Respiratory Failure

Question 1

List 4 conditions which may increase the drive to breathe

Answer 1

Anxiety
Exercise
Metabolic acidosis
Hypoxaemia

Question 2

List 4 conditions which may reduce the drive to breathe

Answer 2

Breath holding
Narcotic overdose
Sedatives

Question 3

What are the 3 main functions of the upper airway?

Answer 3

Humidifies, filters and warms the incoming air
First line of defence
Prevents aspiration

Question 4

What pathological processes may cause obstruction or narrowing of the airways?

Answer 4

Bronchospasm
Increased secretions/sputum
Collapse

Question 5

What pathological processes may result in collapse of the airways?

Answer 5

Lack of elastic support (e.g. in emphysema)

Endoluminal (carcinoma, sputum)

Question 6

What is the role of surfactant?

Answer 6

Improves lung compliance by reducing surface tension of fluid lining alveoli and thereby preventing alveoli collapse

Question 7

How does inspiration occur?

Answer 7

Diaphragm and intercostals contract, greater space produces negative pressure gradient and air moves in

Question 8

How does expiration occur? Is it a passive or active process?

Answer 8

Relaxation of muscles reduces intrathoracic volume which forces air out
Usually a passive process caused by recoil tendency of lungs (due to elastic tissue fibres and alveolar surface tension)

Question 9

When is expiration active?

Answer 9

During periods of high activity

Question 10

How does gas exchange occur?

Answer 10

Difference between partial pressures of O2 and CO2 in alveoli and pulmonary capillaries promotes diffusion across alveolar-capillary membrane
This is normally very efficient (occurs passively via concentration gradients)

Question 11

Normal ABG values (pH, PaCO2, PaO2, HCO3-, SaO2)

Answer 11

pH: 7.35-7.45 (2 SDs)
PaCO2: 40 (35-45) mmHge
PaO2: 100 (>85) mmHg
HCO3-: 24 (22-30)
SaO2: 95-100%

Question 12

What is the A-a gradient?

Answer 12

Difference between the arterial (a) and Alveolar (A) concentration of O2

Question 13

What does an elevated A-a gradient suggest?

Answer 13

A problem with diffusion or a V/Q mismatch (less commonly a shunt)

Question 14

What is the equation to calculate A-a gradient on room air at sea level?

Answer 14

A-a = (150 - (1.25 x PaCO2)) - PaO2

Question 15

What is the normal range for the A-a gradient on room air at sea level for young adults? What is it for the elderly?

Answer 15

7-14 in young adults

Higher in the elderly

Question 16

Define respiratory failure

Answer 16

Impairment of gas exchange between ambient air and circulating blood, occurring in intrapulmonary gas exchange or in the movement of gases in and out of the lungs

Question 17

Describe the pathophysiology of respiratory failure and distinguish between the 2 types

Answer 17

Impaired gas exchange usually results in hypoxaemia (PaO2 50 mmHg, type 2)
Type 1 will eventually progress to type 2 if no intervention, due to fatigue of muscles of respiration

Question 18

List 2 causes of reduced inspired O2

Answer 18

Altitudes

Fires

Question 19

List 2 causes of ventilation-perfusion mismatch

Answer 19

Pneumonia

PE

Question 20

List 2 causes of impaired diffusion

Answer 20

Pulmonary fibrosis

COPD

Question 21

List 5 causes of hypoxaemia

Answer 21

Reduced inspired O2
Ventilation-perfusion mismatch
Impaired diffusion
Shunt
Hypoventilation (as pCO2 increases, pO2 must fall)

Question 22

List 6 causes of hypercapnia

Answer 22

Central depression
Completely blocked upper airway
Primary "pump" failure
Muscle fatigue
Intrinsic lung disease (e.g. severe COPD; most common)
Chest wall abnormalities

Question 23

List 2 causes of central depression

Answer 23

Narcotics

Sedation

Question 24

List 2 causes of primary “pump” failure

Answer 24

NMD e.g. Guillain Barre syndrome, MND

Question 25

List 2 chest wall abnormalities which may contribute to hypercapnia

Answer 25

Obesity | Kyphosis

Question 26

List 4 symptoms of respiratory failure

Answer 26

Usually non-specific: SOB, drowsiness, confusion, headache

Question 27

List 4 additional signs of respiratory failure

Answer 27

Use of accessory muscles Increased RR or irregular breathing Low O2 saturation (does not provide an indication of PaCO2) Signs of the cause e.g. HF

Question 28

Set of Ix for patient presenting with suspected respiratory failure

Answer 28

Basic bloods: FBE, UEC, LFT, CRP, ABG Imaging: CXR, CT chest (CTPA, HRCT), VQ Others: lung function (e.g. spirometry, methacholine challenge), sleep study

Question 29

Outline the 5 basic management principles for any patient with respiratory failure

Answer 29

``` Maintenance of adequate O2 delivery (remembering that too much O2 can be detrimental to CO2 retainers, mostly patients with chronic lung disease) Reduce respiratory workload Maximise ventilation Maintain stable pH/electrolytes Try and target cause ```

Question 30

What general treatments are used for a patient presenting with respiratory failure?

Answer 30

Bed rest initially (may need to be seated) Adequate sleep Analgesia (avoid too much as you may cause sedation) Fluid, calories Humidification Physiotherapy to help clear secretions and to recondition Avoid aspiration (via correct positioning, etc) and fluid overload NIV

Question 31

21 year old male admitted with multiple fractures after MVA 18 hours post-admission develops agitation and respiratory distress O/E: RR 30, HR 130, BP 130/80, afebrile, a few bilateral basal crepitations Initial tests?

Answer 31

Most importantly: ABG: pH 7.48, PaO2 50, PaCO2 30, HCO3- 26 CXR: diffuse bilateral coalescent opacities

Question 32

21 year old male admitted with multiple fractures after MVA 18 hours post-admission develops agitation and respiratory distress O/E: RR 30, HR 130, BP 130/80, afebrile, a few bilateral basal crepitations What is the likely pathological process and how is it causing respiratory failure?

Answer 32

ARDS Increased pulmonary capillary permeability causing gas exchange defect (low V/Q units, shunt) and mechanical defect (increased elastic work of breathing)

Question 33

21 year old male admitted with multiple fractures after MVA 18 hours post-admission develops agitation and respiratory distress O/E: RR 30, HR 130, BP 130/80, afebrile, a few bilateral basal crepitations What is the management?

Answer 33

High flow humidified O2 Careful monitoring (clinical, SpO2, ABG) CPAP/BiPAP Invasive ventilation if unable to sustain adequate PaO2 or if type II ventilatory failure develops

Question 34

25 year old asthmatic in ED with acute attack Initially: generalised wheeze, RR 26, PEFR 30% pred, ABG pH 7.50, PaO2 68, PaCO2 28 2 hours later after standard Rx: sleepy, RR 22, quiet chest, ABG pH 7.32, PaO2 70, PaCO2 52 (40% O2) What is the pathophysiology of the respiratory failure?

Answer 34

Abnormal gas exchange (due to low V/Q units) and increased O2 demand leads to increased ventilation (however abnormal mechanics of breathing lead to increased resistive WOB) Results in development of respiratory muscle fatigue and ventilatory failures

Question 35

25 year old asthmatic in ED with acute attack Initially: generalised wheeze, RR 26, PEFR 30% pred, ABG pH 7.50, PaO2 68, PaCO2 28 2 hours later after standard Rx: sleepy, RR 22, quiet chest, ABG pH 7.32, PaO2 70, PaCO2 52 (40% O2) What is the management?

Answer 35

Assisted ventilation (invasive) for rest O2 Bronchodilators (to reduce WOB) Corticosteroids

Question 36

53 year old woman now day 3 post-laparotomy for Ca bowel Some cough and fever, ongoing abdo pain Crepitations in both lung bases ABG on 40%: 7.48/pCO2 33/pO2 66/23 12 hours later after analgesia and Abx, looks more unwell ABG on 60% now: 7.28/pCO2 46/pO2 50/16 What is going on?

Answer 36

Abnormal gas exchange (likely caused by infection, alveolar oedema or atelectasis) and increased O2 demand causes increased ventilation initially, but because of abnormal mechanics of breathing (and an already reduced respiratory effort due to pain, reduced drive due to possible narcotic OD) results in increased elastic WOB Development of respiratory muscle fatigue and possible narcotic OD results in respiratory failure (and worsening gas exchange)

Question 37

53 year old woman now day 3 post-laparotomy for Ca bowel Some cough and fever, ongoing abdo pain Crepitations in both lung bases ABG on 40%: 7.48/pCO2 33/pO2 66/23 12 hours later after analgesia and Abx, looks more unwell ABG on 60% now: 7.28/pCO2 46/pO2 50/16 What is the management?

Answer 37

Adequate O2 Ensure not narcotised, suitable pain control Reduce WOB with diuretics and physiotherapy Assisted ventilation Sleep

Question 38

45 year old man with known MND, presents with a number of weeks of morning headache, daytime sleepiness and poor memory ABG on RA 7.4/pCO2 50/pO2 77/33 What could be happening?

Answer 38

Normal pH with evidence of compensation because this is a chronic process Likely the result of hypoventilation due to "pump" failure

Question 39

45 year old man with known MND, presents with a number of weeks of morning headache, daytime sleepiness and poor memory ABG on RA 7.4/pCO2 50/pO2 77/33 What is the management?

Answer 39

``` Lung function Diagnostic sleep study Maximise ventilation (likely to need ventilatory support at night, may need to reduce weight) Avoid aspiration (appropriate positioning etc) ```