Continuous wavelet transform of aortic pressure oscillations in anesthetized dogs: effects of 45 degrees tilting

The time-frequency analysis of signals by means of continuous wavelet transform (CWT) was applied to blood pressure oscillations recorded from the aorta of anesthetized dogs. This method yielded two and three-dimensional representations of either the module or phase in function of time, in contrast with the fast Fourier transform (FFT) which gives the spectrum in the frequency domain. From the CWT of arterial pressure oscillations we obtained visual information on aortic valves closure, heart rate, respiratory rate and smooth muscle contractions in arterial and arteriolar walls (very low frequency component). The objective of this study was to analyze the frequency-time behavior in two and three-dimensional cardiovascular changes during 45 degrees head-up and head-down tilts, compared with zero degree supine position. In eight pentobarbitone anesthetized dogs, the postural changes were repeated for more than ten times in each one. Heart rate variability was derived by applying a new mathematical procedure. We utilized the pronounced changes of heart rate during each respiratory cycle (inspiratory tachycardia and expiratory bradycardia) to establish a correlation with the arterial pressure fluctuations during normal and tilting conditions. Significant differences in heart rate were observed between the 45 degrees head-up and head-down tilts, compared with the supine position. The results show that anesthetized dogs might constitute an appropriate model where to study orthostatic hypotension and microgravity blood shifts

Wavelet analysis of arterial pressure and blood velocity pulsations in the aorta of anesthetized dogs

The arterial pressure and blood velocity pulsations were recorded from the aorta of anesthetized dogs by means of micro-tip pressure and velocity transducers. Wavelet transforms (Wt) were obtained by converting the analog signals into digital samples at the rate of 42.7 per second, which were subsequently subjected to an algorithm of WT. An iterative rarefaction (2(0) to 2(-4) resolutions) of the number of samples was followed by a substraction of the high frequency components (wavelet coefficients) from the corresponding resolutions. Analyses of the arterial pulsations revealed that the second WT always yielded four types of systolic apexes, which were apparently devoid of physiological meaning, since they were inherent to the ®triangulation phase® of the WT algorithm. In addition, the third WT occasionally revealed slow amplitude modulations, which could not be identified in the original recordings and whose significance deserves further investigation. This is also valid for the wavelet coefficients, whose biological meaning is still obscure. In summary, the WT operates as a low pass filter, which brings to light the lower frequency components of arterial pulsations and which finally yields the mean values of both arterial pressure and blood velocities