How do I measure?
Medical Flow Meters
In this article we discuss an issue of general interest to all of us – medical flow meters and specifically, blood flow…
Titan’s interest in measuring blood flow was initiated by an enquiry for a set of meters to record the flow of blood substitute around a model of the human circulation system.
Blood is fundamental to life and is pushed around your body by a positive displacement pump with simple flap valves to prevent back flow. The speed of this pump varies from around 50 to 200 beats per minute and the pressure pulse is an unusual shape:-
The main pressure peak is from the heart pulsing, the notch is the aortic valve closing and the remaining pressure is from the elasticity of the arterial system. These pressures induce the flow and the main flow is caused by the resilient structures contracting after being stretched by the pressure pulse and being locked off by the aortic valve closing.
Surprisingly the highest flow range is after the heart contraction and aortic valve closure, this is the resilience in the vessel walls pushing the blood with the “retained” pressure. The changes in velocity are very dynamic and differing parts of the flow curve are affected by different parts of the circulation system. Consequently knowing where the flow is unusual could help diagnosis of circulatory problems. This complete blood pumping cycle is around 600 milliseconds (ms) with the individual elements as short as 4ms or 2ms for a patient with a rapid heart rate. Interestingly the velocity of the blood within the arterial system varies from 0.3 to 400mm per second, which is an incredibly wide velocity range.
Medical Flow Meters & Blood Flow
So how do you measure this? The simple answer is with difficulty. Obviously blood vessels cannot be cut to insert a flow meter so some form of clamp-on system must be used. Electromagnetic technology with capacitively coupled electrodes, light and ultrasonic flowmeters have all been used. Ultrasound has been quite successful in this area initially with Doppler shift technology where reflections from the cells within the blood and later with time of flight systems. To accurately record the more rapid changes in flow, a suitable meter must have a fast response time of at least 200Hz and preferably 4 or 500Hz to catch all the transients. The latest generation of time of flight meters have a clip-on assembly which consists of a pair of ultrasonic crystals placed at an angle and a reflector positioned at the “focal point” of the two waves. These sensors are available in a range of sizes to suit the blood vessel size.
The phase shift between the upstream and downstream signals are effectively the velocity within the vein. On a practical front these sensor assemblies have to be capable of being sterilised or are supplied pre-sterilised and disposable. These very small sensors work down to 0.7mm diameter vessels with typical accuracies of ±5%. Laser Doppler is also proving to be quite successful.
So you can see while we have not yet reached the complex goal of accurately monitoring blood flow around the human circulatory system we have made significant strides forward in being able to measure blood flow in individual veins and arteries.