# impedance

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## impedance

[im-pe´dans]
1. obstruction or opposition to passage or flow, as of an electric current or other form of energy.
2. the resistance in alternating current circuits, represented by the letter Z in mathematical formulas. Medical equipment is often rated according to impedance to allow for optimum performance by matching impedance ratings. A transformer can be used between components to cause the impedances of unequal systems to match.
acoustic impedance an expression of the opposition to passage of sound waves, being the product of the density of a substance and the velocity of sound in it.

## im·ped·ance

(im-pē'dăns),
1. Total opposition to flow. In electricity, when flow is steady, impedance is simply the resistance, for example, the driving pressure per unit flow; when flow is changing, impedance also includes the factors that oppose changes in flow. Thus, deviations of impedance, from simple ohmic resistance because of the effects of capacitance and inductance, become more important in alternating current as the frequency of oscillations increases. In fluid analogies (for example, pulsatile flow of blood, to-and-fro flow of respiratory gas), impedance depends not only on viscous resistance but also on compressibility, compliance, inertance, and the frequency of imposed oscillations.
2. Resistance of an acoustic system to being set in motion.

## impedance

/im·ped·ance/ (im-pēd´ans) obstruction or opposition to passage or flow, as of an electric current or other form of energy. Symbol Z .
acoustic impedance  an expression of the opposition to passage of sound waves, being the product of the density of a substance and the velocity of sound in it.
aortic impedance  the sum of the external factors that resist ventricular ejection.

## impedance

[impē′dəns]
Etymology: L, impedire, to entangle
a form of electric resistance observed in an alternating current that is analogous to the classic electric resistance that occurs in a direct current circuit. It is expressed as a ratio of voltage applied to a circuit to the current it produces, as an alternating current oscillates ahead of or behind the voltage.

## Impedance

Cardiac pacing The total opposition that a circuit presents to alternating electrical current. Cf Resistance.

## im·ped·ance

(im-pē'dăns)
1. Opposition to flow of gases, liquids, or electrical current.
2. Resistance of an acoustic system to being set in motion.

## impedance

any resistance to the flow of fluids moved by a series of pulses, such as blood flow.

## impedance

obstruction or opposition to passage or flow, as of an electric current or other form of energy.

acoustic impedance
an expression of the opposition to passage of sound waves, being the product of the density of a substance and the velocity of sound in it.
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