Acid-Base Disturbances: Treating Respiratory Problems.
Emergencies: A disaster may interfere with breathing as well as the body’s metabolism: respiratory failure raises the level of CO2 (carbonic) acid and metabolic failure raises the level of lactic and other acids. The paragraphs below primarily discuss the treatment of Respiratory Acidosis. Treatment may also be required for Metabolic Acidosis.
Treatment of Respiratory Failure.
Ventilation. Intubating and ventilating a patient carry significant risks as the Covid 19 Pandemic have demonstrated. Accordingly, a physician decides to ventilate a patient based on exhaustion, prognosis, prospect of improvement from concurrent therapy and, only in part, on the level of the PCO2.
Once the clinical decision is made, the PCO2 helps calculate the required ventilation. This is because, the body’s metabolism determines the rate of CO2 production which, in turn, determines the ventilation required.
Calculating Required Ventilation.
This is a rough approximation which neglects dead space:
Where VT equals tidal volume and f equals frequency of ventilation:
PCO2 x Ventilation = Constant
PCO2 x f x VT = K
or f x VT = K / PCO2
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,
To calculate the new Ventilation divide k by the desired PCO2:
New Ventilation = K / Target PCO2
= Old PCO2 x f x VT / Target PCO2
1) Pure Respiratory Acidosis (See Diagram): 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 disturbance, it is usually safe and appropriate to return the PCO2 to normal.
2) Chronic Respiratory Failure (See Diagram): This chronic bronchitic patient normally has a PCO2 of 50 mmHg with partially compensating metabolic alkalosis (SBE=4). 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 to his normal level.
Metabolic level does not change: Note that in both these examples, the change in ventilation alters the PCO2 but the level of the Metabolic Acidosis, the SBE, does not change. The patient moves horizontally as the ventilation is increased.