This is a study published in Chinese looking at the effects of acupuncture at P6 and P5 in healthy volunteers comparing the effects of retained needles to traditional needling. Use of P6 and P5 for cardiac purposes is a time honored prescription. These points are also used in Longhurst’s protocol for for high blood pressure (hypertension.) Measuring HRV after needling P6 and P5 has precedent also.
In this study, they found a cardiac excitation initially (which I also find in practice) and then an increase in RR interval, or decrease in heart rate. They found HFR increased, which can be interpreted as an increase in parasympathetic activity. LFR was unchanged, interpreted to mean no change in sympathetic activity. Sadly, since they were healthy volunteers, there can be no clinical correlation with acupuncture effectiveness. Information on my practice here.

[Effects of retained needling and non-retained needling on regulation of the cardiac autonomic nerve].
Zhongguo Zhen Jiu. 2011 Nov;31(11):993-7.
Luo LP, Shen ZY, Yu P

Department of Physiotherapy, The Second Hospital of Sanming City, Sanming 366000, Fujian Province, China.

To investigate the mechanism of retained needling and non-retained needling on the physiologic regulating effect of the cardiac autonomic nerve in healthy persons.

Sixty volunteers were randomly divided into a retained needling group and a non-retained needling group, 30 cases in each group. They were all received perpendicular needling at Neiguan (PC 6) and Jianshi (PC 5) retaining for 20 min in the retained nee dling group and without retaining in the non-retained needling group. The heart rate variability (HRV) indices, i.e., R-R interval, low frequency (LF), high frequency (HF) and LF/HF were observed.
Compared with before acupuncture, the R-R interval were significant increased during the stimulation period and during the post-stimulation period in the retained needling group, especially during the stimulation period (all P < 0.01), and during the post-stimulation period of 5 min and 10 min in the non-retained needling group (P < 0.01, P < 0.05), then returned to pre-acupuncture level quickly, and the R-R interval in the retained needling group was significant ly higher than that in the non retained needling group at 10 min (P < 0.05). At 5 min, the HF were significantly higher, and the LF/HF were significantly lower than those before acupuncture (P < 0.01, P < 0.05), and there was no obvious change in LF in both groups.
Retained needling and non-retained needling at Neiguan (PC 6) and Jianshi (PC 5) can both excite cardiac vagus nerve briefly and then the acupuncture effects reduce grad ually until disappearance, while no obvious changes can be found with cardiac sympathetic activity. Both of needling can reduce heart rate and the action time of heart rate decrease by retained needling is longer than non-retained needling.