Cardiorespiratory Responses to Exercise

CHAPTER 9: Cardiorespiratory Responses to Exercise

Part A: Acute Responses

Overview of Cardiovascular and Respiratory Responses to Acute Exercise

  • Key themes covered:

    • Cardiovascular responses to acute exercise

    • Respiratory responses to acute exercise

    • Recovery mechanisms following acute exercise

Electrical Cycle of the Heart

  • Phases of Cardiac Cycle:

    • Depolarization-Contraction: Electrical impulses trigger heart contractility, leading to blood ejection from the heart.

    • Repolarization-Relaxation: Heart muscle relaxes, allowing chambers to fill with blood.

  • Components Involved:

    • SA Node: Primary pacemaker that initiates depolarization.

    • AV Node and Purkinje fibers: Play a vital role in conduction system leading to coordinated contractions, especially affecting ventricular contraction.

ECG Components

  • Key Waves:

    • P Wave: Atrial depolarization

    • QRS Complex: Ventricular depolarization

    • T Wave: Ventricular repolarization

  • Intervals:

    • PR Interval: Time from the start of atrial excitation to the start of ventricular excitation.

    • QT Interval: Time from the beginning of ventricular depolarization to the end of repolarization.

    • S-T Segment: Represents the period of complete ventricular depolarization.

Cardiovascular Responses to Acute Exercise

  • Primary Adaptations:

    • Increased blood flow to working muscles.

    • Alterations in heart function and peripheral circulatory dynamics, involving:

    • Heart rate (HR)

    • Stroke volume (SV)

    • Cardiac output (Q)

    • Blood pressure (BP)

  • Cardiac Output (Q): Defined as the amount of blood the heart can pump out per minute.

    • Formula:
      Q=SVimesHRQ = SV imes HR

Heart Rate Dynamics
1. Resting Heart Rate (RHR)

  • Normal Ranges:

    • Untrained: 60 to 80 beats/min

    • Trained: as low as 30 to 40 beats/min

  • Factors Affecting RHR:

    • Neural tone, temperature, altitude.

  • Anticipatory Response: An increase in HR above RHR just before exercise due to:

    • Decrease in vagal tone.

    • Increase in norepinephrine and epinephrine levels.

2. Heart Rate During Exercise

  • Intensity Relationship: HR increases directly proportional to exercise intensity.

  • Maximal HR (HRmax):

    • Defined as the highest HR achieved during maximal exertion.

    • Estimation Formula:
      HRmax=220−extage(years)HR_{max} = 220 - ext{age (years)}

    • Demographic Considerations:

    • May overestimate HRmax in younger individuals.

    • Best predictive formula for those around age 40:
      HR<em>max=208−(0.7imesextage)HR<em>{max} = 208 - (0.7 imes ext{age}) or HR</em>max=211−(0.64imesextage)HR</em>{max} = 211 - (0.64 imes ext{age})

  • Example Calculation: For a 60-year-old:

    1. Using the original formula:
      220−60=160extbpm220 - 60 = 160 ext{ bpm}

    2. Alternative estimation:

    • 208−(0.7imes60)=166extbpm208 - (0.7 imes 60) = 166 ext{ bpm}

    • 211−(0.64imes60)=172.6extbpm211 - (0.64 imes 60) = 172.6 ext{ bpm}

3. Steady-State Heart Rate (HR)

  • Defined Point: The plateau HR optimal for meeting circulatory demands at submaximal intensity.

  • Adjustment Period: It typically takes 2 to 3 minutes for HR to stabilize at a new intensity level with higher intensities prolonging this adjustment period.

Heart Rate Variability (HRV)

  • Definition: HRV measures the fluctuation in heart rhythm, displaying shifts in sympathetic and parasympathetic system activity.

  • Influencing Factors:

    • Body core temperature

    • Sympathetic nerve activity

    • Respiratory rate

  • Analysis Method: Primarily conducted through spectral analysis focusing on frequency rather than time.

Electrocardiogram (ECG) Summary

  • Provides a visual representation of:

    • Atrial depolarization via the P wave.

    • Ventricular depolarization as indicated by the QRS complex.

    • Ventricular repolarization expressed in the T wave.

  • ECG Intervals Recognized:

    • P-R Interval

    • QT Interval

    • S-T Segment

How to Measure Heart Rate from an ECG

  • Process: The heart rate can be calculated based on the frequency of the R-R intervals observed in the ECG readings.

  • Example HR Calculation: Utilizing known intervals such as 859 ms (70 bpm) and others indicates variability in exercise-induced HR fluctuations.

Conclusion

  • Understanding these cardiovascular responses, including maximal HR and heart rate variability, is crucial for assessing fitness levels and structuring safe exercise prescriptions in clinical and sports environments.