Medical Blog

Since 1628, when W. Harvey described for the first time the circular motion of blood through the body, the interactions between heartbeat аnd vascular state have been a topic of interest [Harvey, 1628]. Probably the most important signal for these studies is the blood pressure signal. This signal not only contains systolic [maximum] аnd diastolic [minimum] blood pressure values at every heartbeat, but also incorporates information about the volumetric state, peripheral resistance, heart function [contractility], influence of respiration on blood pressure аnd autonomic nervous system influences. The blood pressure level is an important tool in the evaluation of the condition of a critically ill patient. In the critically ill newborn infant not only the early determination of hypo- or hypertension, but also the determination of fluctuations in blood flow аnd blood pressure were found to be important. Among others, Perlman has shown in 1983 that these fluctuations are involved in the pathogenesis of intracranial lesions [Perlman et al., 1983, Miall-Allen 1989, Goldstein аnd Brazy 1990, 1991]. To monitor fluctuations in the blood pressure a continuous registration is needed. Non-invasive methods for continuous blood pressure registration in neonates are not available. The research group therefore remains restricted to the neonates having a clinical indication for an arterial catheter. This is about half of our NICU population.

Physiology

Blood is pumped around in the body by a rhythmic contraction of the heart. In the system circulation it goes from the heart through arterial vessels to the peripheral tissues аnd organs, аnd circulates back to the heart via venous vessels. In the lung blood circulates through the pulmonary artery to small capillary vessels where oxygen/carbon dioxide exchange takes place, аnd back to the heart in the lung veins.
Because of the rhythmic contraction of the heart the pressure in the arteries also fluctuates. The volume аnd the contour of the arterial pulses not only depend on the left ventricular stroke volume аnd the ejection velocity, but also on the relative compliance аnd capacity of the arterial system. Furthermore, they are composed of forward аnd backward waves, reflected from the peripheral system. The waveform thus contains a lot of implicit information about the contractility of the heart [septum] аnd peripheral resistance. Therefore the arterial pressure waveform depends on the measurement position аnd the quantities mentioned above. In infants, the blood pressure values are influenced by the presence of asphyxia, sepsis, infant respiratory distress syndrome [IRDS] аnd patent ductus arteriosus [PDA]. The influence of these pathologies on the contour of the blood pressure signal has not been investigated yet. In adults, altered waveforms have been reported in case of shock, hypo- аnd hypertension, atherosclerosis, vasodilatation, atrial fibrillation, valvular disease, hypertrophic cardiomyopathy, aortic coarctation аnd chronic uremia [O’Rourke, 1982, Gevers, 1994]. Apart from the waveforms, the fluctuations in blood pressure [beat-to-beat аnd slower] are known to contain information about the function of the autonomic nervous system.