Non-Invasive Measurements of Cardiac Output
Measurements of cardiac output in animal models, and humans as well, can be complicated and usually require the surgical placement of probes, systemic administration of markers and blood sampling.
1. Electromagnetic flow probes can accurately measure flow, but they are bulky and unsuitable for use small animals.
2. Doppler probes are flexible and easier to implant, but they still require substantial surgical skills to implant and are limited to measuring velocity. Consequently they are of limited use for long-term monitoring of flow, or for use in models which display impairments in healing or immunosuppression.
3. Transonic flow probes are small, they can be used to precisely measure true flow, and they quite suitable for chronic implants. However, they are still invasive and may be unsuitable in certain models of compromised healing.
One potential solution is to use a recently refined, non-invasive technique based upon changes in thoracic impedance. The techniques uses "sticky pad" connectors identical to those used in EKGs.
These "pads" are placed on the neck and waist:
A "low level" injection current is introduced across the thoracic plane and used to record changes in impedance. A cardiophone is also used to determine the duration of systole.
By tracing the changes in impedance and the length of systole one can use the formula:
SV = R · (L²/Zo²) · T · dZ/dt
to calculate the stroke volume. The product of the stroke volume and heart rate are the cardiac output. Biopac ® makes a unit optimized for this purpose in humans. However, we have worked vigorously to extend this technology for use in rat models.