Stress Testing

Various factors including vagal tone, body position, blood volume and depth of respiration control the input to the heart. The heart responds by pumping blood into the arterial circulation.The amount per beat in ml. is called the stroke volume (SV). The total cardiac output is the stroke volume multiplied by the heart rate. The total cardiac output increases with the exercise,depending on the efficiency of the system, up to about 30 ltr/minute in a well conditioned athlete.An individuals ability to increase cardiac output is the most important factor limiting the ability to increase physical work capacity.

Heart rate response to exercise

Heart Rate Formula

It has been shown and assumed that a healthy 20 years individual would be able to maximize his cardiac output attaining a heart rate of 220/min. It declines by 10 points for every decade in advancing age. This is considered as maximum heart rate attainable during maximum exercise and termed as maximum heart rate (MHR). However, if the subject can attain 85 per cent of MHR it is considered for practical purpose to have achieved adequate exercise and termed as target heart rate (THR). Hence, in a deconditioned subject one thrives to achieve THR rather than MHR.

Preload and Stroke Volume

When exercise begins, a complex set of events can be measured that set the stage for the events to follow. The first is the increase in the venous tone, which is mediated by autonomic reflexes. This squeezes the blood from the large vein into the right side of the heart increasing the effective filling pressure. Cardiac output increases immediately as the result of the increased filling and tachycardia. The baroreceptor reflexes are progressively inhibited as exercise increases. Stroke Volume and Training

The SV of endurance athletes has been reported to be 50 to 75 per cent higher than that of sedentary individuals.Contractility Heart being a muscular pump the ability to eject blood depends on the ability to contract. Another factor is the resistance in the vascular bed through which the heart must pump. The resistance in the lungs is so low in the healthy subject that it plays very little role as a limiting factor in exercise.

Resistance is the product of blood flow multiplied by the blood pressure. When the heart pumps more blood, the cardiac output increases more than the resistance drops therefore, there is a modest increase in systolic blood pressure during exercise in healthy individuals.

Hypocapnia

Hypocapria produces a remarkable increase in the coronary resistance. Increase in PO 2 in the coronary blood has been demonstrated to decrease and a decrease in PO 2 causes a marked increase in perfusion.

Autonomic Responses

As exercise is initiated, there is an immediate with drawal of vagal tone, creating sympathetic vasoconstriction in the capacitance vessels primarily the veins. This is associated with an increase in heart and respiratory rate. A redistribution of blood occurs where the flow to the skin, viscera and non- working muscles is decreased and the flow to the active muscles is appropriately increased.
 

Myocardial Oxygen Uptake 

VO2 max = Maximum C.O x Maximum A-V Oxygen difference.It is the maximum amount of oxygen that the person can use during dynamic exercise involving large amount of total muscle mass. It represents the amount of oxygen transported or used in cellular metabolism.

Met

It refers to the amount of oxygen uptake while sitting at rest as a unit. This differs from basal metabolic rate.1 MET = 3.5ml O2/min/Kg of body weight. VO2 in ml/min/Kg measured during treadmill test expresses the number of METS associated with that activity. A test can be interpreted accurately if atleast 6 METS is achieved during exercise. Healthy individuals can achieve 10-11 METS and aerobic trainers more that 16 METS.

Predictability of MET Level Achieved

An exercise capacity of < 5 METS indicates poor prognosis in patients younger than 65 years age. If 10 METS is achieved it indicates a prognosis with medical therapy similar to CABG.More than 13 METS indicates a good prognosis despite abnormal test response.

Decline in VO 2 max with age

Rate Pressure Product

The systolic blood pressure is multiplied by the heart rate to provide the double product or modified fusion time index. On presence of an obstructed coronary artery it takes a longer time to reach the same pressure pulse value than it would take in a heart with normal coronary arteries. Because the double product depends on both the SBP and the heart rate, the findings that they appear to be related variables is of some interest.