L8 Matching Lung Ventilation and Perfusion Flashcards
What is perfusion?
Blood flow through any organ e.g. lungs. Perfusion occurs through blood vessels. Measured as flow of blood/unit time
How to basal alveoli differ from apical alveoli?
Apical alveoli are 4 times larger than basal alveoli in the upright position due to gravity. As a result, basal alveoli can expand more than apical alveoli and so can ventilate better.
What are extra-alveolar vessels?
This is at the level before the airway. It supplies the lung parenchyma. Each branch then successively split into smaller branches closely following airway divisions. With each branch of the pulmonary artery from the hilum is the, a different part of the lung is supplied.
What is cardiac output?
Cardiac output = Heart rate * Stroke volume
Stroke volume: the volume of the heart pumping out blood from the right and left ventricle. This is usually 60 - 70ml.
What is the diameter of a red blood cell?
8 microns
What is the diameter of a capillary?
10 microns
What is the zone of dead space?
This is the zone where no gas exchange can occur. This is found at the apex of the lung. This is as the area is poorly perfused despite good ventilation. The area is poorly perfused as the blood vessels have collapsed. The pressure inside the alveolus is very high, compared to that of the material end of the blood vessels (which is higher than the venous end). As a result, air will not travel down a pressure gradient as the pressure in to the alveolus for exchange. Blood pressure is also decreased as the apex of the lung is higher than that of the pulmonary artery and so blood loses pressure as it goes against gravity. The alveoli pressure is high, blood pressure is low and capillaries are squashed in this region.
What is the recruitment zone?
This is the zone in the middle of the lung where gas exchange can occur, but it is inefficient. More capillaries are being recruited. The pressure in the alveolus is the same by the pressure in the arterial end of the capillaries is increased due to increased hydrostatic pressure. There is greater recruitment especially in systole. The pressure in the arterial end then surpasses that of the alveolar end. Gases will diffuse as pressure in the arterial end is greater than that of sth venous end.
What is the distension zone?
This is at the lung bases. Here gas exchange is most efficient. The alveoli are hardly open but the blood vessels are open. When you breath in, the alveoli open and the area of lung participates in gas exchange. There is continuous blood flow.
-arterial > P-venous > P-alveolar
What is the VQ ratio in. the:
(i) Zone of dead space
(ii) Recruitment zone
(iii) Distension zone
(i) 0
(ii) Low
(iii) High
What happens to the dead zone in a haemorrhage?
The dead zone increases. This is as if a patient bleeds and they drop their blood pressure, the upstream pressure will drop below the venous pressure. Part of the recruitment zone will therefore turn into dead space as the alveolar pressure exceeds that of the arterial pressure. It will get to the point where air is still there but there is no ventilation.
What happens to the dead zone in mechanical ventilation?
A patient is a sleep and you ventilate the person using a mechanical pressure. We create negative intrathoracic pressure to ventilate our lungs using our muscles when we breath normally. Without this we would not be able to breath as air flows down a gradient. When they cannot create a negative intrathoracic pressure, we force air into their lungs using a positive pressure ventilator. As a result, the pressure in the alveolus will be higher then normal. This means that some of the alveolus that were initially participating in gas exchange would no longer due to the change in pressure gradient, and so increasing the dead space.
What is alveolar mismatch?
The ventilation-perfusion (V/Q) ratio is commonly used to describe this mismatch.
• If the amount of ventilation (V) to an area of lung is equal to the amount of perfusion (Q) the ratio of V/Q would be 1 (ideal situation) - this is highly efficient gas exchange.
• V/Q ratio varies from 3.3 (high due to low perfusion) at the apex of the lungs to 0.6 (low as the area is highly perfused) in the base.
• Normal average V/Q ratio is 0.8
What is physiological dead space?
Physiological dead space =
Anatomic dead space + Alveolar dead space
Whereby alveolar dead space is the part of the lungs which is not perfused or poorly perfused. The anatomical dead space is the dead space apart fo sth conducting pathway where no gas exchange occurs. This is usually 2ml/kg or approximately 150ml in a 70 kg man. In a healthy individual, the anatomical dead space equals the physiological dead space.
How can a pulmonary embolism increase dead space?
Pulmonary embolism- increases the amount of dead space. This is as a blood clot from the peripheral veins such as in a DVT get lodged inside the blood vessel, blocking blood supply. Can lead to severe hypoxia.
It can lead to a Shunt - poor perfusion, but good ventilation. Therefore V/Q = 0 or very low.
