7 1 respiratory physiology - slcp · 2020-05-12 · sri lanka college of paediatricians 6...

Respiratory Physiologypart 1

EDUCATION & TRAINING SRI LANKA COLLEGE OF PAEDIATRICIANS

SRI LANKA COLLEGE OF PAEDIATRICIANS 1

Objectives

Differences between children and adults

Respiratory Physiology

Pathophysiology

Clinical signs of respiratory failure


respiratory drive in small infants

Nasal congestion can lead to significant resp

distress until 2-6 months

Airway small<9 yrsLarge tongue, small

oropharynx (infants & small children) Relatively little

cartilaginous supportChest wall

extremely compliant (neonates)

Ribs relatively horizontal (early

infancy)

Respiratory muscles may fatigue rapidly



Features of paediatric respiratory system:

Airway is small and therefore small absolute changes in radius (due to oedema or secretions) result a large relative change in cross sectional area and hence resistance. Clinically significant up to age of 8-9 years

Younger children have relatively little cartilaginous support of the airways. Dynamic compression during high expiratory flow may occur.

Neonates chest wall extremely compliant – substantial recession, easily overinflated with mech vent, little opposition to deflating tendency of lungs leading to low FRC and rapid desaturation during apnoea (eg during intubation)

In early infancy ribs are relatively horizontal with the result that the respiratory system is very dependent on diaphragmatic function. In presence of hyperinflation or abdominal distension the diaphragmatic function may be restricted with severe respiratory embarrassment

O2FiO2Alv MVV/Q DLO2

Alv MV

Alv MV = RR x (VT ‐ VDS)

CO2


Respiratory Physiology


Respiratory Physiology: The major function of the lung is to get oxygen in and carbon dioxide outFiO2, the major determinant of PAO2: partial pressure of oxygen in the alveoli, Alveolar ventilation e.g. can climb Everest without oxygen only by extreme hyperventilationV/Q matching

DLO2 = Diffusing capacity: how well gas moves from alveoli to blood

Carbon dioxide removal is largely dependent on alveolar ventilation which, in turn, is dependent on the respiratory rate times the difference between tidal volume and dead space. The latter will vary, depending on ventilation perfusion matchingIt is an active process, there is nothing that we can augment to speed up the removal of carbon dioxide. Also it is not easy to detect ventilatory failure unless you very astute in clinical exam or with the help of a blood gas

CO2 closely related to work of breathing and respiratory reserve

Respiration … the big picture

Ventilation & Gas Diffusion

Hb

Cardiac Output

Diffusion & Metabolic Demand


This slide puts respiration into context –outlining that the purpose is cellular oxygenation. Effective cellular oxygenation relies on the pulmonary and cardiovascular system and briefly outline considerations when evaluating respiration.

Oxygen consumption in infants is 6-8mL/kg/min vs 3-4ml/kg/min adults

Respiratory Drive

Considerations

•hypercarbia/hypocarbia•hypoxia•exhaustion/ sedation •reduced cerebral perfusion(low CO)•intact spinal cord


Anatomy & Physiology

INCREASED AIRWAY RESISTANCE

COMPLIANT CHEST WALL


Oxygenation & Carbon Dioxide

•GAS Exchange occurs at the alveoli•C02 diffuses readily but must be removed by adequate MV

•MV = RR x TV

•For 02 diffuses more slowly; critical that alveoli remain open

•Alveoli need a volume of gas to remain in them after expiration

•Alveolar Collapse • decreased oxygenation

• needs higher pressure to reopen (increased WOB_.Video of Alveoli https://www.bing.com/videos/search?q=alveolar+recruitment&&view=detail&mid=0A1A1BEDB98CFC3B24B90A1A1BEDB98CFC3B24B9&FORM=VRDGAR



The alveolus is the essential component required for gas exchange, diffusion of 02 into the bloodstream and removal of C02. Infants are born with only 10% of their alveoli and continue to develop alveoli until they are 8 years of age. Not only are the number of alveoli reduce but the structure is immature. Young children have reduced pores of kohn and canals of lambert, reducing the communication between alveolar units. It is critical that the alveoli stay open for gas exchange to occur. Alveoli do not open and close but must remain open to ensure that all the blood passing can be oxygenated. If the alveolus collapses reduced oxygenation; a closed / small alveolus requires high pressures to reopen. An open alveolus requires minimal pressure to stay open. Avoiding opening and closing prevents damaging the delicate and fragile alveolar membranes and can prevent ALI.

Looking at the picture from an electron microscope we can see how delicate the alveolar structure is. It requires a pressure (distending pressure) to maintain the opening volume and support the open structure. During ventilation we use CPAP or PEEP to prevent the alveolus collapsing during expiration.

Carbon Dioxide out

Largely dependent on alveolar ventilation

Alveolar Ventilation = Resp Rate x (Vtidal‐ VD)

Vtidal = tidal volume (6‐10ml/kg)

VD = dead space volume (3ml/kg)

Anatomical dead space constant

Physiological dead space variable Increases in sick patients

Equipment dead space



Carbon dioxide removal is largely dependent on alveolar ventilation which, in turn, is dependent on the respiratory rate times the difference between tidal volume and dead space. The latter will vary, depending on ventilation perfusion matching

It is an active process, there is nothing that we can augment to speed up the removal of carbon dioxide. Also it is not easy to detect ventilatory failure unless you very astute in clinical exam or with the help of a blood gas

CO2 closely related to work of breathing and respiratory reserve

V/Q MismatchVentilation /Perfusion

Alveolar collapse, consolidation or obstruction larger volume of poorly oxygenated blood returning from the lungs Sa02

Over distention of ventilated alveolar units can reduce blood flow not contributing to oxygenation.



For effective oxygenation at the lungs good matching of ventilation and perfusion is required.Ideally all areas of the lung would be like the example in the middle – good ventilation of alveolus and good perfusion so that deoxygenated blood from the RV travels past the alveolus and picks up oxygen returning to the RA with 100% Sa02.

Some airways are collapsed or blocked not ventilated, the blood that passed by these will not pick up oxygen then mixes with the oxygenated blood reducing the overall Sa02.

Some areas of the lung may good ventilation but no blood flow (eg airways), however in sick lungs where ventilation may not be evenly distributed, IPPV can result in the volume going to compliant alveoli overstretching and restricting blood flow. This can be a problem as it may shunt blood to poorly ventilated alveoli.

Lung Volumes


Dead space

Anatomical Dead Space

Equipment Dead Space


However not all the volume is involved in gas exchange. Some of the gas goes to alveoli that are not perfused. Because there is blood flow to these alveoli they do not take part in gas exchange. These alveoli take up part of what is known as dead space. The remainder of the dead space is made up by the pharynx, trachea and bronchial tree

Lung Volumes

Closing CapacityVolume at which alveoli in dependent areas of the lung

start to collapse/close.

Functional Residual Capacity Volume remaining after expiration.

IMPORTANT FOR OXYENATION Normally ~30mL/kg

FRC is smaller in proportion to lung volume in children.

Infants & young children FRC drops below the closing capacity when they are supine alveolar collapse

Respiratory disease FRC alveolar collapse

Tidal VolumeVolume of gas with each breath.

IMPORTANT FOR C02 REMOVAL

To keep alveoli open need FRC > CC



When thinking about breathing it is important to understand lung volumes. Tidal volume – ~ 7mL/kg . Tidal volume & minute ventilation are important for ventilation (C02 removal) . Discuss FRC and the relationship between FRC & CC and how CPAP/PEEP can support this. FRC is important for oxygenation.

FRC = ERV (expired residual vol ) + residual vol RVFRC decreased by about 30% in spine vs erect FRC falls during disease, loss of surfactant etc when lungs are more prone to collapsePaeds have poor chest wall compliance relatively lower FRCInfants & children FRC is about the same as CC risk of collapse and increase wobOther factors which drop FRC = anesthetic drugs, sedation, no spontaneous breathing rapid desaturation during intubation etc

C02 diffuses more readily than 02

How to Support our Baby?MANAGE VENTILATION (C02)

Goal = improve minute volume

MV = RR x TV

RR – 80bpm

TV – air entry, chest wall movement

Options:

*decrease resistance (CPAP)

*support inspiration (NIV or IPPV)

MANAGE OXYGENATION (02)Goal = improve FRC

FRC = volume in lungs at expiration

Crackles, Sp02 grunting

Options:

*keep alvoli open (CPAP, PEEP)

*re‐open alveoli (mean airway pressure)

*Fi02 (symptomatic only)



Remember that the aim of therapy is to restore oxygen delivery to tissues and that saturation notPO2 is a major determinant of oxygen delivery. Oxygen delivery is the product of the cardiacoutput, the oxygen content of blood and a correction factor. The oxygen content of blood is mainlydependent on the oxygen saturation and the haemoglobin saturation, with the a very smallcontribution from dissolved oxygen. As a result it is the saturation which is important indetermining oxygen delivery rather than the PaO2

Oxygen Delivery to the Cells

222

2 2

PaO0.0031.37Hbsaturation O content O

10contentOoutput CardiacdeliveryO

HB 140

COPoor perfusion

Cap RefillBP, HR



Remember that the aim of therapy is to restore oxygen delivery to tissues and that saturationnot PO2 is a major determinant of oxygen delivery. Oxygen delivery is the product of thecardiac output, the oxygen content of blood and a correction factor. The oxygen content ofblood is mainly dependent on the oxygen saturation and the haemoglobin saturation, with thea very small contribution from dissolved oxygen. As a result it is the saturation which isimportant in determining oxygen delivery rather than the PaO2

7 1 respiratory physiology - slcp · 2020-05-12 · sri lanka college of paediatricians 6...

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