RESP - Respiratory failure

Question 1

(3) Examples of obstruction/narrowing of conducting airways

Answer 1

• Bronchospasm
• Secretions/sputum

• Collapse
– Lack of elastic support eg emphysema
– Endoluminal (carcinoma, sputum)

Question 2

What does surfactant do? Can you give an example of a deficient state?

Answer 2

– improves lung compliance by reducing surface tension of fluid lining alveoli and preventing alveoli collapse

– Very premature babies do not have enough surfactant

Question 3

How does inspiration occur?

Answer 3

– Created by a negative pressure gradient

– Diaphragm & intercostals contract, greater space causes pressure drop - air moves in

Question 4

How does expiration occur?

Answer 4

– relaxation of muscles = less space air is forced out.

– Usually passive from recoil tendency of lungs (elastic tissue fibres & alveolar surface tension).

– Only active during periods of high activity.

Question 5

How is the Aa gradient calculated?

- usual values

Answer 5

A-a = (150 – (1.25 x PaCO2)) – PaO2

7-14 in young adults, higher in the elderly

Question 6

What does elevated Aa gradient mean?

Answer 6

If elevated it suggests a problem with diffusion or a V/Q mismatch (less commonly shunt)

I.e. concentration of oxygen in Alveolar (A)&raquo_space; concentration of oxygen in arteries (a).

Question 7

Define respiratory failure

Answer 7

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 8

Define type 1 & 2 respiratory failures

Answer 8

Type I: hypoxaemia

• Impaired gas exchange
• PaO2 less than 60mmHg

Type II: hypercapnia

• Hypoventilation (inadequate movement of gases in and out of the lungs)
• PaCO2 >50mmHg

Question 9

What (5) would cause hypoxaemia (hence type 1 respiratory failure)?

Answer 9

Due to impaired gas exchange

• Reduced inspired O2
– Altitude, fires
• Ventilation-perfusion mismatch – Pneumonia, pulmonary embolus
• Impaired diffusion – Pulmonary fibrosis, COPD
• Shunt
• Hypoventilation (as pCO2 goes up, pO2 must fall)

Question 10

What (2) would cause ventilation perfusion mismatch?

Answer 10

Pneumonia

PE

Question 11

What (2) would cause impaired diffusion in respiration?

Answer 11

Interstitial lung disease

COPD

Question 12

What (6) would cause hypercapnia (hence type 2 respiratory failure)?

Answer 12

Due to hypoventilation

• Central depression – Narcotic overdose, sedation
• Completely blocked upper airway
• Primary “pump” failure – Neuromuscular disease eg Guillain Barre Syndrome, MND
• Muscle fatigue – Usually as a consequence of › WOB
• Intrinsic lung disease eg severe COPD (most common)
• Chest wall abnormalities
– Obesity
– Kyphosis

Question 13

(5) symptoms of respiratory failure

Answer 13

• They may be non-specific
• Shortness of breath
• Feeling drowsy
• Feeling confused
• Headache

Question 14

(6) signs of respiratory failure

Answer 14

• Use of accessory muscles
• Increased respiratory rate, irregular breathing
• Low oxygen saturation (but can this tell us the CO2?)
• Drowsiness
• Confusion
• Signs of the cause eg heart failure

Question 15

How do you assess patients with possible respiratory failure and establish a diagnosis?

Answer 15

ABG

–Basic bloods
–CXR
–CT chest (CTPA, HRCT)
–VQ
–Lung function
–Sleep study

Question 16

How do you treat respiratory failure?

Answer 16

• Maintain adequate O2 delivery: (don’t give too much for chronic hypercapnic pts e.g. COPD)
• Reduce respiratory workload – Provide rest for the respiratory muscles
• Maximise ventilation
• Maintain stable pH/electrolytes
• Try and target the cause

– Eg. Asthma vs atelectasis

Question 17

• 21 year old male admitted with multiple fractures after MVA
• 18 hours after admission develops:
– Agitation
– Respiratory distress, respiratory rate 30
– Pulse 130, BP 130/80, afebrile
– A few bilateral basal crepitations

• ABG pH 7.48, PaO2 50, PaCO2 30, HCO3 26 (air)

Rapidly changed CXR 18h post admission

DDx?

Answer 17

ARDS (acute respiratory distress syndrome)

Increased pulmonary capillary permeability causing:
1. Gas exchange defect low V/Q units shunt

2. Mechanical defect › elastic work of breathing

Question 18

Mx of ARDS

Answer 18

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

Question 19

• 25 year old asthmatic in ED with acute attack
• Initially
– Generalized wheeze, respiratory rate 26, PEFR 30% pred
– ABG pH 7.50, PaO2 68, PaCO2 28 (air)
• 2 hours later after standard Rx
– Sleepy, respiratory rate 22, quiet chest
– ABG pH 7.32, PaO2 70, PaCO2 50 (40% O2)

What are the causes of the respiratory failure?

Answer 19

Abnormal gas exchange and increased O2 demand -> Increased ventilation but abnormal mechanics causing increased WOB -> Development of respiratory muscle fatigue and ventilatory failure -> quiet chest

Question 20

Mx of severe asthma attack + type 2 RF

Answer 20

– Assisted ventilation (invasive) for rest
– Oxygen
– Bronchodilators (reduce WOB)
– Corticosteroids

Question 21

• 53yo woman now day 3 post laparotomy for Ca bowel
• Some cough and fever, ongoing abdominal pain
• Crepitations in both lung bases (can you think what this might mean?)
– ABG on 40% 7.48/pCO 33/pO 66/23
• 12 hours later after analgesia, antibiotics
– More unwell
– ABG on 60% 7.28/ pCO2 46/pO2 50/ 16

What is going on and is it just a respiratory problem?

Answer 21

• Abnormal gas exchange
– Infection, alveolar oedema, atelectasis
• Abnormal mechanics (elastic WOB)
• Reduced resp effort due to pain
• Reduced resp drive 

Mechanism:
Abnormal gas exchange and increased O2 demand -> Increased ventilation initially but abnormal mechanics causing increased WOB -> Development of respiratory muscle fatigue, possible narcotic OD and ventilatory failure. Worsening gas exchange also

Type I -> II respiratory failure

Question 22

Rx of respiratory failure post op

Answer 22

–Adequate oxygen
–Ensure not narcotised, suitable pain control
–Reduce WOB: Diuretics, physio
–Assisted ventilation
–Sleep

Question 23

• 45yo man with known motor neurone disease p/w a number of weeks of morning headache, daytime sleepiness and poor memory.
• ABG on air 7.4/pCO2 50/pO2 77/33

What could be happening and how is this different to the previous cases?

Answer 23

• Normal pH so this is chronic
• Evidence of compensation
• Hypoventilation due to “pump failure”

Question 24

Mx of chronic hypoventilation & daytime sleepiness due to motor neurone disease

Answer 24

–Lung function
–Diagnostic sleep study
–Maximise ventilation
• Likely to need ventilatory support at night
• Reduce weight if required
–Avoid aspiration