Ten specific phobics improved during a mean of two sessions of experimental treatment by exposure in vivo in a balanced design. Short-term results replicated those of a previous study that self-control of heart rate with the aid of biofeedback significantly reduced heart rate during treatment, but this did not hasten reduction of subjective anxiety, nor of respiratory rate or skin conductance responses. An hour's pretreatment training in self-control of heart rate with the aid of feedback did not enhance the effect. Mere instructions to lower heart rate without feedback had a significant effect during treatment, but the addition of heart rate feedback to instructions significantly augmented the decline in heart rate.
Nunes JS, Marks IM. Feedback of True Heart Rate During Exposure in VivoPartial Replication With Methodological Improvement. Arch Gen Psychiatry. 1976;33(11):1346-1350. doi:10.1001/archpsyc.1976.01770110074007