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Physiological breathlessness on exertion is the sensation of increased work of breathing which the patient usually recognises as being normal for the degree of exertion.
Pathological dyspnoea on exertion is the sensation of breathlessness at a lower level of exertion than is normal for the patient.
During exertion the requirements for an increased cardiac output result in a tachycardia and consequently, a reduction in the ventricular filling time. These changes plus the effects of any mitral valve stenosis or defects in left ventricular filling cause an increase in the left atrial pressure. It the increase in pulmonary venous pressure that triggers the sensation of breathlessness.