PVR and SVR

Question 1

What is the poiseuille’s law

Answer 1

• Applies to steady laminar flow of homogenous fluid through rigid tube
  o Proportional to pressure difference and radius
  o Inversely proportional to length, viscosity
• Resistance to flow defined as:
  o Sensitive to change in radius

R = (change P)/Q
= 8nL/pir4

Question 2

What is vascular impedance

Answer 2

• Assess vessel caliber and elasticity → resistance and compliant characteristics
• Instantaneous ratio of pulsatile pressure/flow
• Rarely use in clinical situation

Question 3

Equations to calculate vascular resistance

Answer 3

o Driving pressures across pulmonary/systemic vascular beds (mmHg)
 PCWP can be used as surrogate for LAP
o Blood flow across pulmonary/systemic vascular beds (L/min)

Question 4

Units of vascular resistance

Answer 4

o Hybrid resistance units (HRUs): mmHg/L/min → Woods units
o Metric resistance units: dynes-sec-cm-5
 Conversion by x80
o Normalized resistance
 Use blood flow indexed with BSA instead of blood flow
 Not obtained by dividing resistance by BSA

Question 5

how is CO measured for vascular resistance calc

Answer 5

Fick method or thermodilution technique

Question 6

Qs equation

Answer 6

(O2 consumption (ml/min))/(SAO2 content-MVO2 content (ml/L))

Question 7

Qp equation

Answer 7

(O2 consumption (ml/min))/(PVO2 content-PAO2 content (ml/L))

Question 8

Control of SVR

Answer 8

Symp system + local metabolic factors
o Hypotension + ↓CO → baroR, α adrenergic system, vasoconstrictive hormones → ↑SVR

Question 9

Effect of exercise on SVR/PVR

Answer 9

↓SVR with unchanged PVR

Question 10

What diseases can cause ↓SVR can be seen with ↑Qs

Answer 10

arteriovenous fistulas, severe anemia, hyperT4
o Regional differences in vascular resistance

Question 11

Total pulmonary resistance

Answer 11

• Ratio of mean PA pressure/Qp
  o Express resistance to flow transporting blood from PA → LV in diastole
  o Neglecting LV diastolic pressure
• Used when PCWP/LAP cannot be measured

Question 12

What does PVR express

Answer 12

pressure drop across major pulmonary vessels: precapillary arterioles, pulmonary capillary bed
o More precise to assess degree of pulmonary vascular disease

Question 13

Diseases incr PVR

Answer 13

hypoxia, hypercapnia, ↑∑ tone, polycythemia, serotonin release, PTE, precapillary pulmonary edema, lung compression (pleural effusion)

Question 14

Diseases decr PVR

Answer 14

O2, adenosine, isoproterenol, α antagonist (phentolamine/tolazoline), inhaled NO, prostacyclin infusions, nitroprusside, Ca2+ channel blockers

Question 15

SVR vs arteriolar resistance

Answer 15

• Not synonymous of arteriolar resistance → drop in pressure is > at arteriolar level (60%) vs capillaries and small veins (15%) and proximal arterioles (10%)

Question 16

Changes in SVR/PVR can result from

Answer 16

o Changes in length of vascular bed → uncommon
o Altered blood viscosity
o Changes in CSA (radius) of vascular bed