A technique for the monitoring of blood flow during continuous haemofiltration

Objective: To establish a technique for the monitoring and graphic display of blood flow during continuous renal replacement therapy (CRRT). Design and setting: Technique assessment study in a tertiary intensive care unit. Patients: Six ICU patients receiving CRRT. Interventions: A technique was devised to monitor and graphically display blood flow during CRRT. This technique used a mini-ultrasound Doppler probe attached to the blood tubing with link to a lap top computer for quantitative graphic display. Blood flow was measured and displayed during routine treatment using this method in six patients. Measurements and results: Blood flow wave data were monitored and successfully displayed as a real-time wave form and analysed using Windaq data analysis software. This initial analysis over a 6-h period revealed the following facts: (a) blood flow was not the same as set by the machine roller pump on nine occasions, (b) blood flow reductions defined as a drop in the 'diastolic' were 20% (seven) and 30% (two) less than set flow, (c) flow reductions frequently failed to trigger machine alarms, and (d) the blood flow wave displayed had unique characteristics. Complete flow monitoring was then undertaken for the functional life of one haemofilter over 24.5 h. There were 27 blood flow reductions, and blood flow was less than set for a total of 463.9 min or 31.5% of operating time. Conclusions: Blood flow during CRRT can be monitored by an ultrasound Doppler probe and displayed graphically. Preliminary data using this technique suggest potentially serious and undetected problems with blood flow during routine CRRT.