Heartbeat coordination, exercise and ventilation

What you’ll learn

• Trace cardiac excitation and interpret simple ECG timing.
• Calculate cardiac output and minute ventilation.
• Explain neural and hormonal responses to exercise.

Understand the idea

Cardiac muscle is myogenic: excitation originates within the heart. The sinoatrial node initiates a wave that spreads through the atria. A delay at the atrioventricular node permits ventricular filling; excitation then travels through the bundle of His and Purkyne fibres, coordinating ventricular contraction from the apex upwards. An ECG records electrical activity: the P wave corresponds to atrial depolarisation, QRS to ventricular depolarisation and T to ventricular repolarisation. Electrical events precede the associated mechanical events.

Cardiac output is heart rate multiplied by stroke volume. Respiratory minute ventilation is breathing rate multiplied by tidal volume. During exercise, both can rise to deliver oxygen and remove carbon dioxide more rapidly. Chemoreceptor information about carbon dioxide-related pH changes and other inputs reaches control centres in the medulla. Sympathetic activity increases heart rate, parasympathetic activity can reduce it, and motor output adjusts respiratory muscle activity.

Adrenaline contributes to the fight-or-flight response by increasing cardiac activity and mobilising energy supplies. In spirometer traces, the volume swing estimates tidal volume and the number of cycles per time gives breathing rate; an appropriately calibrated trend with carbon dioxide removed can estimate oxygen consumption. Compare resting and exercising data using consistent calibration and account for dead space, leaks and individual differences. ECG patterns can support investigation of abnormal rhythms but one trace is not a complete clinical diagnosis.

Key formulae

$$\text{cardiac output}=\text{heart rate}\times\text{stroke volume}$$

$$\text{minute ventilation}=\text{breathing rate}\times\text{tidal volume}$$

$$\text{heart rate}=\frac{60}{\text{time between successive beats in seconds}}$$

Worked example

During exercise, heart rate is $150\,\mathrm{min^{-1}}$ and stroke volume is $100\,\mathrm{cm^3}$. Calculate cardiac output in $\mathrm{dm^3\,min^{-1}}$.

1. Multiply rate by volume per beat: $150\times100=15000\,\mathrm{cm^3\,min^{-1}}$.
2. There are $1000\,\mathrm{cm^3}$ in $1\,\mathrm{dm^3}$.
3. Divide by $1000$ to obtain $15\,\mathrm{dm^3\,min^{-1}}$.

Answer: $15\,\mathrm{dm^3\,min^{-1}}$.