List the physiological factors that increase respiratory rate. Include an explanation of the mechanism by which each achieves this increase.

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College Answer

Good candidates had a structured approach to this questions. Submitted question
structures took the form of key headings (eg, PaCO2, PaO2, pH, etc) with an 
accompanying explanation, which included diagrams, which were often 
underutilised. Candidate answers that lacked any structure were more likely to have 
omissions and lacked sufficient depth and as a result scored fewer marks. For a 
good answers candidates where expected to list and explain (preferably by including 
diagrams) physiological factors such as PaCO2, PaO2, pH, Exercise, temperature, 
pregnancy and the associated receptors for each mechanism.
Syllabus: B1c 1
Reference: Nunn’s Applied Respiratory Physiology, Lumb, 6th edition 60-68
Principles of Physiology for the Anaesthetist, Power & Kam, 1st edition 92-98

Discussion

The college comments persistently refer to how they would have preferred some diagrams. Of the numerous possible graphs one could use, probably the most relevant would be those representing the relationship between ventilation, hypercapnia and hypoxia:

ventilatory%20responses%20to%20hypercapnia%20and%20hypoxia.jpg

Physiological Factors which Influence the Respiratory Rate

Physiological factor Mechanism Physiological effect
PaCO2

Sensed by peripheral chemoreceptors:
- Carotid bodies (glossopharyngeal nerve)
- Aortic bodies (vagus nerve)

Increased PaCO2 increases the respiratory rate and tidal volume 

Decreased PaO2 increases the respiratory rate
(rapidly acting breath-to-breath control of respiration)
- this response to hypoxia is triphasic

PaO2
pH

Sensed by central chemoreceptors in the medulla

Decreased pH in the CSF increases the respiratory rate and tidal volume
(slow acting, steady state control; adjustments occur over minutes)
Temperature

Increased sensitivity of periphperal chemoreceptors to O2

Increased sensitivity of central chemoreceptors to changes in pH

A rise in temperature will increase the minute volume at any given PaCO2 and PaO2 level

Responses to hypoxia and hypercapnia are
amplified by hyperthermia

Exercise

Descending control of muscle activity simultaneously simulates the central respiratory control centres

The ventilatory response of exercise is increased ventilation, and because this is not a feedback mechanism the increase in ventilation is simultaneous with the beginning of exercise, or actually slightly preceeds it. 
Pregnancy

Progesterone acts directly on central integrative control of ventilation

During pregnancy, minute volume increases and the stable PaCO2  baseline progressively decreases, producing the respiratory alkalosis of pregnancy
Blood pressure

Sensed by aortic chemoreceptors and carotid sinus baroreceptors

Hypertension decreases the respiratory rate and hypotension increases it.

With a sufficient acute hypertensive event, respiration may briefly cease ("adrenaline apnea")

References

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