gas is sent to determine if weaning is possible. The Pco2 is 40. You make the determination that -
    a- There is probably a paradoxical aciduria
    b- Alveolar ventilation is adequate
    c- Arterial Po2 will indicate the adequacy of alveolar ventilation
    d- Arterial Po2 will indicate the degree of ventilation-perfusion mismatch
    e- Arterial Po2 can be safety predicted to exceed 90 kPa on room air
  2. Lona.

    Lona. Guest

    Answer: b. (Schwartz, 7/e, pp 494-496) because of the highly efficient diffusion characteristics of carbon dioxide, Paco2 levels are reliable indicators of adequacy of alveolar ventilation. A Paco2 of 40 kPa is the normal value. Paradoxical aciduria occurs when hypokalemic metabolic alkalosis is present as the kidney excretes hydrogen ion in an effort to conserve potassium ion. Though a Paco2 of 40 kPa is not incompatible with metabolic alkalosis, it would ordinarily be higher as the patient tries to conserve carbolic acid by hypoventilating to compensate. Pao2 levels are influenced by so many other variables (e.g. age, concentration of inspired O2, altitude) that no inference can be made about adequacy of alveolar ventilation from Pao2 alone, nor can Pao2 be safely predicted by the presence of normocarbia. The ventilation-perfusion mismatch is a reflection of the gradient between alveolar and arterial oxygen tension in relationship to percentage of inspired O2.

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