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VASCULAR RESISTANCE
www.propofology.com
Dr. David Lyness
@Gas_Craic
Vascular resistance refers to the resistance that must be overcome to push blood through the circulatory system and create flow.
SVR: Systemic vascular resistance. Represents the load applied to the left ventricular muscle during ejection.
PVR: Pulmonary vascular resistance= index of the resistance offered by the pulmonary capillaries to the systolic effort of the RV
Hypoxic Pulmonary Vasoconstriction = HPV
Systemic vascular resistance (SVR): 80 x (MAP - RAP)/CO = 1000 - 1500 dyne s/cm5
Systemic vascular resistance index (SVRI): 80 x (MAP - RAP)/CI = 1970 - 2390 dyne s/cm5/m2
Pulmonary vascular resistance (PVR): 80 x (MPAP - PAWP)/CO = <250 dyne s/cm5
Pulmonary vascular resistance index (PVRI): 80 x (MPAP - PAWP)/CI = 255 - 285 dyne s/cm5/m2
Systemic vascular resistance index (SVRI): 80 x (MAP - RAP)/CI = 1970 - 2390 dyne s/cm5/m2
Pulmonary vascular resistance (PVR): 80 x (MPAP - PAWP)/CO = <250 dyne s/cm5
Pulmonary vascular resistance index (PVRI): 80 x (MPAP - PAWP)/CI = 255 - 285 dyne s/cm5/m2
• The lung is a low pressure system
• Low pulmonary pressure minimises the work of the right heart
• The pulmonary vessels distend and recruit to maintain low pressures
• Very high and very low lung volumes increase pulmonary vascular resistance
• Hypoxic pulmonary vasoconstriction diverts blood flow away from poorly
ventilated regions of lung
• HPV is obtunded by anaesthesia
• Ventilatory stategies can prevent increases in pulmonary pressures e.g low TV's, permissive hypercapnia
• TIVA does not affect HPV
LOW PRESSURE PULMONARY SYSTEM
HPV = with low PP of alveolar oxygen, the lung will divert its own blood supply to areas of properly ventilated lung tissue.
PVR = depends on the alveolar and extra-alveolar resistances.
Both low and high lung volumes will increase PVR, it is at its lowest at FRC.
Alveolar vessels or pulmonary capillaries will become overly stretched longitudinally at high lung volumes and therefore their resistance will increase.
With extra–alveolar vessels resistance will increase at low lung volumes since they are not encouraged to expand by lung parenchyma.
PVR can also be altered through the distension and recruitment of pulmonary vessels which have a large ability to increase their capacitance.
METABOLIC COMPOUNDS
Endothelin from the lung endothelial cells in response to tissue damage = potent vasoconstrictor
Prostacyclin = a naturally occurring vasodilator Nitric oxide/ AKA: endothelin derived relaxing factor = a potent vasodilator
Blood flow to and around the lung is similar to any other organ but at much lower pressures than the systemic system.
The blood vessels in the lungs continually branch and get consistently smaller very like the branching of the airways.
Pulmonary Arteries = very thin vs. the arteries of main circulation = supply lung up to the terminal bronchioles, splitting into the capillary bed.
Capillary bed = great capability to distend thus enhancing gas exchange and reservoir action.
Once RBC's become oxygenated, capillary bed is drained into venules which then join to form the pulmonary veins.
Blood vessels can distend and be recruited which allows the pressures in the pulmonary system to stay low despite very high blood flow.
The PA’s supply blood flow and oxygen ONLY to the lungs and must have the ability to accept huge blood volumes at times.
The low pulmonary pressures are important to minimise the work of the right heart.
The blood vessels in the lungs continually branch and get consistently smaller very like the branching of the airways.
Pulmonary Arteries = very thin vs. the arteries of main circulation = supply lung up to the terminal bronchioles, splitting into the capillary bed.
Capillary bed = great capability to distend thus enhancing gas exchange and reservoir action.
Once RBC's become oxygenated, capillary bed is drained into venules which then join to form the pulmonary veins.
Blood vessels can distend and be recruited which allows the pressures in the pulmonary system to stay low despite very high blood flow.
The PA’s supply blood flow and oxygen ONLY to the lungs and must have the ability to accept huge blood volumes at times.
The low pulmonary pressures are important to minimise the work of the right heart.
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