Intravenous acid infusion without lowering arterial pH stimulates breathing.

The research investigates how the introduction of acid into the body intravenously, without changes to arterial pH, stimulates breathing. It was found out that the increase in ventilation, related to an increase in breath, could be induced independently of changes in arterial pH.

Research Methodology

• The experiment was performed on six awake ponies. The choice of animals can be justified by the larger body systems, allowing for clearer analysis and the ability to generalize the results to larger mammals including humans.
• The researchers infused HCl (hydrochloric acid) into the right atrium of these ponies and simultaneously infused NaOH (sodium hydroxide) into the left heart to neutralize the acid and maintain normal arterial pH. In this way, they could examine the impact of the acid infusion on ventilation, whilst keeping the effect of acid-base balance constant.

Research Findings

• The primary finding of this research was that total ventilation, meaning an increase in breath, in the ponies increased initially and remained elevated even though the arterial pH was maintained normal.
• This increase in ventilation at the start coincided with an increase in pulmonary arterial PCO2 (the pressure of carbon dioxide in the arteries) and a decrease in pulmonary arterial pH. This suggests that the introduction of acid stimulated breathing by acting on the pulmonary system, irrespective of arterial pH.
• As the experiment progressed, the pH and CO2 level in the pulmonary artery returned to control values due to changes in the arterial blood’s acid-base status resulting from hyperventilation.
• Fascinatingly, the experiment led to a significant increase in pulmonary arterial blood pressure throughout the infusion. It’s worth noting that subsequent infusion of hypertonic saline led to only minor changes in measured variables, demonstrating the specific role of the acid infusion in triggering these changes.

Implications and Conclusions

• This research suggests that acid infusion can increase ventilation, even without a drop in systemic arterial pH, which contradicts the common understanding that decreased pH levels often trigger increased breathing.
• This experiment shows that the stimulation of breathing may not be solely tied to the systemic arterial H+ concentration. The researchers express uncertainty about the mechanism driving the observed ventilatory response, suggesting it might be due to activation of pulmonary chemoreceptors, vascular mechanoreceptors or other unknown factors.