2006A10 List the physiological factors which increase respiratory rate and include a brief
explanation of the mechanism by which each achieves this increase.

 

List:

·       Intro

·       Chemical factors

·       Mechanical factors

·       Increased metabolic rate

·       Other

 

Intro:

·       Respiratory rate (RR) controlled by medullary respiratory centre

·       ↑Afferents -> ↑rate of discharge of pre-Botzinger pacemaker cells -> ↑rate of efferents to respiratory muscles -> ↑RR

·       Afferents may have synergistic effects

 

Chemical factors (most important):

Peripheral chemoreceptors

·  Carotid bodies (sensitive to ↑PaCO2, ↓PaO2, ↓pH)

·  Aortic bodies (sensitive to ↑PaCO2, ↓PaCO2)

·  Stimulus -> K+ channel closure -> afferent via CNIX Hering’s nerves (carotid) and CNX (aortic)

·  5x faster than central – for rapid control

·  Stimuli are synergistic with each other

·  PaO2 important only if low (especially <50mmHg)

·  ↑PaCO2 1mmHg -> ↑minute ventilation 2-3L/min

Central chemoreceptors

·  Ventral medulla

·  Stimuli: ↑PaCO2, CNS acidosis

·  CO2 crosses blood-brain barrier, dissociates into H+ and HCO3- (carbonic anhydrase), decreases pH in CSF and brain ECF -> ↑afferent

·  4x more powerful than peripheral

·  Most important factor normally

 

Mechanical factors:

Baroreceptors

·  Aortic sinus and carotid sinuses

·  Stretch-activated mechanoreceptors

·  ↓mAP -> ↓inhibitory afferent to medulla -> indirect stimulation of resp centre

Pulmonary J receptors

·  Stretch/irritation -> ↑afferent via general visceral afferent, spinomedullary tract

·  Important in dyspnoea induced by acute pulmonary oedema

Airway smooth muscle

·  Tidal volume >1L -> ↑↑Stretch -> ↑afferent to resp centre -> transient ↓RR (Hering Breuer reflex)

Joint and skeletal muscle proprioceptors

·  ↑Limb motion -> ↑Abeta afferent via spinomedullary tract -> unknown mechanism

 

Increased metabolic rate:

Common mechanism

·  ↑Metabolic rate (MR) -> ↑VCO2 -> ↑PaCO2 -> ↑RR

Neonate

·  ↑MR 2x (VO2 7-8mL/min/kg)

·  RR ~40

(WOB minimized at high RR due to low total compliance)

Pregnant

·  ↑MR 1.2x (VO2 300mL/min)

·  RR 16-20

·  ↑Lean mass, ↑cardiac and respiratory work

·  Progesterone also sensitises respiratory centre -> compensated respiratory alkalosis

Hypermetabolic disease

·  ↑Thyroid

·  Sepsis

·  Malignant hyperthermia

Specific dynamic action of food

·  Mostly due to protein deamination and transamination

 

Other:

Central cause

·  Anxiety

·  Stress

·  Pain

·  Voluntary control

·  Exact mechanisms unknown

Exercise-induced

·  Cause unknown

·  Moderate exercise:

o ?↑Activity of descending pathway at onset of exercise

o ?↑PaO2 oscillation during ventilatory cycle

·  Exhaustive exercise:

o ↑H+

o ↓ PaO2 via chemoreceptors