Indirect evidence for respiratory influences capable of changing RR interval independently of baroreflex

Positive phase values between cardiac cycle duration and systolic arterial pressure at respiratory rate are usually found in human subjects in supine position. Under the assumption that heart period lags behind systolic arterial pressure at negative phase values, positive phase values are traditionally interpreted as an indirect evidence of the presence of the feed-forward arm (from cardiac cycle duration to systolic arterial pressure). We propose to interpret the positive phase values between cardiac cycle length and systolic arterial pressure at respiratory frequency as an indirect evidence of influences capable of changing cardiac cycle duration independently of baroreflex. This interpretation does not exclude the traditional one but it stresses that positive phase values can be observed even in presence of a negligible contribution of the feed-forward path.