Final answer:
Phenylephrine improves myocardial oxygen delivery by reducing systemic vascular resistance, not by significantly increasing heart rate or stroke volume. Acetylcholine increases rather than decreases digestion, and epinephrine is a positive inotrope that increases ventricular contraction strength.
Step-by-step explanation:
Phenylephrine improves myocardial oxygen delivery by reducing systemic vascular resistance, which is strongly indicated by its action as a vasoconstrictor. It is not primarily known for increasing heart rate or stroke volume, which are more commonly associated with adrenergic drugs like epinephrine and norepinephrine. While phenylephrine may modestly improve coronary blood flow via reflex bradycardia, this is not its primary mechanism for enhancing myocardial oxygenation. Instead, by increasing vascular resistance, it can indirectly increase vascular return and blood pressure, facilitating better coronary perfusion during diastole.
Among examples of incorrect pairings: acetylcholine typically increases digestion rather than decreasing it, as it promotes parasympathetic activity leading to the stimulation of digestive processes. Regarding positive inotropes, a substance like epinephrine increases the strength of ventricular contraction rather than decrease it. Furthermore, while sympathetic stimulation can increase heart rate, above certain thresholds (such as 160 bpm), increases can actually lead to a decrease in cardiac output due to reduced ventricular filling time.