Which of the following will cause an increase in stroke volume of the heart?

Cardiac output and blood pressure are two important measures of the health and function of the cardiovascular system.  You need to understand these measures as a fitness professional in order to design and deliver safe, effective exercise sessions, and in the case of blood pressure, be able to conduct and interpret blood pressure measurements for your clients.

Cardiac output

Cardiac output (known as ‘Q’) is a measure of the amount of blood that is pumped out of the heart in one minute.

‘Q’ specifically refers to the amount of blood pumped out of the left ventricle as this is the ventricle that supplies blood to the muscles and organs of the body.

Cardiac output is made up of two components, heart rate (HR) and stroke volume (SV). 

This can be easily measured through the use of heart rate monitors or taking ones pulse (counting the ‘pulses’ at the radial artery for example over a one minute period). 

The normal resting heart rate range for an adult is between 60-100 bpm.  However when a resting heart rate is greater than 100bpm it is called ‘tachycardia’ and when it’s below 60bpm it is called ‘bradycardia’.

These can both indicate possible heart conditions or complications and if you notice these in a personal training client of your's you should advise your client to have a medical check.  The exception to this is that bradycardia may be present in extremely fit (international level multisport, triathlon, ironman etc) individuals and not something to be concerned about.

Stroke volume (SV) refers to the quantity of blood pumped out of the left ventricle with every heart beat.  The exact volumes are not easily measured, so they are often estimated based on what we know about stroke volume and the factors that it affects such as blood pressure which we can measure.

The equation for cardiac output is:

HR x SV = Q.  Therefore to calculate Q we must first establish HR and SV.  An example at rest is shown below.

HR  (70BPM) x SV (70ml) = 4900ml/min or 4.9 litres per minute

An increase in HR, SV or both will increase someones Q.  SV on the whole does not fluctuate too much, with only relatively small increases with exercise.  HR on the other hand increases quite dramatically and thus is the biggest influencer of increasing somesones Q.

Increases in Q with exercise are vital, as it is essentially your CV system trying to meet the demands of the body for the supply of oxygen rich blood and the removal of waste.

However it is highly unlikely that you will ever have to measure a client’s Q, but because Q affects blood pressure, which you will measure, its important that you know what HR & SV are and how they along with Q influence blood pressure.

Blood pressure

BP measurements are usually taken on the upper arm with a ‘sphygmomanometer’ and a stethoscope as pictured on the adjacent diagram. 

The sphygmomanometer consists of an inflatable cuff with a pressure gauge.

When inflated the cuff blocks the flow of blood to the arm below the cuff.  As the cuff is allowed to slowly deflate, the measurer listens through the stethoscope to sounds as the artery opens and allows blood flow to continue again.

The measurer is listening for two specific sounds as the blood flows through the artery, as shown on the below image.

The first sound heard as the artery opens enough for the first pumps of blood to come through is known as the ‘systolic’ pressure.  This measures the force the heart has to pump against to get the blood to flow around the body. 

The second sound recorded is known as the ‘diastolic’ pressure.  This measure is recorded at the point where the measurer stops hearing the ‘pump’; essentially it measures the pressure in the arteries as the heart relaxes.

The systolic number is placed over the diastolic number and is always the higher of the two numbers. For example blood pressure of 110 (systolic) and 70 (diastolic) is read as 110/70mmHg.  The measurement of blood pressure is expressed in millimetres of mercury (mmHg).

High blood pressure (known as ‘hypertension) occurs when systolic blood pressure readings consistently exceed 140mmHg and or diastolic readings exceed 90mmHg. 

‘Normal’, healthy BP is regarded as 120/80mmHg or thereabouts. 

BP is also used as a ‘risk factor’ for many diseases and illnesses, such as heart disease.  Doctors and fitness professionals alike use blood pressure to screen for potential problems before making judgements as to what exercise a person can safely take part in.

A person’s BP is determined by the following three factors;

1.      Cardiac output (as we have already discussed)

2.      Blood viscosity (the thickness of the blood)

3.      Total peripheral resistance ‘TPR’ (the resistance the blood encounters on its voyage within the blood vessels)

Cardiac output

Cardiac output as you know is made up of heart rate and stroke volume.  At rest these are relatively constant however with exercise the heart beats faster and more blood is pumped out with each beat.  These factors both contribute to a rise in BP, as would any other factor that caused the heart to speed up.

Changes in the volume of blood within the cardiovascular system will also affect BP.  If a person was severely dehydrated or lost a large quantity of blood through a wound there would be less blood for the heart to pump, thereby reducing cardiac output and BP.

If the volume of blood increased (waste products not being removed to the kidneys due to kidney failure for example) then there would be a greater quantity of blood within the system increasing the pressure within.  Think about putting more air into an already inflated balloon and you’ll get the picture!

Blood viscosity

Blood can thicken for many reasons but the main ones are a lack of water and or a high glucose (blood sugar) concentration. 

For diabetics, if they don’t control their blood sugar levels these can quickly become high and cause problems with high blood pressure.  Low hydration levels can also result in thick blood and therefore higher blood pressure. 

This is why it is important to always remain well hydrated, as it helps to reduce the pressure within the blood vessels and therefore the load on the heart to pump the blood.  For this reason people with a history of heart problems are often prescribed medications to keep their blood thin.

Total peripheral resistance

When we were kids we used to take the garden hose and put our thumb over the end of it to get the water to squirt further (usually to make sure a sibling got wet!).  We increased the pressure by decreasing the space the flow of water could go through.

The same principle applies in the body with blood and the vessels.  In cardiovascular terms this is known as ‘total peripheral resistance’ (TPR). If the area available for blood to flow through is reduced then pressure will increase. 

If pressure remains very high for long periods of time the danger of a vessel bursting increases significantly, in the case of the aorta this would result in a virtually instantaneous death through massive immediate blood loss. 

The major concern with a BP that is consistently elevated is that there may be a potential obstruction within the blood vessel which narrows the available area blood has to flow through.

Such obstructions are usually due to the build up of fatty ‘plaques’ which stick to the walls of blood vessels and build up over a period of time.  This is pictured on the adjacent diagram.

A diet high in fat, low in vegetables and a sedentary lifestyle can all contribute to the build up of these fatty plaques. 

This is known as ‘atherosclerosis’ and if untreated these plaques can build up to completely block a vessel, or a chunk of the plaque can break off and cause a blockage further down the vessel where the vessel narrows.

If the blockage is not cleared quickly then the tissues that receive oxygen and nutrients from that vessel are likely to die.  Depending on where the blockage occurs the effects can range from minor to fatal.

If the blockage occurs in the coronary (heart) arteries then a heart attack (usually fatal) will result, if the blockage occurs in a cerebral (brain) blood vessel then a stroke will occur with usually irreversible damage to the part of the brain that is effected.

Blood vessels can also narrow when stress hormones (e.g. cortisol) are released by the body causing the smooth muscle within the walls of the vessel to contract (vasoconstriction).

This again ‘constricts’ blood flow and increases blood pressure, as can be seen in the adjacent picture ‘C’.

Tense muscles around a blood vessel can also ‘constrict’ the flow of blood and increase BP.  This can occur when someone is very stressed or has a lot of tightened muscles due to exercise stress, as can be seen in the adjacent picture.

In summary, any increases in cardiac output (HR and/or SV), blood viscosity or total peripheral resistance will result in increases in BP. 

As BP is a key measure of the health of the cardiovascular system it should be measured regularly and people whose readings are consistently elevated at rest should be treated accordingly, usually by referral to medical personnel, restriction of exercise to low intensity and a focus on dietary and lifestyle modifications to reduce stress and the intake of fatty foods.

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