How Long Is Long Enough? Controlling for Acute Caffeine Intake in Cardiovascular Research

Figure 1. Working Memory Task. Note. Each trial consists of presentation of a fixation cross on a computer screen for 750 ms followed by a series of nonsense letters consisting of four capital letters presented for 550 ms. The letter series was then masked, with a single blue letter appearing above the masked series for the remainder of the trial. Subjects must then press one of two keys indicating whether or not the blue letter was presented in the preceding letter series. Each trial was a total of 4000 ms long. Subjects completed 14 practice trials before beginning the 3 min task. Subjects were instructed to attempt to respond correctly for at least 90% of 44 total trials for a cash bonus [35,36].

Figure 2. Mean Heart Rate (HR), Condition × Phase. Note. Error bars are omitted from graphs due to inherent inaccuracy in reporting error for repeated measures MANOVA results in SPSS. Experimental phases were as follows: Phase 1: measures taken prior to consumption of coffee. Phase 2: acute measures collected 30 min following intake of coffee, and Phase 3: measures collected six hours following consumption of coffee. ** p < 0.05.

Figure 3. Mean HR Reactivity by Phase. Experimental phases were as follows: Phase 1: measures taken prior to consumption of coffee. Phase 2: acute measures collected 30 min following intake of coffee, and Phase 3: measures collected six hours following consumption of coffee. ** p < 0.05.

Figure 4. Mean RMSSD, Condition × Phase. Note. Condition × Phase interaction. Experimental phases were as follows: Phase 1: measures taken prior to consumption of coffee. Phase 2: acute measures collected 30 min following intake of coffee, and Phase 3: measures collected six hours following consumption of coffee. ** p < 0.05.

Figure 5. Pairwise Comparisons for Mean MAP. Note: Significant differences between Phases 1 and 2 (p < 0.000), and 2 and 3 (p < 0.000), but no significant differences between Phases 1 and 3 (p = 0.052). Main effect of Phase significant at p = 0.000. Experimental phases were as follows: Phase 1: measures taken prior to consumption of coffee. Phase 2: acute measures collected 30 min following intake of coffee, and Phase 3: measures collected six hours following consumption of coffee. **** p < 0.000.

Figure 6. Self-rated Pain Severity for Cold Pressor, Condition × Phase. Experimental phases were as follows: Phase 1: measures taken prior to consumption of coffee. Phase 2: acute measures collected 30 min following intake of coffee, and Phase 3: measures collected six hours following consumption of coffee. ** p < 0.05.