Pulmonary Circulation Lecture Notes (RCSI, Year 1)

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Pulmonary Circulation Class Course Year 1 Respiratory System Title Lecturer Pulmonary Circulation Prof. Christopher Torrens; presented by Dr Ebrahim Rajab Date 27.04.23 Learning Outcomes • Explain differences betwe...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Pulmonary Circulation Class Course Year 1 Respiratory System Title Lecturer Pulmonary Circulation Prof. Christopher Torrens; presented by Dr Ebrahim Rajab Date 27.04.23 Learning Outcomes • Explain differences between the systemic and pulmonary circulations • List pressures in the pulmonary circulation and define hypertension • Describe the local control of blood flow in the pulmonary circulation • Outline the mechanism of hypoxic pulmonary vasoconstriction • Describe the effects of gravity on alveolar perfusion • Describe Starling forces affecting filtration in pulmonary capillaries • Describe the causes of pulmonary oedema Outline of Lecture 1. The pulmonary circulation a. pulmonary pressures b. effect of gravity on flow 2. Pulmonary Resistance a. mechanisms regulating pulmonary resistance b. hypoxic pulmonary vasoconstriction 3. Pulmonary Oedema a. Starling forces in the lung b. mechanisms of pulmonary oedema Blood Supply to the Lungs • There are actually two blood supplies to the lungs – the bronchial and the pulmonary circulations • The bronchial circulation is part of the systemic circulation – arises from the aorta • The bronchial vascular bed supplies oxygen and nutrients to the smooth muscle and interstitial tissues of the lung • It drains into pulmonary veins and returns back to the left ventricle – so pulmonary venous return is slightly greater than cardiac output – it is also why gas exchange will never be perfect Pulmonary Circulation • The pulmonary circulation transports deoxygenated blood from right ventricle to the alveolar capillaries – and returns oxygenated blood to the left atrium • Pulmonary circulation receives the whole of cardiac output as right ventricular output must equal left – ~4-6 l/min at rest and can increase to ~25 l/min in exercise • Though it receives the same volume of blood, the pulmonary circulation is a low pressure system Quiziology • Thinking back to the haemodynamics sessions… • To achieve this low pressure what parameter of the pulmonary circulation must differ from that of the systemic circulation? 1. 2. 3. 4. 5. Decreased flow Decreased resistance Decreased volume Increased flow Increased resistance Pulmonary Circulation • The pulmonary circulation is a low pressure system • Right ventricular pressure • Pulmonary artery pressure • Pulmonary Pulse pressure 25/0 mmHg 25/10 mmHg 15 mmHg • Mean Pulmonary pressure 15 mmHg • Pulmonary capillary pressure ~6 mmHg – systolic - diastolic – diastolic + 1/3 pulse pressure • Left atrial pressure (LAP) (range 6-10 mmHg) ~ 2 mmHg (range 2-6 mmHg) Pulmonary Circulation 120/80 mmHg Mean 100 mmHg 25/8 mmHg Mean 15 mmHg 10 ARTERY ARTERY SYSTEMIC PULMONARY 25/0 RV CAP RA 2 6 120/0 LV 30 20 CAP LA 5 10 VEIN VEIN Measuring Pulmonary Pressure • Unlike systemic pressure, pulmonary pressure cannot be measure by plethysmography • Pulmonary capillary pressure is measured by capillary wedge pressure – is slightly higher than left atrial pressure – used to estimate left atrial pressure • Catheter put through right side into branch of pulmonary artery – it wedges at the pulmonary capillaries and stops flow Effect of Gravity and Volume • As lung expands during inspiration, the extra-alveolar vessels are pulled open – this distension means lung vascular resistance is low at high lung volumes – and conversely vascular resistance is higher at low lung volumes • This is offset by the effect on capillaries where the opposite is true • Capillaries are squashed as lung volume rises meaning that capillary resistance rises as lung volume does – and falls as lung volume does • So resistance varies across the respiratory cycle Effect of Gravity on Flow • Alveolar capillaries are thin so they are distended by blood pressure yet compressed by air pressure • When standing, blood pressure is reduced above the cardiac level – by approximately 0.74 mmHg/cm • Given the height of the lung and its position relative to the heart, there is a difference in pressure between the base and the apex – the lung is ~30 cm in height, so this is ~22 mmHg of difference – apex is -14 mmHg; base is +8 mmHg relative to cardiac level • This difference gives three regions of lung perfusion – zone 1 (apex), zone 2 (middle) and zone 3 (base) Effect of Gravity on Flow -14 mmHg Zone 1 Zone 2: 30 cm Zone 3: +8 mmHg Effect of Gravity on Flow • Zone 1 (apex): no flow – in certain circumstances • Normally there will be flow but if PA > Pa then flow will cease • This can occur if… – Pa drops (e.g. haemorrhage) – PA increases (e.g. positive pressure ventilation) Parterial (Pa) PAlveolar (PA) Pvenous (Pv) Zone 1: no flow (in certain circumstances) PA > Pa > Pv Effect of Gravity on Flow • Zone 2 (middle): intermittent flow • Systemically flow is due to the arterial venous difference – Q = ΔP/R where ΔP = Pa - Pv • Hydrostatic pressure increases due to gravity mean Pa is greater than PA but Pv is not Parterial (Pa) PAlveolar (PA) Pvenous (Pv) – so ΔP here is Pa-PA not Pa-Pv • In exercise, increased pulmonary arterial pressure means continuous flow in lung Zone 2: intermittent flow Pa > PA > Pv (Flow depends on aA gradient not av gradient) Effect of Gravity on Flow • Zone 3 (base): Continuous flow • At the base, hydrostatic pressure is greatest • Both arterial and venous pressure exceed alveolar pressure • Flow is continuous throughout the cycle Parterial (Pa) PAlveolar (PA) Pvenous (Pv) Zone 3: continuous flow Pa > Pv > PA Outline of Lecture 1. The pulmonary circulation a. pulmonary pressures b. effect of gravity on flow 2. Pulmonary Resistance a. mechanisms regulating pulmonary resistance b. hypoxic pulmonary vasoconstriction 3. Pulmonary Oedema a. Starling forces in the lung b. mechanisms of pulmonary oedema Pulmonary Resistance • Pulmonary circuit receives the same volume of blood • Since it is low pressure, it must also be low resistance – Q = ΔP ÷R so rearranging R = ΔP ÷Q • Systemic circulation – R = 100 – 2 ÷ 6 = 98 ÷ 6 = 16.3 mmHg/L/min • Pulmonary circulation – R = 15 - 5 ÷ 6 = 10 ÷ 6 = 1.7 mmHg/L/min (about 10x lower) • The pulmonary circulation can take larger cardiac output without increasing resistance or pressure Pulmonary Resistance • Pulmonary blood volume is approximately 500 ml • Volume can increase to some extent without a change in pressure or resistance – due to the capillary recruitment and distension Recruitment & Distension Resting pressure Increased pressure recruitment distension Pulmonary Resistance • Pulmonary blood volume is approximately 500 ml • Volume can increase to some extent without a change in pressure or resistance – due to the capillary recruitment and distension • In left heart failure pulmonary volume increases – pulmonary congestion • In conditions of SNS activation up to 50% of the pulmonary volume can move to systemic circulation – sympathetic constriction decrease capacitance – dehydration and shock lecture Control of Pulmonary Resistance • Relatively low neural/hormonal influence – there is SNS and PNS innervation, but role is questionable • Little or no myogenic or metabolic effect as in other beds – other than the recruitment and distension • Oxygen is hugely important factor and pulmonary circulation behaves differently to systemic arteries – in systemic arteries there is adenosine-related vasodilatation • In pulmonary circulation, hypoxia leads to constriction – hypoxic pulmonary vasoconstriction Hypoxic Pulmonary Constriction • Pulmonary arteries constrict to hypoxia – this is an intrinsic mechanism • This response controls capillary perfusion – shunting blood away from poorly ventilated areas – matching ventilation (V) and perfusion (Q) • An important factor in hypoxic diseases – COPD/high altitude cause hypoxic pulmonary vasoconstriction – ultimately leads pulmonary hypertension and oedema – may eventually lead to right heart failure Outline of Lecture 1. The pulmonary circulation a. pulmonary pressures b. effect of gravity on flow 2. Pulmonary Resistance a. mechanisms regulating pulmonary resistance b. hypoxic pulmonary vasoconstriction 3. Pulmonary Oedema a. Starling forces in the lung b. mechanisms of pulmonary oedema Pulmonary Oedema • As with the systemic circulation, this relates to the Starling forces • However, values in the lung are quite different – due to the low pressure nature of the circulation • Hydrostatic pressures – low pressure means capillary pressure is low: 6 mmHg – lymphatic pumping mean interstitial pressure lower: -8 mmHg • Colloid Osmotic pressures – leaky capillaries allow more colloid in interstitium: 15 mmHg – plasma colloid about the same as systemic circulation: 26 mmHg Capillary Dynamics - Normal • Net filtration pressure = (Pc − Pi) − (πc − πi) 27 Pi = -2 mmHg 19 ∏i = 7 mmHg interstitium Net Filtration = 8 mmHg Pc = 25 mmHg capillary = 8 mmHg ∏p = 26 mmHg Capillary Dynamics – Pulmonary Circulation • Net filtration pressure = (Pc − Pi) − (πc − πi) 14 Pi = -8 mmHg = 3 mmHg 11 ∏i = 15 mmHg interstitium Net Filtration = 3 mmHg Pc = 6 mmHg capillary ∏p = 26 mmHg Quiziology • Pulmonary oedema is common, secondary to left heart failure. By which mechanism does it occur? 1. 2. 3. 4. 5. Decreased capillary colloid osmotic pressure Decreased interstitial hydrostatic pressure Decreased interstitial lymphatic drainage Increased capillary hydrostatic pressure Increased interstitial colloid osmotic pressure Pulmonary Oedema • Fluid in the alveoli leaves by one of two mechanisms – before being carried away be lymphatics • Active pumping of Na+ creating an osmotic gradient or the negative interstitial pressure sucks it out • As with peripheral oedema, an imbalance in Starling forces or failure to clear fluid leads to oedema • Interstitial oedema increases the diffusion distance for O2 and decreases lung compliance. – if it reaches positive interstitial fluid pressure, fluid crosses alveolar membranes giving alveolar oedema – potentially fatal due to suffocation. Pulmonary Oedema • Major causes of pulmonary oedema are • Rises in pulmonary capillary pressure – left heart failure leads to failure of the circulation and pulmonary congestion increasing capillary pressure – high altitude causes hypoxic pulmonary vasoconstriction leading to HAPE – in chronic conditions lymphatics can expand to compensate • Increases in pulmonary capillary permeability – damage to the capillary in conditions like pneumonia – damage leads to leakiness and a decrease in the colloid osmotic pressure holding fluid in the capillary Summary • The pulmonary circulation is a low pressure, low resistance system • There is limited extrinsic control and resistance is controlled by local factors • Pulmonary arteries constrict to hypoxia – matches perfusion with ventilation – cause of morbidity in pulmonary disease • Changes in Starling forces leads to pulmonary oedema Additional Information • Textbook of Medical Physiology – Guyton & Hall • The Cardiovascular System – Noble, Johnson, Thomas & Bass • An Introduction to Cardiovascular Physiology – R Levick • Medical Sciences – Naish & Court Email: [email protected] Phone: 2269 Twitter: @chris_torrens Blog: https://tweetingstarling.wordpress.com