L30 Breathholding

Question 1

What is the point of hyperventilating when atmospheric PO2 falls

Answer 1

hyperventilating seems pointless we=hen it wont increase atmospheric PO2 - its purpose is to alter the composition of alveolar air
Alveolar air acts as an internal reservoir of O2 and Co2 between breaths. –> without it PaO2 would fall and arterial blood acidity would rise dangerously between breaths.
Hyperventilating hypoxic air causes PCO2 to fall.
The displaced alveolar CO2 is replaced by atmospheric O2 –> results in alveolar PO2 rising slightly + this helps maintain the vital pressure gradient driving O2 into tissues

Question 2

The carotid arterial chemoreceptors are the only way the respiratory system in the humans detects hypoxia

Answer 2

Central chemoreceptors don’t detect hypoxia.
The aortic arterial chemoreceptors have no known effect on breathing in humans.
Carotid chemoreceptors are not necessary for breathing to increase during exercise

Question 3

Arterial chemoreceptor denervation has almost no effect on breathing during exercise

Answer 3

Wasserman 1975 showed that carotid body denervated asthmatics showed normal rapid increase in breathing at start of exercise. Slower to reach o new steady state but did get there.

Question 4

Define breath holding

Answer 4

conscious suppression of breathing. Breakpoint = involuntary

Question 5

What mechanism causes the breakpoint of breath holding?

Answer 5

Blood gases at breakpoint - PaO2 falls below normal level of 100 mmHg, PaCO2 rises above normal level of 40 mmHg –> 48-55
- The longer you hold your breath, the worse the gases partial pressure.

Exercise shortens breath hold time if the increase metabolic rate will decrease breath hold time.

Question 6

What is the evidence against PaO2 and PaCO2 threshold for cause the breakpoint of breath holding

Answer 6

O2
- Oxygen cant be holding the threshold. In hyperoxia the breakpoint was 553 mmHg compared to the normal 100. But there still was a breakpoint. Hyperoxia prolongs breath holding time. Similarly hypoxia shortens breath hold time.

CO2
- Hypercapnia shortens breath hold time. Hypocapnia before hold, lengthens breath hold time?

Question 7

What happens to lung volume during breath holding

Answer 7

BH abolishes the Pa gradient driving Co2 into the lungs so the net effect is that lung volume shrinks during BH. But the breakpoint cant be a simple function of lung shrinkage

Question 8

what happens to the central respiratory rhythm during breath holding ? and how can you measure this

Answer 8

Rhythm does reappear before the breakpoint of breath holding .
Measure using respiratory sinus arrhythmia - your heart period decreases in inspiration –> heart rate increases when you breathe in