by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology
Emergency therapy: The body's metabolism produces respiratory (carbonic) acid and, in cardiorespiratory failure also produces metabolic (lactic) acid. In emergencies, therefore, it is usual to find that correction is required for metabolic or respiratory acidosis.
For this reason, and in the interest of simplification, the following paragraphs primarily discuss acidosis and its correction:
Respiratory acidosis. A physician decides to ventilate a patient to reduce the PCO2 level based on exhaustion, prognosis, prospect of improvement from concurrent therapy and, only in part, on the PCO2 level. Once the clinical decision is made, the PCO2 helps calculate the required ventilation.
The PCO2 reflects the balance between the production of carbon dioxide and its elimination. Unless the metabolic rate changes, the amount of carbon dioxide produced is roughly constant and determines the amount of ventilation required to maintain a given PCO2 level.
This is a rough approximation which negelects dead space:
Where VT equals tidal volume and f equals frequency of ventilation:
This equation means that roughly the same number of carbon dioxide molecules are eliminated by high ventilation at a low PCO2 as by low ventilation at a high PCO2,"Just divide K by the target PCO2"
The Target Ventilation
Calculate the new Ventilation by dividing k by the desired PCO2:
1) Pure Respiratory Acidosis (Click on Picture on Right):: This patient has a pure (acute) respiratory acidosis with a PCO2 = 70 mmHG (9.8 kPa) and is ventilating at 4 L/min. The constant is 4 x 70 = 280. To obtain a PCO2 of 40, the ventilation required would be 280 / 40 = 7 L/min. This should correct the PCO2 to 40 mmHg (5.7 kPa). For an acute distubance, it is usually safe and appropriate to return the PCO2 to normal.
2) Chronic Respiratory Failure (Click on Picture on Left): This chronic bronchitic patient normally has a PCO2 of 50 mmHg with partially compensating metabolic alkalosis (SBE=3). Acute respiratory failure due to pneumonia has raised his PCO2 to 70 mmHg (9.8 kPa) despite his ventilating at 8 L/min. His constant is 70 x 8 = 560. The target is to return him to his customary PCO2. The required ventilation is 560 / 50 = 11.2 L/min. This returns the PCO2 that is normal for him.
Metabolic level does not change: Note that in both these examples, the change in ventilation alters the PCO2 but the the level of the Metabolic Acidosis, the SBE, does not change. The patient moves horizontally as the ventilation is increased.
Alan W. Grogono
|Copyright Mar 2018.|
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