Pulsus paradoxus refers to a condition where the pulse weakens significantly or disappears during inhalation, caused by a decrease in the left ventricular stroke volume. Normally, the strength of the pulse is not affected by the respiratory cycle. However, when cardiac tamponade, significant pericardial effusion, or constrictive pericarditis occurs, inhalation can restrict the relaxation of the right heart, leading to a reduced volume of blood returning to the heart and affecting the right heart’s output. Consequently, the amount of blood the right ventricle pumps into the pulmonary circulation is reduced. Furthermore, the pulmonary circulation is affected by the negative thoracic pressure during inhalation, causing pulmonary vessels to dilate and reducing the volume of blood returning from the pulmonary veins to the left atrium. Therefore, the output of the left heart also decreases. These factors lead to a weakened pulse during inhalation, which can sometimes be imperceptible, also known as pulsus paradoxus. This condition can be identified by a significant drop in systolic pressure by more than 10mmHg during inhalation compared to exhalation.