Respiratory mechanics
Elastic Forces and Lung Volumes
SOME IMPORTANT ASPECTS TO START WITH
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1. Any movement of the lungs is passive in nature and the result of changes to pressures within or outside the lungs
2. An isolated lung will collapse until there is no further air in it. Its properties give it a tendency to deflate
3. On opened thoracic cage on the other hand will expand due to its properties (recoil)
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In a subject with an open, unobstructed airway the respiratory system (Chest wall and lungs) will equilibrate itself at a specific level as a result of the opposing forces. The chest wall has a tendency to inflate while the lungs have a tendency to deflate. If no air is flowing, the respiratory system will remain in a balanced state. At the end of normal expiration this state correlates to the functional residual capacity (FRC).
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- The way the lungs respond to changes in pressure around it is governed by the impedance
- Lung impedance (ZLdyn) is the combined effect of the elastic and resistive loads on the lungs
Lung Impedance
Non-Elastic Resistance
Elastic Resistance
- Frictional resistance through gas flow
through the airway
- Frictional resistance from deformation of thoracic tissue
- Inertia associated with movement of
gas and tissue
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THIS OCCURS AND IS MEASURED WHILE GAS IS FLOWING ONLY!
THE ENERGY USED TO OVERCOME THIS PART OF RESISTANCE IS LOST AS HEAT!
- Elastic resistance of lung tissue and
chest wall
- Resistance from surface forces at the
alveolar gas-liquid interface
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THIS IS MEASURED WHILE GAS IS NOT FLOWING! THE ENERGY USED TO OVERCOME THIS PART OF RESISTANCE IS STORED AS POTENTIAL ENERGY WHICH THEN ACTS AS A SOURCE OF ENERGY DURING EXPIRATION!
THE CONCEPT OF TRANS-MURAL PRESSURE
Transmural pressure refers to the pressure inside relative to outside of a compartment. Under static conditions, the transmural pressure is equal to the elastic recoil pressure of the compartment. The transmural pressure of the lungs is also called trans-pulmonary pressure.
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Transpulmonary pressure can be increased by either:
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1) increasing the pressure inside relative to the pressure outside the lungs or
2) by decreasing the pressure outside relative to the pressure inside the lungs
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At FRC, the respiratory muscles are relaxed and the elastic recoil of the lungs is equal in magnitude but opposite in direction to the elastic recoil of the chest wall and intrapleural pressure is subatmospheric, at about - 5 cmH20.
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Within the respiratory system there a multiple trans-mural pressure that can be measured:
Trans-Alveolar pressure
As an individual fills its lungs during inspiration the transmural pressure (=trans-alveolar pressure = pressure difference between the alveolus and the pleural space) increases as lung volume increases. The slope of the curve is equivalent to the lung's compliance. Airways begin to collapse at the closing capacity. At residual volume there is already widespread collapse.
Trans-Alveolar pressure during spontaneours Breathing
Tidal Volume
Pleural Pressure
Air Flow
Alveolar Pressure
Inspiration
Expiration
At the start of inspiration, the diaphragm contracts and descends, expanding the thoracic volume. The descent of the diaphragm compresses the abdominal contents and decompresses the contents of the thoracic cavity.
With expansion of the thoracic cavity and its decompression, both intrapleural pressure and alveolar pressure decrease. Alveolar pressure decreases to a sub-atmospheric level and the pressure gradient for the flow of air into the lungs is established.
Air flows into the lungs and lung volume increases until the alveolar pressure rises to the atmospheric level [0 cm H2O] when the pressure gradient for flow of air into the lungs ceases to exist.
At the end of quiet inspiration, intrapleural pressure reaches about - 7-8 cmH20, and the transpulmonary pressure distending the lungs increases to 8 cm H2O (see above for formula)
During quiet expiration, the cycle is reversed, the inspiratory muscles relax and the inward elastic recoil of the lungs results in deflation of the lungs. During deflation, the lungs and chest wall move as one unit. Airflow out of the lungs ceases when alveolar pressure equals atmospheric pressure (0 cm H2O)
Busting the myth around ETT Tubes and resistance
Many of us believe that breathing through a small ETT must be hard for a child.
The opposite is true - let me explain!
Let's do the math:
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A 3 kg infant accepts a 3.0 mm ID ETT whereas an adult of 60 kg can tolerate a 9.0 mmID ETT -- a 20 times increase in body size but only a 3 times increase in ETT size.
The subglottic area of the infant is ~ 7 times greater in proportion to body size than that of an adult.
The inverse 4th power relationship of resistance to radius tells us that the infant ETT has a much higher “resting” resistance. This is irrelevant because of the shorter tube length and low flows generated by the infant compared to the adult. The net effect is that the infant is breathing through a hose rather than a straw when compared to the adult.
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The main determinants of ETT resistance are internal diameter and length. At physiologically relevant flow amounts the "small" size of paediatric ETTs do not contribute to any significant increase in resistance.
Peak and mid-inspiratory flows in humans are approximately 0.5 L/kg/min . When related to a 60 kg adult, this gives flows of about 30 L/min with a resistance of 10 cmH2O/L/sec. A 3 kg infant breathing through a 3.0 mmID ETT has inspiratory flows of about 1.5 L/min and a resistance of 15–20 cmH2O/L/sec, almost double that of the adult but clinically and physiologically irrelevant when considering the inspiratory resistance in the normal infant is already 80 – 90 cmH2O/L/sec.
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Jarreau and colleagues found that flow in smaller ETTs of 2.5 – 3.5 mmID was laminar, not turbulent. Flow limitation in ETTs was studied by Hammer and Newth in Rhesus monkeys. They showed that even in the smallest ETT studied (3.0 mmID), limitation of flow occurred only at about 400 mL/s. This is the equivalent of 24 L/min or 8 L/kg/ min in a 3 kg infant – well above the 1.5 L/min peak and mid-inspiratory flows normally achieved by infants of this size.
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Willis et al. looked at the pessure-rate product of 22 patients. They found no difference between CPAP and PS of 5 cmH2O. Both provided a decreased work of breathing than either T-piece or the extubated patient. Patients on T-piece had less work of breathing than when extubated. Takeuchi et al. showed that the work of breathing through an ETT for infants was only marginally higher than that after extubation. They also showed that 4 cmH2O PS was more than enough to offset the marginal increases in work of breathing through a 3.5–4.5 mmID ETT and was equivalent to breathing without the ETT .
--> The evidence shows the increased resistance in small ETTs is minimal and the additional work of breathing negligible
--> If an infant or young child cannot sustain a SBT on CPAP or a T-piece for several hours they are unlikely to fail extubation
--> Adding PS is likely to mask respiratory insufficiency and contribute to a higher failed extubation rate
USING OHM'S LAW ON THE ABOVE STATEMENT
We can calculate the delta pressure needed to generate the flows in a small and a larger ETT as a surrogate for the WOB a patient need to come up with in order to generate the flow given the resistance.
This shows us that the delta pressure (gradient) needed to generate the flow is~ 10 times greater in the adult! Small ETTs diameters need therefore to be interpreted in relation to body size and physiologic amounts of flow during ventilation!