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Physics, Pharmacology and Physiology for Anaesthetists - 4

Thermodilution và pha loãng thuốc nhuộm Một chất đánh dấu được tiêm vào tĩnh mạch trung ương. Một dòng động mạch ngoại biên được sử dụng để đo lượng của chất trong hệ thống động mạch. Một đồ thị tập trung so với thời gian được sản xuất và các thuật toán cấp bằng sáng chế dựa trên phương trình | Cardiac output measurement The Fick principle The total uptake or release of a substance by an organ is equal to the product of the blood flow to the organ and the arterio-venous concentration difference of the substance. This observation is used to calculate cardiac output by using a suitable marker substance such as oxygen heat or dye and the following equation Vo2 CO Cao2 - Cvo2 so CO V o2 Cao2 - Cvo2 where Vo2 is the oxygen uptake CO is cardiac output Cao2 is arterial O2 content and Cvo2 is mixed venous O2 content. Thermodilution and dye dilution A marker substance is injected into a central vein. A peripheral arterial line is used to measure the amount of the substance in the arterial system. A graph of concentration versus time is produced and patented algorithms based on the Stewart-Hamilton equation below are used to calculate the cardiac output. When dye dilution is used the graph of concentration versus time may show a second peak as dye recirculates to the measuring device. This is known as a recirculation hump and does not occur when thermodilution methods are used. Stewart-Hamilton equation If the mass of marker is known and its concentration is measured the volume into which it was given can be calculated as V M C If concentration is measured over time flow can be calculated as Flow M C.At where M is mass V is volume and C is concentration. A special form of the equation used with thermodilution is Cardiac output measurement 65 Flow Vinj Tb - Tt .K Tblood t t where the numerator represents the mass of cold and the denominator represents the change in blood temperature over time K represents computer constants. Dye dilution graphs Draw a curve starting at the origin that reaches its maximum value at around 5 s. The curve then falls to baseline but is interrupted by a recirculation hump at around 15 s. This is caused by dye passing completely around the vasculature and back to the sensor a second time. 0 5 10 15 20 Time s Demonstrate that the semi-log .