IMPLANTABLE ULTRASONIC BLOOD FLOWMETERS* 
James D. Meindl* 
Accurate measurement of pulsatile blood flow can be 
achieved on a chronic basis in research animals through 
the use of totally implantable ultrasonic flowmeters. 
The continuous wave Doppler flowmeter provides a tech- 
nique for measurement of flow velocity at a particular 
location such as the center of the lumen; the pulsed 
Doppler flowmeter is useful for measurement of flow 
velocity distribution or profile across the vessel and 
lumen diameter and hence volume flow. Both instru- 
ments can be electronically precalibrated and exhibit 
no baseline or scale factor changes during chronic expe- 
riments. Custom designed silicon monolithic integrated 
circuits off'er significant advantages in reduced size 
and power drain as well as improved reliability in these 
instruments. 
INTRODUCTION* 
Accurate measurement of pulsatile blood flow 
is of major importance in cardiopulmonary re- 
search and development. It is particularly use- 
ful to perform this measurement on a chronic 
basis in research animals under a variety 
of normal and abnormal conditions. The 
most promising instruments available for this 
purpose are ultrasonic flowmeters. This paper 
briefly reviews their principles of operation as 
well as their primary capabilities and limita- 
tions. In addition, a discussion is presented of 
two new chronically implantable ultrasonic flow- 
meters which are designed using unique families 
of silicon monolithic integrated circuits. Fol- 
lowing initial applications in animals, it is con- 
ceivable that these implantable flowmeters may 
be useful in heart surgery patients. For ex- 
ample, by attaching a remotely removable trans- 
ducer to the aortic wall and connecting it to a 
subcutaneous electronics package, accurate mon- 
itoring of cardiac output (i.e., instantaneous 
volume of blood flow) might be safely ac- 
* Department of Electrical Engineering, Stanford University, Stan- 
ford, California 94305. 
** This investigation was supported by the U. S. Public Health 
Service Research Grant 1-PO1-GM17940-01. 
complished during the most critical phase of 
recovery. 
CHRONIC PULSATILE BLOOD FLOW 
MEASUREMENT 
The most desirable features of a chronically 
implantable blood flowmeter are listed in Figure 
1. To begin with, the instrument should ex- 
hibit no measurement ambiguity; it should 
measure a well defined physiological quantity 
to a specific degree of accuracy. Ideally, it 
should require no calibration in vitro or in vivo 
by the user ; it should come to him electronically 
precalibrated by the producer. During chronic 
experiments, the instrument should exhibit no 
significant changes in its baseline or its scale 
factor. For implantable instruments, small size 
is an obvious necessity and low power drain is 
essential to long operating life. To avoid trauma, 
non-constricting non-erosive flow transducer 
cuffs are necessary. Finally, to assure full ex- 
IMPLANTABLE BLOOD FLOWMETERS 
• NO MEASUREMENT AMBIGUITY 
• NO USER CALIBRATION 
• NO ZERO DRIFT 
• CONSTANT SCALE FACTOR 
• SMALL SIZE 
• LOW POWER DRAIN 
• NONTRAUMATIC 
• AUTOMATIC DATA COLLECTION 
Figure 1. — Major desirable features of chronically 
implantable blood flowmeters. 
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