Cardiorespiratory mechanics Flashcards
Describe the mechanics of ventilation for one breath cycle
As alveolar pressure decreases A pressure gradient is created Causes air to flow in More gas particles in the lungs Diaphragm recoils and pressure returns to starting point
What is flow rate proportional to?
Flow rate is proportional to the pressure gradient
If airway radius is the same
Describe the protocol for performing pulmonary function tests
Patient wears noseclip
Patient wraps lips round mouthpiece
Patient completes at least one tidal breath
Patient inhales steadily to TLC
Patient exhales as hard and fast as possible
Exhalation continues until RV is reached
Patient immediately inhales to TLC
Visually inspect performance and volume time curve and repeat if necessary. Look out for:
- Inconsistencies with clinical picture
- Interrupted flow data
What are the key pieces of information that can be taken from flow-volume loops
Peak respiratory flow rate
Vital capacity
What would an abnormal flow-volume loop look like in a patient with COPD?
Lungs in COPD are fuller
Loops shift to the left
Air that is accessible is much less
Vital capacity decreases
Narrowing of the airways cause the peak to lower
Coving worsens- indented exhalation curve
What would an abnormal flow-volume loop look like in a patient with restrictive disease?
Displaced to the right
Narrower curve
What would a flow-volume loop look like in a patient with a variable intra-thoracic obstruction?
Blunted expiratory curve
Otherwise normal
What would a flow-volume loop look like in a patient with a variable extra-thoracic obstruction?
Blunted inspiratory curve
Otherwise normal
What would a flow-volume loop look like in a patient with a fixed airway obstruction?
Blunted inspiratory curve
Blunted expiratory curve
Otherwise normal
What is poiseuille’s law
𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒= 8ƞƖ/〖𝜋𝑟〗^4
What is resistance proportional to?
Inversely proportional to the fourth power of the radius
What is conductance?
Willingness of the airways to conduct fluid transpire (Air)
What happens to airways as lung volume increases?
They dilate
Conductivity increases
What is the role of smooth muscle in the walls of small arteries and arterioles?
to regulate their diameters and the resistance to blood flow
What is blood pressure equal to?
Cardiac output x Resistance
Why is the blood pressure relationship approximate?
steady flow (which does not occur due to the intermittent pumping of the heart)
rigid vessels
right atrial pressure is negligible
Physiologically, how is regulation of flow achieved?
Variation in resistance in the vessels
What is the importance of poiseuille’s equation?
emphasises the importance of arterial diameter as a determinant of resistance. Relatively small changes in vascular tone (vasoconstriction/vasodilation) can produce large changes in flow.
What does resistance of a tube to flow depend on?
Fluid viscosity (, eta)
- The length of the tube (L).
- Inner radius of the tube (r)
What would halving the radius to the flow?
Decrease it 16x
At rest what is cardiac output?
5L/min
During exercise what is cardiac output?
20L/min
What is laminar flow?
Velocity of the fluid is constant at any one point and flows in layers
Blood flows fastest closest to the centre of the lumen.
What is turbulent flow?
Blood flows erratically, forming eddys, and is prone to pooling
Associated with pathophysiological changes to the endothelial lining of the blood vessels..
What is pulse pressure equal to?
SBP-DBP
What is MAP equal to?
DBP + 1/3 pulse pressure
What is compliance?
The tendency to distort under pressure
What is elastance?
The tendency to recall to its original volume
Not the same as elasticity
What is the elasticity of a vessel related to?
Compliance
What happens one the aortic valve closes?
ventricular pressure falls rapidly but aortic pressure falls slowly
What can explain the events that occur after the aortic valve closes?
the elasticity of the aorta and large arteries which act to “buffer” the change in pulse pressure.
What occurs during ejection?
blood enters the aorta and other downstream elastic arteries faster than it leaves them (40% of SV is stored by the elastic arteries)
What causes ejection to cease?
Aortic valve closes
Due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in the downstream circulation
What can cause arterial compliance to decrease?
arteries become stiffer; e.g. with age
What happens when arterial compliance decreases?
the damping effect of the Windkessel effect is reduced and the pulse pressure increases.
How can the failure of valves present?
dilated superficial veins in the leg (varicose veins
How can the prolonged elevation of venous pressure present?
Prolonged elevation of venous pressure (even with intact compensatory mechanisms) causes oedema in feet
Define the law of Laplace
When the pressure within a cylinder is held constant the tension on its walls increases with increasing radius
How does an aneurysm occur?
Over time, vessel walls can weaken causing a balloon-like distension
What is a pathological example of the law of Laplace
Vascular aneurysms increase radius of the vessel. This means that for the same internal pressure, the inward force exerted by the muscular wall must also increase.
What happens if the muscle fibres of the aneurysm weaken?
the force needed cannot be produced and so the aneurysm will continue to expand …
often until it ruptures.
What is compliance?
The relationship between the transmural pressure and the vessel volume is called the compliance and depends on vessel elasticity
What is the relationship between venous and arterial compliance?
Venous compliance is 10 to 20 times greater than arterial compliance at low pressures.
Why is there less ventilation at the top of the lungs?
PPL is more negative (-8 cmH2O)
Greater transmural pressure gradient (0 vs. -8)
Alveoli larger and less compliant
Less ventilation
Why is there more ventilation at the bottom of the lungs?
PPL is less negative (-2 cmH2O)
Smaller transmural pressure gradient (0 vs. -2)
Alveoli smaller and more compliant
More ventilation
Why is there a lower flow rate at the top of the lungs?
Lower intravascular pressure (gravity effect)Less recruitment
Greater resistance
Lower flow rate
Why is there higher intravascular pressure at the bottom of the lung?
Higher intravascular pressure (gravity effect) More recruitment Less resistance Higher flow rate
What occurs to overcome variation in ventilation and perfusion?
Ventilation perfusion matching
