Cardiac Output

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Skill:

•  Measurement and interpretation of the heart rate under different conditions 

•  Interpretation of systolic and diastolic blood pressure measurements

    
Cardiac output describes the amount of blood the heart pumps through the circulatory system in one minute

  • It is an important medical indicator of how efficiently the heart can meet the demands of the body


There are two key factors which contribute to cardiac output – heart rate and stroke volume

  • Equation:  Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)


cardiac output


Heart Rate

Heart rate describes the speed at which the heart beats, measured by the number of contractions per minute (or bpm)

  • Each ventricular contraction forces a wave of blood through the arteries which can be detected as a pulse
  • The typical pulse rate for a healthy adult is between 60 – 100 beats per minute


Heart rate can be affected by a number of conditions – including exercise, age, disease, temperature and emotional state

  • Additionally, the body will attempt to compensate for any changes to stroke volume with a corrective alteration to heart rate


An individual’s heart rate is controlled by both nervous and hormonal signals:

  • Heart rate is increased by the sympathetic nervous system and decreased by parasympathetic stimulation (vagus nerve)
  • Heart rate can also be increased hormonally via the action of adrenaline / epinephrine


Blood Pressure

Stroke volume is the amount of blood pumped to the body (from the left ventricle) with each beat of the heart

  • It is affected by the volume of blood in the body, the contractility of the heart and the level of resistance from blood vessels


Changes in stroke volume will affect the blood pressure – more blood or more resistance will increase the overall pressure

  • Blood pressure measurements typically include two readings – representing systolic and diastolic blood pressures
  • Systolic blood pressure is higher, as it represents the pressure of the blood following the contraction of the heart
  • Diastolic blood pressure is lower, as it represents the pressure of the blood while the heart is relaxing between beats


Blood pressure readings will vary depending on the site of measurement (e.g. arteries have much higher pressure than veins)

  • A typical adult is expected to have an approximate blood pressure in their brachial artery of 120/80 mmHg to 140/90 mmHg
  • Blood pressure can be affected by posture, blood vessel diameter (e.g. vasodilation) and fluid retention or loss


Blood Pressure Changes in the Circulatory System 

blood pressure