It refers to the decrease in the systolic blood pressure by more than 10mm Hg during inspiration
Seen in

cardiac tamponade
constrictive pericarditis
obstructive pulmonary disease

The reason for the decrease in systolic blood pressure during inspiration is explained in this article : Inspiratory decrease in systolic blood pressure and pulse volume – Mechanism
Mechanism of pulsus paradoxus:

In cardiac tamponade

Tense fluid in the pericardial sac impairs ventricular filling
Also, the negative intrathoracic pressure is not well transmitted to the pericardial sac
Fall in pulmonary venous pressure is more than the fall in left atrial pressure
Hence filling of the …

The systolic blood pressure and pulse volume decreases during the inspiratory phase of respiration.
Mechanism

During inspiration there is increased venous return (due to negative intrathoracic pressure)
right ventricles expands more
interventricular septum is pushed to the left side (Reverse  Bernheim effect)
This decreases the left ventricular volume
Also the pooling of blood in the pulmonary circulation occurs, decreasing amount of blood reaching left ventricle

This causes decrease in stroke volume of left ventricle, thus decreasing the systolic blood pressure. The maximum decrease in blood pressure that is considered normal is 10 mm Hg. If the decrease …

Compromise of the left ventricular filling due to bulging of the interventricular septum into the left ventricle
Can occur due to

Physiologically during inspiration due to increased filling of the right ventricle (because of increased venous return)
pulmonary embolism

Right ventricular failure occurring due to the mechanical obstruction caused by the bulging of the ventricular septum into the right ventricle
First described by Bernheim in 1910
Can occur due to

hypertrophy of the ventricular septum (as in aortic stenosis)
septal aneurysm (following infarction)

Straw coloured pericardial fluid aspirated into a syringe. Straw coloured fluid indicates an exudative effusion and tuberculosis is an important cause.

Cardiac index (CI) is an index of cardiac function. It is ratio of Cardiac Output (CO) to Body surface area (BSA).
CI = CO/BSA

Abrupt cessation of circulation due to failure of the heart to pump effectively during systole.

Cardiac Ouptut – acronym

AAA is the acronym for: Abdominal Aortic Aneurysm
abbreviation, expansion

Abdominal aorta.
Abdominal aortic aneurysm is known by the acronym: AAA