Respiratory Failure Flashcards
Types of respiratory failure
Type I failure or hypoxemic respiratory failure (disorders of oxygenation) is characterized by a PaO2 of less than 8 kPa with a normal or low PaCO2
Type II respiratory failure (disorders of ventilation) is characterised by a PaO2 of less than 8kPa (hypoxaemia) with an increase in PaCO2 levels greater than 7kPa (hypercapnia).
Either may be acute or chronic
What can cause respiratory failure type 1?
- Hypoxic hypoxaemia (diffusion disorder) - most common and can be treated directly by physiotherapeutic techniques
- Ischaemic hypoxaemia (pulmonary disorder)
- Anaemic hypoxaemia (O2 carrying capability disorder)
- Toxic hypoxaemia (O2 unable to bind to Hb, i.e. CO poisoning)
What can cause respiratory failure type 2?
Any pathology causing hypoventilation. Problems with:
- Respiratory centre: Depression of Hypercapnic and hypoxic ventilatory drives , By drugs/GA/normal occurrence in sleep
- Medulla/spinal cord: trauma, neoplasm
- Innervation of respiratory muscles: phrenic nerve paralysis
- Respiratory muscles: Weakness or fatigue- MND/GBS/SCI/COPD/Chest wall disorders
- Upper airway obstruction: Foreign body/OSA
- Excessive WOB: Added load on the mechanics of breathing, Acute severe asthma/acute exacerbation of COPD
- loss of chest wall movement/poor ventilatory effort: acute condition (rib #) or chronic condition (scoliosis)
Consequences of respiratory failure are:
Respiratory muscle fatigues
Hypoventilation
Sputum retention
Hypoxaemia
If respiratory failure continues may lead to:
Cardiac arrhythmias Cerebral hypoxaemia Respiratory acidosis CO2 narcosis (hypoxic drive patients) Coma Cardiac arrest (worst case scenario)
In type 1 respiratory failure the processes compromised are:
transfer of oxygen across the alveolus, transport of oxygen to the tissues
In type 2 respiratory failure the processes compromised are:
- transfer of oxygen across the alveolus,
- transport of oxygen to the tissues
- removal of carbon dioxide from blood into the alveolus and then into the environment.
Hypoxic hypoxaemia is associated with:
Diffusion limitation
V/Q mismatch
Airflow limitation: asthma (acute bronchoconstriction)
Decreased FiO2
Primary respiratory disease: COPD, pulmonary fibrosis, CF, pneumonia, sputum retention, decreased gas transfer across thickened (fibrotic/ oedematous) membrane
Primary cardiac disease: heart failure, congestive cardiac failure, pulmonary oedema (causing a diffusion limitation across the respiratory membrane)
Ischaemic Hypoxaemia is associated with:
Inadequate blood flow through the lung:
Pulmonary embolus, low CO
Destruction of the lung capillaries (Emphysema/ lung trauma)
Anaemic Hypoxaemia is associated with:
Reduction in the O2 carrying capacity of the blood
Shock (significant blood loss with a reduced Hb)
Primary blood disease: sickle cell crisis, anaemia
Toxic Hypoxaemia is associated with:
Difficulty in the utilisation of oxygen: patients admitted with inhalation burns/ smoke inhalation injuries
carbon monoxide poisoning, cyanide
Consequences of hypoxaemia:
Tissue hypoxia due to O2 affinity of Hb being dependent on the amount of O2 available
Increased CO and redistribution of blood flow to vital organs at the detriment of other tissue
Causes of V/Q mismatch
- Wasted ventilation(perfusion limited): Perfusion defect prevents inspired Oxygen from reaching arterial blood. E.G PE
- Wasted Perfusion: Small airway closure to:
smoking COPD/Pulmonary oedema/increased age - ↓FRC, ↑ CV: ↓ volume to:
UAS/Cardiothoracic surgery/obesity/ascites/atelectasis/supine position/ILD - Cardiac shunt - deviation of blood flow
Causes of diffusion limitation
- Emphysema: ↓SA
- Low FiO2 -high altitude: Decreased diffusion gradient
- Scarring or fluid in interstitial space – pulmonary oedema/ILD: Increased thickness of alveolar/capillary membrane
Sputum or inflammation: Increased thickness of alveolar/capillary membrane
Transfer of oxygen across the alveolus is governed by:
Fick’s law of diffusion:
Diffusion is directly proportional to: Surface area of the lungs, Pressure gradient of gas
Inversely proportional to: Respiratory membrane thickness
CO2 diffusion is not as easily as affected as O2 diffusion due to:
Henry’s Law = The amount of gas in solution depends on the partial pressure of the gas and it’s solubility
Carbon dioxide in air has a low partial pressure but high solubility therefore still able to diffuse across thicker membranes or when there is less SA whereas Oxygen has lower solubility but a higher partial pressure
CO2 retention is mainly caused by:
Mechanical problems
Type 2 respiratory failure is often seen as an imbalance between:
strength and load of the respiratory system
Strength, i.e. muscle weakness, impaired neuromuscular transmission, diminished drive
Load, i.e. chest wall abnormalities, airway and lung abnormalities
State load factors that will cause respiratory failure in patients with emphysema:
Increase load on respiratory muscles:
- Increased airways resistance (airway collapse, sputum, bronchospasm)
- Static and dynamic hyperinflation (alters mechanics of ventilation)
- Intrinsic positive end expiratory pressure (peep) - intraalveolar pressure at end of exhalation
- Increased resistance to airflow
- Increased elastic load overtime as chest changes shape
State strength/capacity factors that will cause respiratory failure in patients with emphysema:
Decreased strength/capacity of respiratory muscles
- Hyperinflation (diaphragm is impaired)
- Respiratory muscle fatigue
- Poor muscle oxygenation
- Poor muscle nutrition
- General fatigue
- Over use of accessory muscles (too much oxygen used)
How do we treat Type 1 Respiratory Failure:
Hypoxic hypoxaemia (diffusion disorder) - increase lung volumes and/or sputum clearance techniques Ischaemic hypoxaemia, Anaemic hypoxaemia, Toxic hypoxaemia - optimise V/Q matching
How do we treat Type 2 Respiratory Failure:
Disorders of ventilation (depth or rate) - Acute condition (opiate overdose): support respiratory system until we can sort out of the cause, chronic condition (COPD - Emphysema): optimise by increasing strength of respiratory muscles or strategies to decrease load placed on respiratory muscles
Consequence of Chronic Respiratory Failure:
Chronic conditions, i.e. emphysema, whereby they are always in Type 2 respiratory failure. The persistent high levels of CO2 may have an effect on the respiratory processes causing:
- Hypoxic Drive - Instead of high levels of CO2 causing an increase in rate and depth of breathing, low levels of O2 drive the respiratory pattern.
- Therefore, if too much O2 is given the body thinks it has enough and the patient stops breathing.
- In some circumstances this can lead to cardiac arrest
- CO2 levels start to rise because respiration has stopped
