MECHANISM OF HEART FAILURE
Heart failure means a condition when the heart cannot provide sufficient blood flow to meet physiological needs of the body. Systolic function of the heart is controlled by four important factors.
- The contractile state of myocardium.
- The preload of the ventricle i.e. the end diastolic volume and the resultant fibre length of the ventricle therefrom just prior to the onset of contraction.
- The after load applied to the ventricles i.e. the impedance to ventricular ejection.
- Rate of the heart.
Normal heart can increase the cardiac output from 5L to even 20L by adjusting stroke volume and heart rate but in cardiac failure it is not possible particularly in presence of exercise. To compensate the decreased cardiac output several physiologic mechanisms come into action. These include increased sympathetic activity, redistribution of blood flow to essential organs, increased renin, angiotensin II, aldosterone production, cardiac hypertrophy, Frank-Starling mechanism and increased peripheral oxygen extraction. All these mechanisms will not only have some favourable effects for improving cardiac output and circulation but also have some unfavourable and deleterious effects. Compensation in the state of heart failure may be decompensated by dietary increase of salt, exposure to heat, humidity, effort, anemia, pregnancy, thyrotoxicosis, sudden cardiac dysrhythmia, avoidance of prescribed medicines, etc. Heart failure may be of two types.
Low output failure
Here the cardiac output is low as occurs in hypertension, coronary vascular disease, myocardial infarction, some valvular disease of the heart.
High output failure
Here the cardiac output is elevated as occurs in emphysema, thyrotoxicosis, severe anemia A1, arteriovenous shunt, beri beri and Paget’s disease etc. The mechanism responsible for the genesis of heart failure in such cases is not clearly understood. Isolated or predominant diastolic dysfunction of the heart may also manifest features of cardiac failure. It is due to abnormal filling of the ventricles either due to impaired relaxation of the myocardium or due to non-compliant cardiac chamber.
Clinically, however, heart failure may be manifested as left ventricular failure, right ventricular failure and biventricular or mixed heart failure.
Precipitating factors of heart failure
- Active carditis.
- Emotional upsets.
- Excessive salt intake.
- Infection and infective endocarditis.
- Myocardial infarction and systemic hypertension.
- Pregnancy etc.