Modifying the rate of breathing is the basis for a particular technique called kapalabhati, which is a high frequency yoga breathing technique (with a breath rate of 1.0 to 2.0 Hz) [3].

This high frequency yoga breathing (HFYB) technique could be expected to alter cardiac functioning, and indeed this was seen in a study on twenty-four volunteers who practiced the technique at the rate of 2.0 Hz for 15 minutes, analyzed as three, five minute periods [4]. A 0.1 Hz rhythm was present in the record of R-R intervals and the blood pressure during the HFYB. The results were taken to support the hypothesis about the integrative role of cardiovascular and respiratory rhythms when the respiratory frequency is altered. The same authors further assessed the effects of the HFYB (at 2.0 Hz) as three, five minute periods, as in their other study, in seventeen advanced yoga practitioners [5]. The EKG, respiration and blood pressure were recorded continuously during the three, five minute periods of HFYB, as well as before and after the practice. The beat to beat series of the R-R intervals as well as systolic and diastolic blood pressure were analyzed by a spectral analysis of the time series. All frequency bands of R-R interval variability were reduced in HFYB. These results as well as the changes in other variables point to decreased cardiac vagal modulation during HFYB. The changes in R-R interval variability, as well as in the blood pressure, were interpreted by the authors as suggestive of decreased cardiac vagal modulation during HFYB.

A comparable result was reported when twelve experienced practitioners practiced the HFYB technique at 2.0 Hz for one minute and a frequency domain analysis was carried out [6]. Following one minute of HFYB there was a significant increase in the low frequency power and in the LF/HF ratio, while the HF power was significantly lower following the HFYB. The results suggested that the HFYB modifies the autonomic status by increasing sympathetic activity with reduced vagal activity.

The effect of HFYB on the cardiac autonomic control was hence reported to have comparable effects in two studies which differed in the duration of the practice i.e., fifteen minutes [5] as compared to one minute [6].

The same HFYB practice was shown to improve the performance in a cancellation task requiring both focused and selective attention [7]. The participants here were forty-six medical students, forty-eight middle-aged adults (30 to 59 years), and sixteen older adults (all of whom were over 60 years of age). The HFYB was practiced at rates varying between 1.0 and 2.0 Hz, for 15 minutes.

Apart from the performance in a paper and pencil cancellation task [7], the HFYB at 2.0 Hz for one minute also improved the performance in a P300 event related potential task [8]. The P300 is an event related potential which is generated when attending to and differentiating between auditory stimuli [9]. The P300 was recorded in two groups with fifteen participants in each group. One group practiced the HFYB at 2.0 Hz for one minute, while the other group practiced being aware of their breath, for the same duration of time. The P300 changed after both practices. A decreased latency of the P300 after the HFYB suggested a decrease in the time required to complete this attentional task. In contrast following breath awareness an increase in the P300 amplitude which was recorded, suggested that greater neural resources were needed to complete the task.

The time domain measures (i.e., the mean R-R interval, NN50 and pNN50) are recognized to be strongly dependent on the vagal modulation [11,12]. The decrease in the mean RR interval, NN50 and pNN50 during and after HFYB were significant whereas the changes were significant only after breath awareness but not during the practice. Hence the decrease in vagal modulation occurred during and after HFYB and after breath awareness but not during breath awareness. The decrease in vagal modulation described above for HFYB and breath awareness could be related to the fact that these practices are associated with increased vigilance and better performance in an attention task [7]. This is relevant since attention and vigilance are associated with higher sympathetic modulation in human volunteers and animal models [13,14].

There were no changes in variables obtained by frequency domain analysis during or after HFYB. In contrast the LF significantly increased during and after breath awareness whereas the HF decreased during and after breath awareness. The LF band of the heart rate variability is mainly related to sympathetic activation when expressed in normalized units [12], while efferent vagal activity chiefly contributes to the HF band [15].

The absence of change in the frequency domain analysis variables during HFYB could suggest that HFYB was associated with parasympathetic withdrawal but not with simultaneous increase in sympathetic modulation. In the present study it is also essential to remember that the HF band was extended from the conventional 0.15 - 0.40 Hz to 0.15 - 1.50 Hz. This was done to match the breathing frequency during HFYB where breathing frequency was approximately 1.0 Hz. While the breathing frequency was not modified for breath awareness, the band was extended for analysis of heart rate variability during breath awareness as well. This change may have influenced the results.

Concurrent monitoring of respiration and the heart rate variability are considered important. This has been supported by an acute increase in LF and total spectrum heart rate variability and in vagal baroreflex gain corrected with slow breathing during biofeedback [16]. Biofeedback training used to increase the amplitude of respiratory sinus arrhythmia maximally increases the amplitude of heart rate oscillations at approximately 0.1 Hz [17]. In the present study during HFYB the respiratory frequency was maintained at approximately 1.0 Hz, however there was no increase in HF power during HFYB, hence the changes observed in the present study appear to be related to changes in autonomic modulation rather than being a result of the change in respiratory rate.

The study has two main limitations. (i) It would have been ideal to test the subjects after a time interval with no intervention as the study included two yoga practices, but no control. (ii) In the present study the breath frequency was close to the frequency of the heart rate during HFYB. This could have resulted in interference between two close frequencies and given rise to artifact, particularly increasing the LF (which was not seen).