***4 lung volume, 4 lung capacities
TLC = VC + RV (Normal: VC = 5L, TV = 500mL) TLC = IC + FRC
VC = TV + IRV + ERV FRC = ERV + RV IC = IRV + TV
Nitrogen wash out test (Determine FRC)
Keep Breathing in pure O2 (no nitrogen)
—> wash out all N2 in lungs
[N2] x FRC = [N2 in bag] x volume of expired gas
Other methods of determining FRC:
- Body plethysmography: Boyle’s law
- Helium dilution method
Measure uneven ventilation
- Single breath nitrogen washout
—> full inspiration of pure oxygen from residual volume to TLC
—> breathe out slowly and evenly to RV
—> measure N2 concentration changes
Phase 1:
- very short
- 0 N2 concentration (upper airways)
- pure O2 expired
Phase 2:
- short
- rapid increase in N2 concentration
- N2 washed out from anatomical dead space
Phase 3:
- alveolar gas
- flat slope normally
- slope = % increases in N2 concentration per volume of expired gas
- measure of UNEVEN ventilation (normal: 0-1.5%)
Phase 4:
- abruptly increase in N2 concentration
- when breathing out to RV, only alveoli at the top of lung continue to empty
- alveolar at the base closed
- measure closing volume of basal alveoli
- 10% VC in young adult
- increase in premature airway closure (emphysema, asthma)
- basal alveoli 提早 close (during phase 3) —> 拉闊距離 —> increase closing volume
- Multiple breaths nitrogen washout test
- breathe pure oxygen at constant tidal volume for 7 minutes
- plot N2 concentration against number of breaths
- exponential curve
- even ventilation —> uniform washout rate —> straight line on semi-log paper
- uneven ventilation —> different wash out rate —> no straight line
- causes:
—> regional disturbance in expansion (oedema)
—> regional changes in elasticity/CL (change in time constant: time for filling alveoli)
—> regional obstruction (bronchoconstriction)
—> regional check valve (radial traction —> Raw)
Maximal expiratory pressure
Relationship between lung volume and maximal expiratory pressure:
—> Volume↑ —> P↑
Length-tension relationship of skeletal muscle
—> tension depends on optimal length of muscle
—> at TLC: expiratory muscle at optimal length —> greatest tension
—> at RV: inspiratory muscle at optimal length —> greatest tension
Maximum flow rate
Maximum flow rate = Lung recoil pressure / Raw upstream of EPP
FEV1/FVC
FEV1: volume comes out in 1st second
FVC: total volume comes out after forcing
Normal: FEV1: 4L, FVC: 5L —> FEV1/FVC = 80%
Fibrosis: FEV1 no change/↓, FVC ↓↓ —> FEV1/FVC = 90%
Asthma: FEV1 ↓↓, FVC no change/↓ —> FEV1/FVC = 45%
***Maximal expiratory and maximal inspiratory flow-volume curve
Ascending limb: effort dependent, Raw dependent
Descending limb: effort independent, Raw dependent
Emphysema: - TLC ↑ (curve shifts left) - RV ↑ (curve shifts left) - VC no change/↓ (curve 無變/窄左) - Ascending limb —> gentler slope —> poor effort (muscle longer than optimal length) —> Raw ↑ —> lower peak - Descending limb —> lower flow rate —> scooped out appearance: early closure of airway due to dynamic airway compression - Inspiration —> smaller flow —> easier lung recoil —> poor effort —> Raw ↑ ***- ↓FEV1 + no change/↓FVC —> FEV1/FVC <80%
Fibrosis: - TLC↓ (curve shifts right) - RV↓ (curve shifts right) - VC↓ (curve 窄左) - Ascending limb —> similar slope to normal —> poor effort (muscle longer than optimal length) —> but Raw↓ (in favour) —> lower peak - Descending limb —> higher flow rate —> bigger lung recoil pressure to force air out —> Raw↓ (in favour) - Inspiration —> smaller flow —> larger lung recoil —> poor effort —> Raw↓ (in favour) ***- no change/↓FEV1 + ↓FVC —> FEV1/FVC>80%
Blood pressure reading
Auscultatory (Korotkoff sound) —> Gold standard —> directly measure SBP, DBP
Phase nothing (cuff pressure > systolic pressure)
Phase 1: Regular tapping (cuff pressure = systolic pressure)
Phase 2: tapping + murmur (intermittent / turbulent pulsatile flow)
Phase 3: loud, banging sound
Phase 4: sudden softening (cuff pressure = diastolic pressure —> return to continuous blood flow)
Phase 5: cessation of all sound (slightly below diastolic pressure —> continuous laminar flow)
Electronic / Oscillatory method —> “calculation” of BP
Cuff deflating —> amplitude of pressure wave by pulse detected by monitor —> when amplitude is maximum —> mean arterial pressure —> device estimate systolic and diastolic value from mean arterial pressure
Advantage:
- can measure BP in critical care patients with muted Korotkoff sounds
- external noise not a problem
Disadvantage:
- sensitive to patient movement
- erroneous determination of mean BP would produce inaccurate value for systolic and diastolic BP
Variation in BP
- Between-readings:
- white coat hypertension (SBP may be higher initially, DBP no difference) - Between-events
- Difference between left and right arm should be <5mmHg
- Inter / intra-observer variation
- Different accuracies for different equipment
Effect of posture in change in BP
- Lying to upright
- Gravitation pressure act on lower limb blood —> venous return ↓ —> BP ↓ (very short, cannot be detected unless catherterisation
—> detected by baroreceptors —> cardiovascular centre
—> ↑ sympathetic activity —> ↑ TPR —> ↑ DBP more —> mean BP after standing up ↑
—> venomotor tone slowly increase —> ↓ venous compliance —> ↑ venous return
—> ↑ cardiac output - Arm position (effect of gravitational pressure on weight of blood column above heart)
Every 1.3 elevation above heart —> 1 mmHg ↓ (affected SBP, DBP equally) - Activity:
SBP, DBP both ↑ by 10 mmHg
Urine flow and blood pressure
- Effect of saline infusion —> neural, neurohormonal, hormonal control of BP
2. Effect of carotid artery occlusion —> ↓ stimulation of baroreceptor —> ↑ SNS on heart —> ↓ HR, force —> ↑ SNS on arteriole —> ↑ TPR —> ↑ SNS on vein —> ↑ venous return —> ↑ SV
- Effect of adrenaline
Low dose: only stimulate beta receptor in heart —> ↑ HR, ↑ force —> ↑ CO —> ↑ BP
Medium dose: stimulate alpha receptor in vessel —> vasoconstriction —> ↑ TPR —> ↑ BP
High dose: stimulate renal sympathetic nerve —> ↑ renin release —> ↑ water retention —> ↓ urine output
4. Effect of osmotic diuresis Filtered concentration > tubular maximum —> glucose remain in collecting duct —> ↓ osmotic gradient between collecting duct and interstitial space —> Diuresis —> no change in BP
- Loop diuretic
—> inhibit NKCC —> inhibit Na reabsoption
—> ↓ osmolarity of interstitium
—> ↓ water reabsorption from collecting duct
—> Diuresis
6. ADH —> ↑ permeability of collecting duct to water —> ↑ water reabsorption —> ↓ urine output —> slight ↑ BP
- Effect of blood loss / volume depletion
—> Atrial volume receptor —> neural + neurohormonal control
—> Baroreceptor —> neural control
—> Atrial muscle —> hormonal control
