Ventilation- Perfusion Relatioships Flashcards
What is the typical partial pressures of O2 and CO2 at the beginning of pulmonary circulation? End-capillary?
O2- 40 mmHg, CO2- 45 mmHg; O2- 100 mmHg and CO2- 40 mmHg
What are the two extremes of ventilation-perfusion mismatch?
Alveolar dead space and intrapulmonary shunts
If there is more ventilation than perfusion what is the effect on arterial CO2 and O2?
PO2 will increase and PCO2 will decrease
What is the normal V/Q ratio?
1
If the V/Q ratio is decreased, what will happen to PO2?
It will decrease
What is represented by the alveolar plateau phase of the closing volume test?
The plateau represents high resistance pathways which get less of the pure inhaled oxygen and therefore have more nitrogen. The plateau is the slow gradual emptying of those slow alveoli
What is the relationship between high resistance small airways and nitrogen washout time?
The more highly resistant airways, the longer it will take to washout the nitrogen
How can pulmonary blood flow be tested?
Pulmonary angiograms, or lung scans using labeled macroaggregates of albumin
What vessels contribute to the normal anatomic shunt?
Part of bronchial venous drainage, thebesian veins (coronary a. into L. ventricle) and pleural veins all enter left-sided arterial blood
What is the physiologic shunt?
It corresponds to physiologic dead space and consists of anatomic shunts plus intrapulmonary shunts
What are the two classes of intrapulmonary shunts which describe the severity?
Absolute shunts and shunt-like states
What are absolute intrapulmonary shunts?
When mixed venous blood perfusing pulmonary capillaries associated with totally unventilated or collapsed alveoli
What is a intrapulmonary shunt-like state?
Alveolar-capillary units with low V/Q that have low PO2
What is the Shunt Equation and for what is it used?
Says the ratio of shunt flow (flow going by unventilated alveoli) to the total cardiac output is equal to (end-capillary oxygen content minus arteriole oxygen contents) (contents in ml O2/100 ml blood) divided by (end cap oxygen content minus mixed venous oxygen content); Used to quantify how big a physiologic shunt is
What is the problem with shunts?
Venous blood is going directly into arterial circulation, decreasing pO2 and causing hypoxemia if severe enough
True or false: True shunts can be compensated for using high FIO2?
False
What is the complete opposite of intrapulmonary shunts?
Alveolar dead space
Is arterial PO2 equal to alveolar PO2 normally?
No- arterial PO2 is always a little lower
Why is arterial PO2 typically a bit lower than the calculated ideal alveolar PO2?
1- the normal anatomic shunt; 2- there is normally some degree of V/Q mismatch in some parts of the lung; 3-some fluid collects in lower parts of the lung making oxygen diffusion more difficult
What is the typical normal value for the PO2/FIO2 ratio? What values indicate problems?
Nearly 500; Less than 2 or 3 hundred indicates serious trouble
Where does the most ventilation take place in the lung? Why?
Lower parts because intrapleural pressure is more negative at top and so there is greater transmural pressure differences and so the alveoli are bigger and less compliant
Where in the lung does the most perfusion take place? Why?
In the lower lung because there is greater pressure in the vessels at the bottom leading to more distension and recruitment
In the very lowest part of the lungs what is the V/Q ratio? Why?
Less than one because while there is more ventilation in the lower lung, there is way more perfusion in the lower lungs
Where in the lung is PO2 highest? PCO2 highest?
PO2 is highest in the uppermost segment of the lungs; PCO2 is highest in lowest segments of the lung
Where in the lung is the diffusion gradient for PO2 and PCO2 the greatest?
Upper parts of the lung
