(1)
Department of Cardiovascular, Neural and Metabolic Sciences, Istituto Auxologico Italiano, Milan, Italy
8.1 Invasive Methods
Until a few years ago, pulse wave velocity, in little experimental animals (mice, rats, rabbits, etc.), could only be measured by invasive methods. The carotid and femoral arteries were surgically isolated and catheterized to assess intra-arterial pressure (Fig. 8.1).
Fig. 8.1
Invasive assessment of carotid–femoral pulse wave velocity in rats. Carotid and femoral arteries are surgically isolated. A pressure micro-transducer is inserted via the carotid artery into the aortic arch, and a second catheter is placed just above the aortic bifurcation through the femoral artery. Both carotid and femoral signals are recorded simultaneously and then analyzed by a specifically designed software
A pressure micro-transducer was inserted via the carotid artery into the aortic arch for the clinical recording of the intravascular central arterial pressure wave. Another catheter was inserted into the peripheral end of the abdominal aorta and placed just above the aortic bifurcation through the femoral artery. The carotid and femoral signals were recorded simultaneously. The signals were digitized and stored using a data acquisition system, and the delays between proximal and distal pressure waveforms were evaluated by a specifically designed software. After the recordings were completed, the animals were euthanized and the full length of the aorta was exposed. The distance between the proximal and distal pressure transducers was determined by measuring the length of a damp cotton thread stuck onto the aorta between the tips of the two pressure transducers and marked.
However, this operation would cause the death of the small laboratory animal, which was then dissected to measure the distance between the two catheters. Pulse wave velocity was calculated in this way. The biggest problem with this procedure is the impossibility of carrying out longitudinal studies for the evaluation of arterial distensibility. Actually, longitudinal studies in small laboratory animals are particularly interesting because, as their life is so brief, a few months correspond to decades of life in human beings. This is the reason why even follow-up over a few weeks (i.e., after a predetermined diet or after taking some drugs) can show significant thrombogenic action or change in the viscoelastic properties of arteries. It is also important to note that particular rat strains simulate specific pathological conditions of human beings; for example, spontaneous hypertensive rats (SHRs) are very useful for the evaluation of cardiovascular risk and for the evaluation of antihypertensive drugs and obese Zucker rats simulate a condition which is similar to severe metabolic syndrome.
8.2 Non-invasive Methods
Non-invasive approaches to assess pulse wave velocity have already been discussed; nevertheless, these methods are not suitable for large and longitudinal studies. At present, magnetic resonance imaging and Doppler ultrasound-based transit time approaches have been considered to estimate local arterial PWV at different sites in mice and rats.
PulsePenLab® (DiaTecne s.r.l., Milan) has recently been validated. It is an instrument derived from the first version of the PulsePen® tonometer (widely used in clinical practice for human beings), specifically designed for the study of central blood pressure and acquisition of pulse wave velocity in small laboratory animals. In the light of the high heart rate in rats (average 350–400 beats/min), the sampling rate of the tonometer was increased up to 1 kHz, therefore, improving the definition of the signal (16 bits). The transducer was changed as well; its contact area was reduced, with a significant increase in sensitivity. The PulsePen® uses two tonometers, able to record carotid and femoral (or tail) pulse waves simultaneously.
To sum up, a non-invasive measurement of aortic pulse wave velocity and pulse wave analysis in rats and little animals can be possible today, using a very efficient device called PulsePenLab®. Longitudinal studies for assessing the action of drugs or diet on aortic distensibility are now easily and quickly feasible in small laboratory animals. PulsePenLab® tonometers are currently used in studies involving several groups of rats: spontaneous hypertensive rats (SHRs), Wistar, lean Zucker Fa/Fa rats, fatty insulin-resistant Zucker fa/fa rats, Sprague–Dawley, etc.
8.2.1 Aortic PWV in Little Experimental Animals
Aortic PWV may be described as the time taken by the blood pressure wave to propagate from the common carotid artery, considered as central artery, to the peripheral artery. PWV is defined as the ratio of the distance between the two locations on which blood pressure wave was measured as the time delay of the peripheral pulse wave in relation to the carotid pulse wave. Aortic PWV can be assessed in two ways:
Carotid–femoral PWV (Fig. 8.2). This method requires simultaneous recording of pulse waves by placing one probe on common carotid artery and one probe on femoral artery.
