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# MP - Lecture 24

### Mechanics of Breathing II

Medical Physiology – Lecture 24 Mechanics of Breathing II
5 Critical Variables in Respiratory Mechanics Flow, volume, compliance, pressure, and resistance
Pressure = Pressure = (V/C) + (Q x R)
Alveolar pressure is always more ___ than pleural pressure. Positive
Slope of pressure (x-axis) vs. volume (y-axis) curve represents: Compliance
Compliance is high at what volumes and pressures? Low volumes and pressures
Compliance is low at what volumes and pressures? High volumes and pressures
Hysteresis Forward path is different than reverse path
Main hysteresis is observed in lungs at: First breaths, with collapsed lungs
Compliance is always: Positive
In a column of water, pressure is ___ at the surface. Atmospheric
Gravity makes pleural pressure ___ at base than at apex. Less negative (Ppl = -10 cm H2O vs. +2 cm H2O)
Local compliance at apex is ___ than local compliance at base. Lower
Apex has lower compliance because: Inspiration causes greater expansion in base than apex
Typical compliance of lung is: 200 mL/cm H2O
Specific Compliance Compliance divided by initial volume (value independent of size)
When compliance is low, elastic recoil is: High (i.e. high lung volumes, fibrotic conditions)
When compliance is high, elastic recoil is: Low (i.e. low lung volumes, emphysema)
If no lungs present and atmospheric pressure at airway opening: Thorax is at relaxed volume, which is greater than FRC
If no lungs present, what pressure must be applied to bring thorax volume back to FRC? -5 cm H2O (counteract thorax elastic recoil in absence Ppl = -5 cm H2O)
FRC of lungs is caused by: Balance between inward lung recoil and outward thorax recoil
Passive elastic properties of thorax: FRC (3 L) @ -5 cm H2O, resting volume at P = 0 cm H2O
Passive elastic properties of lungs: FRC (3 L) @ +5 cm H2O
Passive elastic properties of thorax and lungs: FRC (3 L) @ 0 cm H2O
Slope of combined passive elastic properties compared to individual curves is: always less
Pressure required to inflate both chest wall and lungs is: The sum of the respective pressures required to inflate individually
Compliance of lung and chest wall combined: (1/CT) = (1/CL) + (1/CCW)
If either CL or CCW are very small, than total compliance is: Very small
Low compliance of either lung or chest wall causes: Restrictive breathing (low total compliance)
Typical compliance values: CL = 200, CCW = 200, CT = 100
Increase in pressure at airway by +10 cm H2O yields only a +5 cm H2O transpulmonary pressure because: Combined compliance is less than individual compliances
Pressure needed to increase FRC by +1 with normal compliance is: P = 10 cm H2O (P = V/CT = 1000/100)
Pressure needed to increase FRC by +1 with low compliance is: P > 10 cm H2O
Obese people often sleep with thorax higher than abdomen because: Reduced thorax compliance
During inspiration (VL, PA, Ppl): VL increases, PA becomes negative, Ppl becomes more negative
At the end of inspiration (PA, Ppl): PA = 0 cm H2O, Ppl = -10 cm H2O
During expiration (VL, PA, Ppl): VL decrease, PA becomes positive, Ppl becomes less negative
At the end of expiration (PA, Ppl): PA = 0 cm H2O, Ppl = -5 cm H2O
Airflow = (Pmouth – Palveolus) / R (R = airway resistance)
Laminar lung flow: Unidirectional flow with fastest flow in the middle
Turbulent lung flow: Forward net direction flow, but many local eddy currents
Causes of turbulent flow: Large radius, high flow velocity, high fluid density, and low fluid viscosity
Main sites of airway resistance are: Nose and large/medium bronchi(oles)
Disease in small airway is difficult to detect early because: minor contribution to airway resistance
If lung volume increase, resistance: Decreases
Bronchial smooth muscle relaxers: B2 adrenergic and nitric oxide
Bronchial smooth muscle contractors: Acetylcholine, histamine, and prostaglandin F2a
If resistance increases, work: increases
If compliance increases, work: decreases
Resistance work is increased by Obstructive diseases
Resistance work is minimized by: Slow breathing (difficult with dyspnea)
Compliance work is increased by Restrictive diseases (prevent volume increase)
Compliance work is minimized by: Shallow breathing
Work of breathing can be increased by: Increased airway resistance, reduced lung compliance, and reduced thorax compliance
Created by: emyang

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