Integration of Blood Viscosity into a Clinical Concept
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According to Starling’s Law of the heart the cardiac output (CO) is determined by the degree of filling of the left ventricle (PCWP). According to Guyton, another important factor which helps to determine the CO, alongside the systemic filling pressure (Psf), is the resistance to venous return(RVR). Still a third factor is the cardiac contractility. On this basis, one can distinguish between preload, contractility and afterload. Under normal circumstances the cardiac contractility is neither a limiting nor a regulating factor, the heart simply pumps away what is offered to it. As a result, the CO is determined to a great extent by the wedge pressure (as a measure for left ventricular filling) and the RVR, while the RVR is determined by the vascular diameter (r) and by the viscosity (η).
Based on Guyton’s analysis of the circulation and Starling’s Law of the heart we propose a new diagram for clinical use in which the cardiac and peripheral factors are both represented and which is built up on the basis of clinically measurable data: cardiac output, wedge pressure and viscosity. Previous experimental work and some representative clinical cases are presented.
It has been demonstrated that an isovolemic reduction in viscosity (e.g. with Arvin) is accompanied by a decrease in wedge pressure (PCWP) while an isovolemic increase in viscosity (exchange transfusion with packed cells) is accompanied by an increase in wedge pressure (PCWP).
It seems likely that the regulation of the vascular diameter and viscosity are related to each other.
Under circumstances of low viscosity the relative hypovolemia which develops can be disadvantageous in relation to low-flow states such as shock and post-stenotic flow. It is essential, therefore, that reduction in viscosity is accompanied by transfusion of extra volume, resulting in normovolemia.
KeywordsCardiac Output Pulmonary Capillary Wedge Pressure Blood Viscosity Rate Pressure Product Peripheral Factor
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