Pulmonary vascular resistance

What is a normal value for pulmonary vascular resistance? Outline physiological factors that influence pulmonary vascular resistance. (00A3)(02A4)

Normal value of pulmonary vascular resistance

Pulmonary vascular resistance (PVR) is about 1/8 to 1/10 of the systemic vascular resistance

Mean pulmonary blood pressure = 15

Left atrium blood pressure = 5

Pulmonary blood flow = 5~6

PVR = Pressure difference / blood flow
= (15-5)/5 or (15-5)/6
= about 1.7~2.0 mmHgL^-1min
= about 100 dyne.sec.cm^-5

Alternatively,

PVR = (Mean pulmonary artery pressure - mean pulmonary capillary wedge pressure) x 80 / cardiac output
= (in dyne.sec.cm^-5)

All 3 of these variables can be measured with a Swan-Ganz catheter

Physiological factors influencing pulmonary vascular resistance

General factors

Factors that influence vascular resistance, both pulmonary and systemic:

• Blood viscosity
• (inversely proportional to 4th power of) vessel radius

Factors unique to lung

1. pulmonary blood flow
   - distension and recruitment
2. lung volume
   & its effect on alveolar and extra-alveolar vessels
3. hypoxic pulmonary vasoconstriction
4. others
   => include drugs, hormonal, pH, CO2

1. Pulmonary blood flow

As pulmonary blood flow increases, PVR drops because of:

• recruitment - some capillaries, which were closed or open but with no blood flow, begins to conduct blood
• distension - capillaries change from near flattened to more circular

Both mechanisms contribute, but:

• at low pulmonary arterial pressure, recruitment dominate
• at high pulmonary arterial pressure, distension dominate

2. Lung volume

At high lung volumes

Resistance is increased because:

stretching of alveolar walls

=> decreased caliber of alveolar capillary

=> increased resistance

At low lung volumes

Resistance is increased because:

1. reduction in radial traction by lung parenchyma
   => decreased caliber of extra-alveolar capillary
   => increased resistance
2. hypoxia-induced vasoconstriction in collapsed alveoli

Lowest PVR occurs at functional residual capacity.

3. Hypoxic pulmonary vasoconstriction (HPV)

• Occurs with decreased alveolar PO2 (PAO2)
• Locally mediated => smooth muscle contraction in arteriole
• Mechanism uncertain
  => ? inhibition of K channel
  => ? increased cytoplasmic [Ca2+]
  => contraction

NB: Hypoxia in all other tissues cause vasodilation not vasoconstriction.

NB: HPV reduces V/Q scatter, and responsible for pulmonary vascular redistribution to upper zones in cardiac failure

4. Others

Factors causing contraction of smooth muscles

(thus increasing PVR)

• (major effect) low PAO2 (i.e. HPV)
• acidosis (drop in pH)
• (weak effect) sympathetic stimulation
• serotonin
• histamine (H1)
• norepinephrine (alpha-1)
• arachidonic acid
• thromboxane A2, endothelin-1 (ET-1)

Factors causing relaxation of smooth muscles

(thus decreasing PVR)

• acetylcholine
  (via release of endothelium-derived relaxing factor, mostly nitric oxide NO)
• isoproterenol
• histamine (H2)
• prostacycline

 

Additional notes

As per examiner's comment: Pulmonary vascular impendence is a more appropriate term for pulmonary circulation due to the relatively great pulsatility.

 

Hypoxic pulmonary vasoconstriction

Net effect - diverting blood away from poorly ventilated area
=> reduce V/Q scatter

Critical at birth

• during fetal life, pulmonary vascular resistance is high
• with breathing, PAO2 increase, and pulmonary resistance decrease (due to reverse of HPV as well as lung expansion)
  => blood flow increase.

Examiner's comment

• Need to state specific range of PVR, not just a value.
• Need to use the right unit (mmHg/L/min) or dyne/sec/cm5
• Even if exact value is not known, need to state that it is 1/8~1/10 of systemic vascular resistance
• Resistance is directly proportional to blood viscosity (influenced by haematocrits), and inversely proportional to 4th power of the radius (due to laminar flow)
• Other factors uniquely to lung: lung volume (and its effect of extra and intra-alveolar vessels), distension and recruitment (secondary to pulmonary arterial pressure), hypoxic pulmonary vasoconstriction
• Lowest PVR being at FRC
• (extra) effects of pH, CO2, endogenous vasodilator/vasoconstrictor (e.g. nitric oxide), effect of hormone (serotonin,histamine), autonomic factors, effect of posture
• Pulmonary vascular impendence is a more appropriate term for pulmonary circulation due to the relatively great pulsatility
• No need to have details discussion of West's zones, laminar flow vs turbulent flow, benefits of hypoxic vasoconstriction