In this article we will discuss about:- 1. Definition of Blood Circulation Time 2. Normal Values of Blood Circulation Time 3. Factors Affecting 4. Importance.
Definition of Blood Circulation Time:
Blood circulation time is the time taken by a particle of blood to flow from one point in the circulation to the other. It measures the average linear velocity of blood.
The principle is to inject some substance intravenously and then to note the arrival of the substance at the point in question. Generally, the cubital vein is selected but other veins can also be used. Many substances are used, such as histamine, fluorescin, potassium ferrocyanide, calcium chloride, calcium gluconate, magnesium sulphate, saccharin, ether, sodium dehydrocholate (decholin), radioactive substances, sodium cyanide, etc.
Other factors remaining constant, circulation time will depend upon the length of the circuit and the method employed. Single estimations are not of much value. Repeated estimations, using the same method, give more correct information. The time is measured with a stop-watch, from the beginning of the injection to the signal of its arrival.
Each substance produces a characteristic effect by which-the arrival can be detected. For instance, decholin—by bitter taste; ether—by smell; magnesium sulphate—by a feeling of heat; sodium cyanide—by respiratory stimulation; histamine—by flushing of face and so on.
Normal Values of Blood Circulation Time:
i. Arm to Tongue. Decholin:
5 ml of 2% decholin is injected quickly into the cubital vein. As soon as the drug reaches the tongue, the patient feels a bitter taste. This is the end point —13 sec. (8—16 sec.).
ii. Arm to Lungs. Ether:
5 minims of ether mixed with 5 minims of normal saline are given intravenously, (IV). Smell of ether in the expired air indicates the end point—6sec. (4—8sec.).
iii. Arm to Face. Histamine:
0.001 mgm of histamine phosphate per Kg body weight is given I.V. Onset of flushing of the face indicates end point—[The time should be more or less same as decholin time. But it is nearly double. Probably vasodilatation involves longer time.] 24 sec.
iv. Arm to Heart: (Radioactive Method):
6.6 sec. (2—14sec.).
v. Arm to Carotid Body: (Sodium Cyanide):
12-15sec.
vi. Total Circulation Time:
25 sec. (12-26sec.).
Dye injection and radioactive methods are of much wider use. In the radioactive method, radioactive isotopes can be easily detected by isotope counters.
Factors Affecting Blood Circulation Time:
i. Increased cardiac output,
ii. Exercise,
iii. Excitement,
iv. Adrenaline, and
v. Rise of basal metabolic rate (B.M.R.), etc.
They will increase the velocity of blood and reduce circulation time. It is not affected by height, weight, pulse rate or blood pressure provided other factors remain constant. In animals, it is seen that smaller the animal, higher is the heart rate and shorter is the total circulation time.
On the whole, the number of total heart beats for one complete circulation time is same in all species. The significance of alteration in circulation time is still a debatable matter. It is a poor indicator of vasomotor change, or blood flow or blood volume.
Clinical Importance of Blood Circulation Time:
1. Circulation time is reduced (i.e., blood flow rapid) in the following conditions:
i. Increased B.M.R., such as:
a. Hyperthyroidism,
b. Fever.
ii. Anaemia (due to increased cardiac output, low viscosity, etc.).
2. Circulation time is increased in the following conditions:
i. Heart Failure:
a. In right-sided failure—Ether time is lengthened.
b. In left-sided failure—Decholin minus Ether time is lengthened. In bronchial asthma arm to tongue time is normal. But in cardiac asthma it is lengthened. Hence, circulation time is a good guide in differentiating between right and left heart failure, also between cardiac and bronchial asthma.
ii. Hypertension.
iii. Myxoedema.
iv. Polycythaemia vera.
v. Postoperative conditions.
vi. Shock.
vii. Peripheral failure, etc.
3. Conditions of the peripheral vessels affect circulation time as follows:
i. Circulation time is lengthened:
a. If the limb is immersed in cold water, and
b. By sympathetic stimulation leading to vasoconstriction.
ii. It is shortened in:
a. Elevation of the limb above the heart,
b. Sympathectomy,
c. Application of the heat,
d. Exercise of the limb, etc.