Physiology 11 Flashcards
What are the important spirometric variables?
What is the difference between a volume and a capacity?
TV: Tidal volume IRV: Inspiratory reserve volume ERV: Expiratory reserve volume RV: Residual volume TLC: Total lung capacity VC: Vital capacity FRC: Functional residual capacity
A capacity is the sum of two or more volumes.
What equipment can be used to measure lung volumes?
- Water sealed spirometer
- Dry spirometer
- Body plethysmograph
- Helium dilution
- Nitrogen washout
Which techniques for measuring lung volumes can calculate residual volume/FRC/TLC?
What are the relevant benefits/drawbacks of these techniques?
Body plethysmography - most accurate
Helium dilution - Does not include collapsed/poorly ventilated areas
Nitrogen washout - Also does not include collapsed/poorly ventilated areas. Requires patient to be able to breathe 100% O2 (ie. not chronic T2RF)
Why is helium used for measuring lung volumes?
Its solubility in blood is very low
What are the important functions of the functional residual capacity (FRC)?
- Oxygen reservoir
- Airway splinting
- Optimal lung compliance
- Optimal pulmonary vascular resistance
What is a typical FRC?
What is the volume of oxygen present in the FRC?
2500ml
2500 x 0.15 = 375ml O2
How long does the O2 reserve in the FRC last during apnoea in normal conditions?
90 seconds
Given adequate proxygenation, how long will the O2 reserve FRC last?
2500 x 0.9 = 2250ml O2
2250 / 250 = 9 mins
What factors affect how long a patients O2 reserves will last?
- Reduced FRC (Obesity)
- Increased consumption (Sepsis, childhood)
What is closing capacity?
Residual volume + closing volume = closing capacity
What may cause increased closing capacity?
- Smoking
- Asthma
- Advanced age
Where does FRC usually sit on the lung compliance curve?
Usually at the steepest part of the curve (ie. most compliant)
What is the impact of restrictive lung disease on lung compliance at FRC?
FRC will be below the optimal point on the curve and compliance will decrease, increasing work of breathing
What factors increase FRC?
Height Male gender Asthma Emphysema IPPV
What factors decrease FRC?
Obesity Anaesthesia Supine position Kyphoscoliosis Lung fibrosis
Does age have an effect on FRC?
No (though CC is increased)
What are the constituents of dead space in the ventilated patient?
- Apparatus
- Anatomical
- Alveolar
Outline Fowler’s Method of calculating anatomical dead space
- Patient breathes normally then takes a VC respiratory breath of 100% O2 (from FRC)
- Patient exhales slowly down to RV, exhaled N2 is measured throughout
Initially following expiration the [N2] will be 0, representing the anatomical dead space. The midpoint of the initial steep rise in [N2] represents anatomical dead space volume.
What is a normal dead space:tidal volume ratio?
0.8 in most mammals
What is the shunt equation?
Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)
What is hysteresis?
The phonomenon of two limbs of a curve following different courses eg. the inspiratory and expiratory limbs of a respiratory P-V curve
How is lung compliance measured?
By calculating the gradient of the pressure-volume curve
Define compliance
What units are used to express lung compliance?
What is a normal value for lung compliance?
Compliance is increase in volume per unit pressure.
Lung compliance is measured in ml/cmH2O
Lung compliance in the normal range is 200ml/cmH2O
Define specific compliance
This is compliance per unit volume of the lung.
This is relevant because lung compliance varies with lung size (volume/capacity). Therefore specific compliance is independent of body size
What is Laplace’s Law for a sphere?
What is this relevant to physiologically?
P = 4T / r
P = pressure inside sphere T = surface tension r = radius of sphere
This is relevant to surface tension in alveoli
What is Laplace’s Law for a single interface (rather than a sphere)?
P = 2T / r
or T = rP / 2
What is surfactant?
- Phospholipid containing dipalmitoyl phosphatidyl choline
- Reduces alveolar surface tension by opposing normal cohesive forces between surface molecules
- Removes the effect of Laplace’s Law on alveoli
- Resists collapse of alveoli at low lung volumes due to repellent forces between surfactant molecules
What is the most relevant property of a gas in terms of its resistance to laminar flow?
Viscosity
What is the most relevant property of a gas in terms of its resistance to turbulent flow?
Density
How is Reynolds number calculated?
How does it determine flow?
Re = 2vdnr
v = velocity d = density n = viscosity r = radius
Re > 2000 indicates high likelihood of turbulent flow
What is a normal FEV1/FVC ratio?
75-80%
What factors affect respiratory function tests?
Age, gender, height, ethnic origin
Outline the components of the system of control for breathing
Sensory information from central and peripheral chemoreceptors (and lung reflex afferents) feed into the central pattern generator (CPG) in the medulla. This gives afferents to the muscles of the larynx and the muscles of respiration
Where are the central chemoreceptors that control breathing located?
Ventral surface of medulla (rostral and caudal chemoreceptors), near the origin of CN IX + X
Where are the peripheral chemoreceptors located?
- Carotid bodies: At bifurcation
- Aortic bodies: Above and below aortic arch
Which receptors form lung reflex afferents?
Pulmonary stretch receptors Irritant receptors Juxta-capillary receptors Nose and upper airway receptors Proprioceptors Arterial baroreceptors Pain and T receptors
What are the components of the respiratory centre?
Dorsal and ventral medullary groups (main groups)
Also pontine group
What happens to breathing pattern if the vagus nerves are transected?
Deep breathing (stretch receptor afferents lost)
What happens to breathing if the brainstem is transected at the mid-pontine level?
What is the effect of vagotomy following this?
Depth of breathing increases (upper pontine group signals to terminate inspiration)
Subsequent vagotomy triggers apneustic breathing (prolonged inspiration with sporadic expiration)
What happens to breathing if the brainstem is transected at the level between the pons and medulla?
What is the effect of subsequent vagotomy?
Breathing is maintained, though irregular and deep.
No effect of subsequent vagotomy seen
What happens to breathing if the brainstem is transected below the medulla?
Apnoea
Outline the location, structure and function of the dorsal respiratory group (DRG)
- Located in floor of 4th ventricle, next to tractus solitarius where afferents from CN IX + X terminate
- Predominantly inspiratory neurones (phrenic and intercostal)
- Supplies UMNs to contralateral anterior horn cells
- Primarily concerned with timing of respiratory cycle
Outline the location, structure and function of the ventral respiratory group (VRG)
- The VRG runs the length of the medulla, starting in the lower pons and comprises four nuclei:
1. Botzinger’s complex: Within the nucleus retrofacialis in the pons. Widespread expiratory function including inhibition of inspiratory neurones.
2. Nucleus para-ambigualis: Controls contralateral inspiratory muscles
3. Nucleus ambiguus: Ipsilateral larynx, pharynx and tongue dilator function.
4. Nucleus retro-ambigualis: Most caudal nucleus, predominantly contralateral expiratory function.
Outline the location, structure and function of the pontine respiratory group (PRG)
Comprises two areas within the pons:
- Pneumoaxic centre: Terminates inspiration, causing higher RR and lower TV
- Poorly defined apneustic centre
Outline the sequence of neurological signals in the respiratory cycle
- Early inspiratory group fire - Airway dilators activated
- Inspiratory augmenting group fires - Inspiratory muscles contract
- Late inspiratory group fires - Relaxation of inspiratory muscles [expiration then begins]
- Expiratory decrementing group fires - Airway adductors activated
- Expiratory augmenting group fires - Expiratory muscles activated
- Late expiratory group fires - Inspiration triggered
Outline the cellular mechanism of central breathing pattern generation.
Slow membrane depolarisation of inspiratory augmenting neurones (via Ca2+ and K+ channels) causing spontaneous depolarisation, triggering inspiration. Membrane then repolarised (following calcium influx) by Ca2+ dependent K+ channels
How does cortical control of breathing occur?
Some cortical fibres bypass the respiratory centre altogether, allowing voluntary control of breathing, however the central pattern generator continues to function throughout voluntary activity
What are the inputs to the motor fibres to respiratory muscles?
Three groups:
- From central pattern generator
- From the cortex (voluntary control)
- Involuntary, non-rhythmic control (eg. hiccups, swallowing, sneezing)
What do the central chemoreceptors detect?
PaCO2, via CSF [H+]
What do peripheral chemoreceptors detect?
PaO2, PaCO2 and [H+]
Which chemoreceptor area is the most important driver of ventilation?
The central chemoreceptor area (responsive to PaCO2)
Outline the anatomy of the central chemoreceptors
- Found in anterolateral medulla, 200-400um below surface
- Rostral zone (crossed by anterior inferior cerebellar artery) and caudal zone, with an intermediate zone where the areas interconnect
How do the central chemoreceptors respond to changes in PaCO2?
CO2 diffuses freely into CSF across BBB (unlike H+/HCO3-)
Becomes hydrated in the CSF, producing H+
CRs sensitive to [H+] produce a minute ventilation response within 1-3 mins
What is the normal pH of CSF? Why?
7.32
Lower protein content reduces buffering capacity
pH shows exaggerated response to PaCO2 compared to blood
What happens to central chemoreceptors in response to chronic raised PaCO2?
- Compensatory transport of HCO3- into CSF within hours to days. May be active but could be due to passive distribution
- Causes chronic loss of central ventilatory response to increased PaCO2
What factors other than chronic T2RF lead to reduced central chemoreceptor response to PaCO2?
Sleep Increased age Genetic/racial factors Training (eg. athletes, divers) Drugs (eg. opioids, barbiturates)
