Mechanical properties of chest wall Flashcards
Describe the various neural centres that control respiration, their function and their components
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Respiratory Centre
- Located in the medulla
- Drives ventilation rate and volume via afferents to respiratory muscles’ motor nerves ^[causes contraction]
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Neuronal Groups
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Dorsal Respiratory Group
- Primarily inspiratory neurons - sending out signals
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Ventral Respiratory Group
- Caudal: Mix of inspiratory and expiratory neurons
- Rostral: Airway dilator functions
- Pre-Botzinger Complex: Likely central pattern generator site ^[pattern of breathing, start and stop]
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Botzinger Complex
- Expiratory neurons
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Pontine Respiratory Group
- Fine control of respiration, influences medullary respiratory center
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Dorsal Respiratory Group
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Cortex
- Voluntary breathing interruption, e.g., singing, talking ^[aka influence pattern of breathing]
Describe how neuronal firing is linked to generation of respiratory activity
Respiratory Cycle and Neuronal Firing
- No single pacemaker is responsible for generating respiratory activity ^[c.v. cardiac]
- Likely a complex interaction of different groups of neurons (6 - half and half):
- Early inspiratory, inspiratory augmenting, late inspiratory
- Expiratory decrementing, expiratory augmenting, late expiratory
- Firing of neurons results in three respiratory phases: - Inspiratory ^[turn on signals, get muscles involved]
- Expiratory Phase 1 (passive) ^[i.e. recoil of lungs]
- Expiratory Phase 2 (active) ^[turning on signals, get muscles involved]
What is the most important factor that influences respiratory rate and how does it exert influence?
CO2 being the most important influence on respiratory rate
- CO2 influences central chemoreceptors
LIST the two main controllers of ventilation
Central and peripheral chemoreceptors
Describe central chemoreceptors
Central Chemoreceptors
- Neurons that are separate from the respiratory center, in the medulla
- Stimulated by H+ concentration in CSF determined by paCO2 (H+ cannot cross BBB, CO2 diffuses easily) - hence why ‘most important factor in determining ventilation’
- CO2 reacts with H2O to form H2CO3, dissociating into H+ and HCO3
- Increased paCO2 leads to increased afferent firing from chemoreceptors to respiratory center
- Respiratory centre controls efferent output to the respiratory muscles (effector), to increase minute ventilation
- Maintains paCO2 under tight control (+/-3) around 40mmHg: most important determinant of minute ventilation
Describe peripheral chemoreceptors
Peripheral Chemoreceptors
- Carotid Bodies
- Respond to paO2, paCO2, and pH
- Afferent signals via glossopharyngeal nerve
- Aortic Arch
- Respond to paO2 and paCO2 ^[potential c/c question within, and compared to centrals]
- Afferent signals via vagal nerve
- Fast acting compared to central chemoreceptors ^[responds to oxygen tension? within vessels] -> drives 20% of response (central 80%), ^[big driver to hypoxia response?]
- Histologically - Glomus or type I cells in contact with synaptic nerve endings
- Activation results from hypoxia, hypercapnia, or acidosis
- Inhibition of K+ channels leads to decreased efflux, and depolarisation, opening VGCCs (and calcium influx)
- Ca2+ triggers neurotransmitter release (dopamine, probably) and afferent signaling via nerves above to respiratory centre (controller), sending efferent signals to respiratory muscles (Effector) to increase minute ventilation
List and briefly describe other controllers of respiration
- lung receptors
- pulmonary stretch - respond to over distention
- juxtacapillary receptors - respond to interstitial fluid APO
- bronchial C - irritants in blood e.g. histamine
- irritant receptors - irritants in gas e.g. cold and dust - baroreceptors - increased mv with hypotension
- muscle and joint receptors - increased mv ^[minute ventilation] with movement
- SNS -increased MV with increased SNS activity
- cortex - can voluntarily override respiratory centre
- pregnancy and progesterone - directly stimulates respiratory centre
- exercise increases MV with exercise
Provide an overview of respiration
- Breathing controlled by medullary respiratory centre
- Efferent signals to respiratory muscles cause volume and pressure changes
- Contraction leads to volume and pressure changes which draw air into lungs
- Lungs lie in thorax, separated from chest wall by intrapleural space
- Lungs tend to collapse; chest wall tends to expand
- Balance leads to negative intrapleural pressure (approx. -5cmH2O)
Describe surface tension, surfactant and its role
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Surface Tension (ST)
- Force across liquid surface (across imaginary line 1 cm long)
- Develops at air-water interfaces
- Greater forces between water molecules than water and gas molecules: liquid surface area becomes as small as possible
- Result: alveolus has tendency to collapse on itself ^[like a bubble]
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Law of Laplace
- Pressure = 2 x ST/radius
- radius inversely proportional to pressure
- alveoli would collapse if it weren’t for surfactant, reducing surface tension as radius decreases
- Pressure = 2 x ST/radius
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Surfactant
- Lipid (90%) fluid from type II alveolar cells
- Majority phospholipid: mainly dipalmitoyl phosphatidyl choline (DPPC): hydrophilic end faces alveolar fluid lining alveoli, hydrophobic end faces gas filled alveolus
- Acts as detergent, reducing water molecule attraction, thus reducing surface tension and preventing collapse
- Decreases ST as lung volume decreases: as lung vol decreases DPPC squeezed together, decreases water-water interaction and thus decreases surface tension
Note: relevant in compliance
Breakdown the respiratory cycle in terms of the key pressures and volumes
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Starting Point: FRC (Functional Residual Capacity)
- Intrapleural pressure = -5cmH2O
- Alveolar pressure = Atmospheric (0cmH2O) - no net movement of air
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Inspiration
- Intrapleural pressure falls to -8cmH2O
- Alveolar pressure falls to -1cmH2O
- Gas influx into alveoli (~500ml)
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Expiration
- Intrapleural pressure rises to -5cmH2O (i.e. more positive)
- Alveolar pressure rises to +1cmH2O before baseline
- Gas exits alveoli to atmosphere (~500ml)
Define compliance, factors that contribute to it
- Definition: Ratio of volume change to corresponding pressure change (C = ∆V/∆P) - slope of PV relationship
- Extent to which lungs expand for each unit increase in transpulmonary pressure (if enough time allowed to reach equilibrium)
- Two determinants of compliance in respiratory system:
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Lung Compliance
- Determined by lung’s elastic recoil (connective tissue, surface tension)
- Normal value: 200ml/cmH2O
- Influenced by factors: Surfactant (most important), age (shape and size), posture, size, lung volume, fibrosis (Stiff, more difficult to expand)
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Chest Wall Compliance
- Normal value: 200ml/cmH2O
- Impaired by factors affecting chest wall expansion (e.g., scoliosis, obesity ^[more tissue on chest wall,m ore pressure])
- Total Lung Compliance: Combination of lung and chest wall compliance (around 100ml/cmH2O)
Describe flow and factors that contribute to it
Flow
- Flow Definition: Substance passing point per unit time
- Flow governed by Ohm’s Law
- Two main types of flow:
- Laminar Flow: Straight, unbranched tube; fastest center flow ^[i.e. in middle of tube]
- Turbulent Flow: Irregular or branched tubes; eddies, higher resistance
- n.b. Transitional Flow: Mixture of laminar and turbulent
- in lung: mix of laminar, turbulent and transitional
- Flow Equation: Flow = Pressure difference (pressure coming in - going out) / Resistance
- Resistance Formula: R = 8nl / πr ^4
- n = viscosity ^[usually fixed]
- l = length ^[usually fixed]
- r = radius
- radius most important e.g. half radius = 16 fold change ^[relevance in pathology]
- Laminar Flow
- ‘series of concentric cylinders sliding over each other’
- Faster in center, slower at edge
- ‘even front’
- Turbulent Flow
- higher flow rates/flow through branched or irregular tubes
- causes concentric circles to breakdown
- flow becomes small currents or eddies, higher friction
- Reynolds Number (Re) - describes tendency towards turbulent flow
- Dimensionless number indicating likelihood of turbulent flow
- Re = ρDv / η
- ρ = Gas density
- D = Tube diameter
- v = Flow velocity
- η = Viscosity of gas
- >2000 associated with turbulent flow
Describe some clinical implications of respiratory mechanics
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Flow in Large Airways
- Predominantly turbulent flow
- frictional forces influences flow (come from lining of airway wall– increases with scarring)
- note: driving pressures really high, so overall frictional forces do not influence flow
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Gas Density
- Different gas densities (e.g., Heliox - gas mixture) to improve flow and oxygen delivery to alveoli
- more laminar flow due to He low density– get into alveoli more easily
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Flow in Small Airways
- Predominantly laminar flow, resistance most important factor
- Viscosity and length are essentially fixed
- radius of airway is the most important factor determining resistance
- Predominantly laminar flow, resistance most important factor
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Factors Affecting Airway Radius
- Internal: Fluid, smooth muscle hypertrophy/contraction
- External: Lung volume, external compression of airway ^[e.g. haemothorax]
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Compliance
- Extent of lung expansion per unit increase in transpulmonary pressure
- Factors influencing decreased compliance
- Physiological: Age, posture (lying flat is worse), decreased lung volumes
- Pathological: Fibrosis, alveolar overdistension (COPD over PEEP), chest wall deformities, obesity
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Time Constants (τ)
- if initial rate of change continued, at what time would process have been completed
- alveolar filling and emptying is an exponential process, measured by t
- one t = 63%
- 3t = 95%
- τ = resistance x compliance
- normally in 0.2s, alveolar filling emptying 95% complete at 0.6s
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Fast and Slow Alveoli (imp)
- resistance and compliance not uniform across lung, therefore t varies
- Fast: Low resistance, compliance, or both (e.g., pulmonary fibrosis): empty and fill quickly
- Slow: High resistance, compliance, or both (e.g., COPD): empty and fill slowly
Breakdown the work of breathing
- Work = Force x Distance
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Respiratory Work: Work = Pressure x Volume
- Normal work small (0.3-0.6J/L), <2% of metabolic rate or 3ml/min O2
- Note: respiratory muscles are very inefficient and with increased work of breathing this becomes much higher
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Inspiratory Work
- Elastic and non-elastic work
- Compliance work against lung recoil, non-elastic work against airway resistance
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Expiratory Work
- Primarily elastic? work
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Factors Influencing Work
- Increased work with decreased compliance or increased resistance
- Elastic work
- 65% of work
- Compliance work: to overcome recoil of the lungs
- Work against elastic forces stored as potential energy which is used during expiration
- Any factor that decreases compliance will increase elastic work (i.e. inversely related)
- Non-elastic work
- 35% of work
- Work to overcome airway resistance
- Non-elastic work lost as heat
- Any factor that increases resistance will increase non-elastic work (directly related)
Pathology and breathing:
- normal: most energy efficient point - more elastic and resistive work: optimal work of breathing
- energy efficient point at higher respiratory rate (greater pressure for ventilation, ventilate at higher respiratory rate)
- air flow resistance: energy efficient point at lower respiratory rate ^[greater gap between breaths, long expiration time; ventilation sets respiratory rate lower]
Review lung volumes and capacities
- Volumes: Directly measured
- Capacities: Sum of two or more volumes
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Lung Volumes
- Tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume
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Lung Capacities
- Total lung capacity, vital capacity, inspiratory capacity, functional residual capacity
Functional Residual Capacity (FRC)
- Amount of gas after tidal expiration
- Balance point between chest wall tendency to expand and lung tendency to collapse
- Functions of FRC
- Minimizes work of breathing, pulmonary vascular resistance, V/Q mismatch, airway resistance
- **Primary oxygen store ^[pre-oxygenation, keeps oxygenated]
- prevents atelectasis (certain volume inside)
- Maintains steady arterial pO2, buffers changes in alveolar pO2 during respiratory cycle
