15.2 Applications and skills

15.2.4 Conditions related to the heart and circulation

Artificial pacemakers are used to regulate the heart rate

artificial pacemakerFigure 15.2.4a – An artificial pacemaker

  • An artificial cardiac pacemaker is a small, implantable device that monitors the heartbeat and maintains a normal heart rate for patients with weak myocardial electrical function.
  • The most common type of pacemaker is about 3–4cm in diameter with electrode leads that extend into the right atrium and ventricle via the superior vena cava.
  • When the device senses a missed heartbeat, it discharges a low voltage pulse, which stimulates cardiac muscle to contract.

Defibrillation is used to treat life-threatening cardiac conditions

ecgFigure 15.2.4b – ECG showing ventricular fibrillation (top) vs normal sinus rhythm (bottom)

  • Ventricular fibrillation is the irregular and uncoordinated contraction of cardiac muscle that can result from a heart attack or other traumatic disturbance of heart function.
  • Fibrillating ventricles do not contract fully. Instead they quiver – this results in little to no blood circulation and can lead to cardiac arrest.
  • A defibrillator has two paddles that are placed diagonally across the chest. If fibrillation is detected, the paddles discharge a therapeutic dose of electricity.
  •  The jolt depolarises cardiac muscle, and allows the SA node to resume pacemaking of the heartbeat.

Causes and consequences of hypertension

  • Hypertension, or chronic high blood pressure, develops over time, and is associated with the same risk factors as coronary heart disease:
    • sedentary lifestyle
    • advanced age
    • diet high in salt/saturated fats
    • smoking
    • family history
    • obesity (or being overweight)
    • gender (males are at greater risk).
  • Consequences of hypertension are summarised here:

Narrowing of arteries

Kidney damage

  • Chronic high blood pressure weakens the ultrafiltration apparatus

Aneurysm

  • Occurs when weakened arterial walls bulge and fill with blood
  • The aneurysm could burst and cause internal bleeding

 

aneurysm

Figure 15.2.4c – Types of aneurysms

Skill: Interpretation of blood pressure measurements

  • Blood pressure is the pressure exerted on arterial walls during systemic circulation, measured in mmHg and expressed as two values, for example ‘130 over 80’.
  • The first number is the systolic pressure (arterial wall contraction) and the second number is the diastolic pressure (arterial wall relaxation).

blood pressure valuesFigure 15.2.4d – Blood pressure values

  • Blood pressure is measured using a sphygmomanometer (blood pressure cuff) placed around the upper arm and inflated to constrict the brachial artery.
  • Pressure on the artery is slowly released, and a stethoscope, placed under the cuff, is used to listen for five specific sounds relating to the closing and re-opening of the artery.

artery soundsFigure 15.2.4e – Blood pressure measurement

Causes and consequences of thrombosis

  • Atherosclerosis is a common cause of hypertension. Blood flow is slowed as a result of narrowing arteries, and this puts pressure on the arterial walls.
  • When circulation is severely restricted, a blood clot, or thrombus, may develop at the site.

thrombusFigure 15.2.4f – Atherosclerosis can lead to thrombosis

  • Thrombosis blocks arteries and results in ischemia. If this occurs in the coronary arteries, it causes a heart attack. In the brain, it causes stroke.

Skill: Analysis of epidemiological data relating to coronary heart disease (CHD)

104, 693 cases of CHD were monitored over 10 years in Tianjin, China. The results are summarised in Figure 15.2.4g.

HD dataFigure 15.2.4g – Incidence of CHD in Tianjin, China, 1999–2008

Analyse the graph and answer the questions:

  1. Define epidemiology and distinguish it from other branches of research science.
  2. Researchers have used chi-square statistics to analyse their data. State the null hypothesis.
  3. Estimate the difference in mortality between urban men and women in the year 2003.
  4. Suggest reasons and for the differences in mortality of:
    1. urban and rural populations
    2. males and females.

AEDFigure 15.2.4h – AED
Automated external defibrillators (AEDs) give visual and audio instructions and can also be deployed by non-specialists.

Study hint

Help yourself to remember the risk factors for hypertension using the acronym ‘SAD SFOG’.

blood pressure measurementFigure 15.2.4i – Blood pressure measurement
Skill and experience are needed to take blood pressure measurements using analogue equipment (Figure 15.2.4i) but automatic cuffs are available for non-specialists (Figure 15.2.4j).

electronic BP cuffFigure 15.2.4j – Electronic BP cuff

Concept help

  • Cardiac arrest is the sudden stop in effective blood circulation to the heart. One of the causes is ventricular fibrillation.
  • Ischemia is the restriction of blood supply to the tissues, causing a shortage of oxygen and glucose needed for cellular respiration.

Nature of Science

Developments in scientific research follow improvements in apparatus: The stethoscope is an acoustic device that is used to listen to heart, lung and abdominal sounds. Its invention led to improved knowledge of the physiology of the heart.

exerciseFigure 15.2.4k – Exercise

In the lab

Heart rate is affected by exercise, body position, breathing patterns and exposure to cold. Measure and analyse heart rate data under different conditions.

Course links

  • Saturated fats, like LDLs, can accumulate as plaque in the arteries. See 6.2.2.
  • Review 6.2.4 for a good summary of the goals and methods of epidemiology.