MEDI7111 Lecture Notes - Lecture 10: Pneumoconiosis, Dlco, Polycythemia
Respiratory 10
Investigations
Arterial Blood Gases
Arterial blood gas analysis provides information about the pH, oxygen and carbon dioxide
parameters of the blood. They provide a small amount of information that is of enormous
help when determining management of acutely and chronically ill patients. The assessment
IS NOT DIAGNOSTIC! It only provides information about the patient’s gas exchange,
ventilatory control and acid-base balance.
Interpretation of each parameter
pH
pH indicates acidosis or alkalosis but in chronic conditions the pH is usually buffered to
within normal ranges.
PaCO2
PaCO2 is a function of ventilation. CO2 is incredibly efficient at diffusion therefore any
changes in the PaCO2 are the result of a change in ventilatory pattern. Hyperventilation
leads to respiratory alkalosis due to the loss of CO2 from the blood resulting in a left shift of
the bicarbonate buffer system. Conversely, hypoventilation causes respiratory acidosis from
a right shift of the buffer system (type II respiratory failure).
PaO2
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PaO2 is a function of ventilation/perfusion matching. Clinically a PaO2 that is lower than
expected is hypoxia and generally requires oxygen therapy. V/Q mismatch is case specific
and is determined by the alveolar gas equation.
P(A-a)O2
The alveolar air difference is used to determine the cause of the hypoxia, either
hypoventilation, V/Q mismatch or both. All causes of hypoxemia (except
hypoventilation) cause an increase in the alveolar air difference. Combining this detail
with the other parameters on the report can assist in the diagnosis of the cause of
respiratory failure.
HCO3-
Bicarbonate is the primary compensatory mechanism for pH and PaCO2. With a renal
disorder than increases/decreases the amount of bicarbonate in the blood; respiration can
be altered to maintain bicarbonate concentration within the normal range with virtually
immediate effect. However, when there is a respiratory derangement in PaCO2, it can take 3-
5 days for the kidney to compensate with additional/less bicarbonate. Therefore when
there is derangement of both PaCO2 and HCO3- it is assumed that the respiratory issue has
been metabolically compensated.
ROME is an acronym that helps to recognise whether the abnormal values are respiratory
or metabolic in origin. RO – respiratory is opposite directions of change, ME – metabolic is
the same direction of change.
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Document Summary
Arterial blood gas analysis provides information about the ph, oxygen and carbon dioxide parameters of the blood. They provide a small amount of information that is of enormous help when determining management of acutely and chronically ill patients. It only provides information about the patient"s gas exchange, ventilatory control and acid-base balance. Interpretation of each parameter ph ph indicates acidosis or alkalosis but in chronic conditions the ph is usually buffered to within normal ranges. Co2 is incredibly efficient at diffusion therefore any changes in the paco2 are the result of a change in ventilatory pattern. Hyperventilation leads to respiratory alkalosis due to the loss of co2 from the blood resulting in a left shift of the bicarbonate buffer system. Conversely, hypoventilation causes respiratory acidosis from a right shift of the buffer system (type ii respiratory failure). Clinically a pao2 that is lower than expected is hypoxia and generally requires oxygen therapy.
