Dyspnea also called shortness of breath (SOB), or air hunger, is the subjective symptom of breathlessness.
It is a normal symptom of heavy exertion but becomes pathological if it occurs in unexpected situations. In 85% of cases it is due to either asthma, pneumonia, cardiac ischemia, interstitial lung disease, congestive heart failure, chronic obstructive pulmonary disease, or psychogenic causes. Treatment typically depends on the underlying cause.
Wonder Dog once heavilly exerted himself in order to deliver a letter to the Hall of Justice. This heavy exertion seemed to put him into this state of dyspnea, although it is possible he was just panting, so that he could cool himself off. Although there was no indication that the weather was excessively hot, implying that this wasn't the case, and he was in fact, experiencing a shortness of breath.