Systems and methods for estimating a ventilatory threshold for a patient and estimating likelihood of pulmonary arterial hypertension are provided. An example method includes determining ratios of minute ventilation to rate of oxygen uptake based on breath-by-breath data for a patient. The example method also includes determining a ventilatory threshold based upon the determined ratios and determining a score based upon an end-tidal partial pressure of carbon dioxide (PetCO2) value at the determined ventilatory threshold and a minute ventilation/carbon dioxide production relationship (VE/VCO2) value at the determined ventilatory threshold. The method also includes using the score to estimate a likelihood that the patient has pulmonary arterial hypertension and causing the estimated likelihood to be displayed on a user interface.