by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology

The objective is to restore the PCO2 to its customary position - which, for chronic problems, may different from "normal".

**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 PCO_{2} level based on exhaustion, prognosis, prospect of improvement from concurrent therapy and, only in part, on the PCO_{2} level. Once the clinical decision is made, the PCO_{2} helps calculate the required ventilation.

The PCO_{2} 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 PCO_{2} level.

Where **V _{T}** equals tidal volume and

**f** equals frequency of ventilation:

This equation means that the same number of carbon dioxide molecules are eliminated by high ventilation at a low PCO_{2} as by low ventilation at a high PCO_{2},

**The Target Ventilation**

Calculate the new Ventilation by dividing **k** by the desired **PCO _{2}**:

**1) Pure Respiratory Acidosis (Click on Picture on Right)::** This patient has a pure (acute) respiratory acidosis with a PCO_{2} = 70 mmHG (9.8 kPa) and is ventilating at 4 L/min. The constant is 4 x 70 = 280. To obtain a PCO_{2} of 40, the ventilation required would be 280 / 40 = 7 L/min. This should correct the PCO_{2} to 40 mmHg (5.7 kPa). For an acute distubance, it is usually safe and appropriate to return the PCO_{2} to normal.

**2) Chronic Respiratory Failure (Click on Picture on Left):** This chronic bronchitic patient normally has a PCO_{2} of 50 mmHg with partially compensating metabolic alkalosis (SBE=3). Acute respiratory failure due to pneumonia has raised his PCO_{2} 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 PCO_{2}. The required ventilation is 560 / 50 = 11.2 L/min. This returns the PCO_{2} that is normal for him.

**Metabolic level does not change**: Note that in both these examples, the change in ventilation alters the PCO_{2} but the the level of the Metabolic Acidosis, the SBE, does not change. The patient moves horizontally as the ventilation is increased.

Acid-Base Tutorial Alan W. Grogono |
Copyright Mar 2018. All Rights Reserved |