Ventilation: Physiology Made Simple (although may still be a little boring)

Dr M Bloch

Anaesthetics Consultant

NHSG

Overview

• Why?• Basic anatomy, breathing basics & basic

physiology• Oxygen delivery

Why?

• To develop a foundation of knowledge of the physiology of the respiratory system that will enable systematic and considered approach to the care of the critically ill patient.

• To understand the impact of acute and chronic pathological processes on their presentation and treatment.

E X T R A T H O R A C IC IN T R A T H O R A C IC

A IR W A Y S(flo w )

P U L M O N A R Y *(A L V E O L I &

P A R E N C H Y M A )

C H E S T W A LL

C O M P L IA N C E(m e ch a n ics)

W O R K o f B R E A T H IN G(n o te c lin ica l s ig n s)

R E S P IR A T O R Y

C N SP N SC V S *

N O N -R E S P IR A T O R Y

A syste m a tic a p p ro a chto ae tio lo gy

R E S P IR A T O R Y F A IL U R Elo w O 2 & h ig h C O 2

le ad ing toh yp ox ia a n d a c ido s is

mbloch@nhs.net

D DANGER (scene, self, patient) 4 DIMENSIONS: up/done, front/back, left/right & time

R RESPONSE(Patient, Help)

RELATION(MOI, SAMPLE)

ReassureReassess

Hc

Ac AIRWAY with C-SPINE O2 ‘do airway think spine’Safe, maintainable / at risk (position / manoeuvres / adjuncts), unmaintainable (secure / definitive)Effort (signs of WOB esp. RR with TV, recession), Efficacy (depth / AE / O2 / CO2), Effect. (HR / LOC / colour / hyponia / FiO2), Exhaustion‘twelve FLAPS’ – open / expanding PTx, flail, HTx. NGTA&B on scene – good 1st principles

B BREATHING

C CIRCULATION Assess: clinical vs. measured (cough, movement, LOC,pulse, CFT, RR, temp. difference [creeping proximally], colour [pink, pale, mottled], urine vs. SaO2, BP, ETCO2, BD, lactate)‘On the floor and four more’.Manage: Controllable vs. uncontrollable haemorrhage: arrest & fluids.C en-route

D DISABILITY DRUGS:

analgesia, & sedation

appropriate antibiotics

DON’T

EVER

FORGET

GLUCOSE

AVPU (GCS + modifications), PEARL, focal, recurrent seizures, ‘protecting airway’ (prevent 20 injury)

E EVALUATEEMOBILISEEVACUATEEXPOSURE

10 20 survey vs. ‘time-critical’

Hypothermia

F ‘FEELING’ Pain, anxiety and fear

G GUIDANCE ‘ask for help’

H HARM e.g. NAI

I IMMUNE / INFECTION

mbloch@nhs.net

Oxygen demand and supply:

Supply of O2 to the tissues requires the integration & regulation of 3 systems: the lungs the blood the circulation

A B

C

Myocardial O2 Supply vs. demand

Decrease O2 delivery

Decreased coronary flowTachycardiaDiastolic hypotensionHypocapnia (vasoconstriction)Coronary artery spasmIncreased preload / afterload (wall tension)

Decreased O2 contentAnaemiaArterial hypoxaemiaShift of ODC to left

Increased O2 demand

Sympathetic stimulation

Tachycardia

Increased myocardial contractility

Increased preload / afterload (wall tension)

Some ANATOMICAL differences are more difficult to deal with than others…

Anatomy (contd.)

AnatomyTrachea

Right & Left main bronchus

Lobar, segmental and terminal bronchii

Respiratory bronchioles

Alveolar ducts

Alveolar sacs

Conducting airwaysDeadspace of ~150mlGas moves by inspiration

Respiratory airways. Just a few millimeters thickContain 2-3L at rest = most of the lung volumeGas movement is by diffusion, Occurs within a second

1

16

23

Structure and innervation

• Thoracic structure– 12 pairs of ribs.– Articulate with vertebrae posterior.– Drop inferior at anterior aspect at rest in expiration.– Articulate at the front with the sternum via costal cartilage.– Inferior aspect of thoracic cage bordered by diaphragm.

• Innervation – Diaphragm: phrenic nerve C3-5 used in quiet respiration.– Intercostal nerves on inferior aspect of rib supplies the

intercostal muscles which are used in forceful respiration.– Direct lung innervation limited because of paucity of fibres. – Sensitive to humoral influences.

Compliance: Volume and pressure relationship

respiratorycentres in medulla

chemoreceptorson aorta and carotid artery

heart

brain

intercostal nerve to externalintercostal muscles

phrenic nerve to

diaphragm

diaphragm

ribs

Control of Respiration

The breathing basics

Magill video

Lung volumes• VT (tidal volume) 500ml• Anatomic deadspace 150ml• Alveolar gas volume 3000ml• Ventilatory rate ~15• Total ventilation 7.5L/min• Alveolar ventilation = (500-150).15 = approx 5L/min

= Cardiac output.

E ffort

E fficacy

E ffect

• VA/Q mismatch is an inequality in either ventilation or perfusion.

• Ratio of ventilation to perfusion varies• Range from zero to infinity!• Ideal ratio is 0.8 (0.5-2.0)

Ventilation-perfusion mismatch and the alveolar-arterial pO2 difference.

V/Q mismatch

• Ventilation increases gradually as you move from apex to base

• Perfusion poor in apices better at bases• V/Q ratio decrease as you move down from apices• A) Well-ventilated poorly perfused (V>Q)• B) Underventilated well perfused (V<Q)• C) Optimally ventilated + perfused (V=Q)

Gas exchange

Ideal Alveolar Gas Equation.

P PP

RP F

R

RA I

A

A IO O

CO

CO O2 2

2

2 2

1

Clinically Useful Form:

Complete Form:

P PP

RA I

A

O O

CO

2 2

2

A-a Difference

• PAO2 - PaO2 A-a = FiO2 – PaCO2 – PaO2

• Normally up to +/- 25 mmHg– Because of normal anatomical shunt– Ventilation/Perfusion mismatching.

• A-a difference increases with pulmonary disease.

21

13

516

50

5

12

45

80

575

18

333 57

a/A ratio

• Normally averages just over 0.8• a/A ratio falls with pulmonary disease.• Lower limit normal:

– young (room air) : 0.74– older (room air) : 0.78– Both groups (100% O2): 0.82

Oxygen carriage

• Is either carried bound to Hb or dissolvedOxygen dissociation curve

• Henry’s Law for dissolved blood– Amount of oxygen dissolved in the blood is equal to

the partial pressure of oxygen– For each mmHg pO2, there is 0.003ml 02/100ml– Therefore at 100mgHg (alveolar pO2), there is 0.3ml

O2/100ml blood.

Oxygen flux = cardiac output x arterial oxygen content

Oxygen flux = C.O. (1,39 Hb SaO2) + (0,003 PaO2)

 

C.O. = cardiac output = stroke vol. X heart-rate where SV depends on preload, contractility and afterload

Hb = haemaglobin

SaO2 = saturation

PaO2 = arterial partial pressure O2

Oxygen dissociation curve

The perfect lung – oxygen cascade showing V/Q Inequality

Hypoventilation

Shunt

Diffusion

20

15

10

5

0

kPa

Hypoxic mixture