Question

(a) Referring to Figure $20.34$, find the time systolic pressure lags behind the middle of the QRS complex. (b) Discuss the reasons for the time lag.

Short answer

1. In time the difference is obtained s: $0.25s$.
2. Till where we measure, the maximal pressure has to travel.

Step-by-step solution

Define Resistance

Resistance is a measurement of the resistance to current flow in an electrical circuit. Resistance in ohms is denoted by the Greek letter omega$(\mathrm{\Omega })$.

Evaluating the difference in time

1. The difference in the time is between middle of the QRS complex and the systolic pressure can be read off from the diagram as about $0.25s$.

Reason for time lag

b. The left ventricle is maximally constricted $0.25s$ after the centre of the QRS complex, which is why maximum (systolic) pressure is measured at that time. It is because we normally use a rater on the upper left arm rather than directly measuring the pressure on the heart. It then takes the time for this maximal pressure to propagate to the place where we measure the pressure (higher pressure is manifested as higher kinetic energy of the blood which travels down the artery).

Therefore, we get:

1. Difference in time is: $0.25s$.
2. Maximal pressure has to travel to where we measure it.