Intrathoracic Vascular Pressures

Question 1

Define compliance

Answer 1

a measure of the ease of expansion of the lungs and thorax, determined by pulmonary volume and elasticity

Question 2

What is the formula for compliance?

Answer 2

V = volume

P = Pressure

Question 3

Short hand terms for compliance problems

Answer 3

C_L = lung compliance

C_T= thoracic compliance

C_LT = Total Lung compliance

V_T = Tidal volume

P_AL= Alveolar pressure

P_PL = Pleural pressure

P_AMB = Ambient pressure

P_AW = Airway pressure

Question 4

Which compliance formula would you use if you where given Tidal volume, Alveolar pressure, and pleural pressure?

Answer 4

this would indicate you need to find C_L

C_L = V_T / (P_AL - P_PL)

Question 5

How would you find thoracic compliance if you were only given values for tidal volume and pleural pressure?

Answer 5

C_T = V_T / (P_PL - P_AMB)

NOTE: unless stated otherwise, P_AMB = 0

Question 6

How would you calculate Total Lung compliance is you where given only tidal volume and airway pressure

Answer 6

C_LT = V_T / (P_AW - P_AMB)

NOTE: unless stated otherwise, P_AMB = 0

Question 7

What are the normal values for C_L,C_T, & C_LT

Answer 7

C_L = 200 ml/cmH₂O

C_T = 200 ml/cmH₂O

C_LT = 100 ml/cmH₂O

Question 8

Which is the only compliance that can be measured by the Anesthesia Machine?

Answer 8

total compliance (C_LT )

Question 9

how do lung, thoracic, and total compliance mathematically relate to each other?

Answer 9

by ohm’s law (P = IV)

the chest wall and Lungs are in series , thus

Question 10

what’s the difference between dynamic compliance and static compliance

Answer 10

Static compliance is the compliance measured when there is no gas flow into or out of the lung

Question 11

when during a spontaneous respiratory cycle do you make hemodynamic measurements?

Answer 11

end expiration

when there is a balance of forces (lungs are ready to collapse and chest wall is ready to expand)

Question 12

what is transmural filling pressure (P_TM)?

Answer 12

referring to right atrial filling pressure, it’s the difference in pressure between two sides of a wall (for example, the pressure inside the atria and the pressure outside the atria - the pleural pressure.

P_TM = P inside - P oustide = P_direct - P_PL

Question 13

how many cmH₂O = 1 mmHg

Answer 13

1.36 cmH₂O = 1 mmHg

Question 14

how would you calculate the % of P_AW is transmitted into alveolar pressure

Answer 14

C_L

C_L + C_T

Example: C_L = 200 C_T = 400

200 / (200 + 400) = 0.33 (33%)

NOTE: this is also know as factor (f) in the formula to calculate P_PL

Question 15

formula for P_PL

Answer 15

P_PL = P_AW x f

f = factor

Question 16

how will increased transmission of intrathoracic vascular pressure (i.e. > 50 %) effect B/P on induction

Answer 16

can have significant decreases in B/P due induction agents plus the decrease in transmural filling pressures. the decrease in transmural filling pressures leads to decreased Cardiac output (ultimatel decrease in HR and B/P)

the decrease in transmural filling pressure is related to the increased transmission of intrathoracic vascular pressures

Question 18

what pathology causes increased lung compliance

Answer 18

emphysema

Question 19

what are some pathologies that decrease lung compliance?

Answer 19

pulmonary fibrosis

infiltrates

(diseases that cause the lung to be “stiff”)