Lecture 31 - exercise physiology PDF

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Lecture 31- learning outcomes 

What are the typical breath-hold times in normal subjects under specified conditions and their variability – Schneider 1930, Ferris 1946 etc.

Typical breathold time was 1.1 minutes +- 0.02 Others say normal to hold for longer 2.7 minutes +0.2 

Does exercise shorten breath-hold time – lin 1974

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

Does PaCO2 and PaO2 change during breathing holding and what are their typical values at breakpoint – lin 1974

As expected during breath-holding, paO2 falls below normal level of 100mmHg and paCO2 rises above normal level of 40mmHg to 48-55mmHg. 

Hyperoxia prolongs breathold time but a PaO2 threshold cannot account for the breakpoint – Ferris 1946

Although hyperoxia prolongs breathold time the breakpoint was higher than normal. They still have a breakpoint so the pO2 threshold is invalidated. 

Hypoxia shortens breathold time but a PaO2 threshold cannot account for the breakpoint – Ferris 1946

Although hypoxia shortens breath-hold time the breakpoint is lower than normal. This invalidates the idea of a pO2 threshold level. 

Explain The contributions of the peripheral chemoreceptors to breath-hold time – gross 1976

Denervation of the peripheral arterial chemoreceptor does not prolong breath-hold times. It also did not affect breath-hold time in hyperoxia. Therefore it cannot be the case that a paO2 threshold exists beyond which cannot breath-hold. 

Hypercapnia shortens and hypocapnia lengthens breathold time but PaCO2 cannot account for the breakpoint – kleman 1971

Although hypercapnia shortens breathold time, breathold time was reduced but PCO2 levels at breakpoint rose which is not consistent with breakpoint regulated by a PCO2 level. This invalidates the idea of paCO2 threshold level. Similarly, although Hypocapnia before hold lengthens breath-hold time, in this case the same PCO2 at breakpoint from normocapnia as from hypocapnia. 

The effects of rebreathing asphyxiating gas mixtures on the ability to continue to breathold – fowler 1954

Rebreathing from asphyxiating gas mixture made paO2 fall even lower and made paCO2 rise even higher. They could still breath-hold for another 20 seconds while blood gases got steadily worse. 

The change in lung volumes during breath hold – Stevens 1946

It does use up O2 but breatholding abolishes the partial pressure gradient driving CO2 into the lungs so the net effect is that lung volume shrinks during breath-hold. The best evidence for this is underwater weighing. As breath-hold lung volume must decrease, we also know that increased lung volume does prolong breath-hold time. 

Can changes in lung volume or pulmonary afferents cannot account for the breakpoint? – godfrey 1969

No. Breakpoint cannot be a simple function of lung shrinkage. It cannot be the case that breakpoint occurs when lungs shrink to a minimum volume because this predicts that cutting pulmonary branches of vagus nerve should enable indefinite breath-holding but this is not so. 

That sensation from abdominal afferents cannot explain the breakpoint (Eisele & Guz 1968).

The breakpoint is not due to sensations from abdominal afferents coming from below the first thoracic. Spinal anaesthesia to T1/C6 causes no change in duration of breath-hold nor a change in sensation. 

Humans don’t have the diving response of diving mammals.(Scholander 1941, Craig1968, Scholander 1962, Parkes 2012, Parkes 2014, Pingatore 2007, Langdon 1997, Guaraldi 2009)

Driving mammals can breath-hold under water for more than 45 minutes and swim around hunting. They do this through a decrease in skeletal and intestinal muscle blood flow. It is reduced to zero. There is vasoconstriction so if no oxygen delivery they must decrease o2 consumption. The secondary response is a decrease in cardiac output by decreasing stroke volume and heart rate - decreased heart rate is a consequence of the diving response not a cause. Evidence for this is that flippers do not bleed and muscle lactate accumulation. Humans can’t do this as they cannot shut down skeletal muscle blood flow to zero. Heart rate does not decrease below 50bpm and humans do not decrease metabolic rate or cardiac output. 

What happens to the central respiratory rhythm during breath-holding (Agostoni 1963, Cooper 2003)

The central respiratory rhythm appears to continue throughout breath-holding. 1. Humans cannot voluntarily stop their central respiratory rhythm 2. Breath-holding is achieved by voluntarily supressing expression of the central respiratory rhythm

3. At the breakpoint some involuntary mechanism overrides voluntary suppression of the expression of the central respiratory rhythm



The effects of diaphragm paralysis on breath-hold time (Campbell1967, Noble & Guz 1971,Banzett 1990, Gandevia 1993).

In a study with a paralysed diaphragm conscious subjects mechanical ventilated and switched off until they signalled to be switched on again. Found that breath hold time increases to a duration of 4 minutes. There was no discomfort which suggests breakpoint is to do with respiratory muscles. 

The possible role of extra pulmonary afferents travelling in the vagus nerve (Guz 1966).

Blocking the whole vagus nerves bilaterally doubles breath-hold time and it alleviates distress of breath holding. So crucial afferents may travel up the vagus nerve. 

The dangers of hypoxia and breath-holding (Schneider 1930)

It is practically impossible for a man at sea level to voluntarily hold his breath until he becomes unconscious. It is safest to study breath-hold time from hyperoxia. 

The dangers if breath-holding is not permitted to restart! (Cumin 2000)

Breath-holding for short periods during vigorous restraint could cause severe hypoxia. It can cause deaths. 

The clinical applications for radiotherapy (Parkes 2014)

Recent interest in using repeated deep inspiratory breath-holds or prolonged single breathholds to improve radiotherapy delivery but there are risks. There is little clinical awareness of the pronounced blood pressure rise and the potential for gradual asphyxia that occur during breath-holding....