Respiratory Failure Flashcards
VA
the amount of air entering or exiting the alveoli and participating in gas exchange each minute
A-a gradient
the difference between PAO2 and PaO2
and hence the difference between hypoxia and hypoxaemia
V/Q
ventilation divided by perfusion
Hypoxic Pulmonary Vasoconstriction
constriction of pulmonary arteries in the presence of alveolar hypoxia to re-direct blood flow from areas that are poorly ventilated to areas that are well ventilated and more oxygen rich; no point to direct blood flow to areas of poor ventilation
ABG Measurement
a sample of blood is taken from an artery, usually the radial artery as that is the most accessible artery
the sample of blood is analysed in a few minutes in an ABG Analyser
acute
sudden deterioration
chronic
gradual and usually permanent change (eg:COPD)
acute on chronic
worsening of existing abnormalities eg: infective exacerbation of COPD
Generally lung failure results in
gas exchnage failure so hpoxaemia
Generally pump failures results in
ventilatory failure so hypercapnoea
Type 1 respiratory failure (hypoxaemic)
due to disease of the lungs, which prevents adequate oxygenation of the blood
Lungs still able to excrete CO2
This results in decrease O2 but normal or lower CO2 levels
State the five mechanisms of hypoxaemia.
Hypoventilation
Low inspired oxygen FIO2
Diffusion impairment
Shunt
Ventilation/perfusion (VQ) mismatch
Hypoventilation causes only Type 1 respiratory failure.
True or False?
False
Type 1 & Type 2
Hypoventilation is caused by
inadequate alveolar ventilation resulting in low alveolar pO2 and high pCO2
Hypoventilation occurs when
respiratory drive is impaired
When is respiratory drive impaired? (6)
- head injury
- drugs that suppress the respiratory center (morphine)
- respiratory muscle weakness
- COPD
- neuromuscular disease
- musculoskeletal disease
At higher altitudes the partial pressure of inspired oxygen increases or decreases?
Decreases
Low inspired oxygen causing type 1 RF slide
Low inspired oxygen FIO2 results in low alveolar PO2
How do we manage Type 1 Rf caused by hypoventilation or low inspired oxygen?
Supplemental oxygen
Can be given through nasal cannula, Venturi mask, re-breathe mask, CPAP, NIV, intubation and ventilation
Diffusion impairment causing Type 1 RF
disease or damage to the basement membrane causes a reduction in the amount of oxygen that diffuses across the interstitium and this results in hypoxaemia;
normal PAO2 but reduced PaO2
Diffusing capacity is also called
Transfer Factor (TLCO/DLCO) and can be measured by using a small amount of carbon monoxide
What gradient is used to determine if there is a problem with diffusion in the lungs?
Alveolar-arterial gradient
Alveolar - arterial gradient equation
4 factors affecting the diffusion of gases
- surface area of the basement membrane (emphysema)
- thickness of the basement membrane (pulmonary fibrosis)
- diffusion coefficient of the gas
- partial pressure and gradient of the gas
Management of type 1 respiratory failure due to diffusion impairment
- treat the underlying condition if possible
- supplemental oxygen
Type 1 respiratory failure caused by shunts
- pulmonary shunt
- anatomical shunt resulting in the mixing or venous and arterial blood
- cardiac shunt can be congenital (cyanotix heart diseaSE) and acquired
Pulmonary shunts
Passage of deoxygenated blood from the right side of the heart to the left side without participating in gas exchange in the pulmonary capillaries
Is a physiological shunt:
- consolidation eg pneumonia (poorly ventilated areas bypassed due to the principle of pulmonary vasoconstriction)
- arteriovenous malformation AVM
Normal V/Q
0.8
Ventilation
Volume of gas inhaled and exhaled over a given time period (flow of oxygen into the alveoli)
Ventilation equation
Alveolar ventilation rate x respiratory rate
Alveolar respiration rate =
Tidal volume - alveolar dead space
Perfusion definition and equation
- refers to the total volume of blood reaching the pulmonary capillaries in a given time period
perfusion = Cardiac output = heart rate x stroke volume
Hypoxia definition
inadequate level of O2 in tissues for cellular metabolism despite adequate perfusion
Hypoxaemia definition
arterial oxygen level below normal which can result in hypoxia
VE
the amount of air entering or exiting the lungs
Hypoxaemia caused by shunts: anatomical shunts: 2 examples:
- deoxygenated blood from the bronchial circulation goes into the pulmonary veins
- deoxygenated blood from coronary circulation goes into the thebesian vein and into the left ventricles
- only 2% of cardiac output
Is ventilation equal throughout the lungs?
no
Where is ventilation lowest in the lungs and why?
- at the lung apex
- because more negative intrapleural pressure and higher transpulmonary pressure causing more distending pressure on the alveoli
- alveoli have a lower compliance at this higher volume so less airflow during inspiration
- ventilation progressively increases moving lower down the lung
- ventilation is highest in the lung base
Gradient of ventilation diagram
Is perfusion equal in the lungs?
No
Where is perfusion lowest?
- the apex lowest
- middle sporadic
- base of lungs has constant perfusion
Where is perfusion lowest?
- the apex lowest
- middle sporadic
- base of lungs has constant perfusion
Where is perfusion lowest?
- the apex lowest
- middle sporadic
- base of lungs has constant perfusion
Where is perfusion lowest?
- the apex lowest
- middle sporadic
- base of lungs has constant perfusion
Gradient of perfusion in the lungs diagram
gradient of perfusion in the lungs diagram
Is ventilation greater than perfusion at the lung base?
no
at the base perfusion is greater than ventilation
Is ventilation greater than perfusion at the lung apex?
Yes
ventilation is greater than perfusion at the lung apex
Is ventilation greater than perfusion in the middle of the lung?
No
ventilation and perfusion are equal
Lung apices are relatively over ventilated/perfused?
ventilated
Lung bases are relatively over ventilated/perfused?
perfused
is there V/Q mismatch in healthy lungs?
Yes
In healthy lungs how is V/Q mismatch minimised?
By hypoxic vasoconstriction, which directs blood away from poorly ventilated areas
Type 2 respiratory failure is
hypoxaemia (pO2<8kPa) and hypercapnoea (pCO2 > 6.5 kPa)
Type 2 respiratory failure occurs due to failure of
ventilation, resulting in alveolar hypoventilation
Acute type 2 respiratory failure occurs within
minutes to hours, renal buffering does not have time to act, so bicarbonate level remains normal and pH drops (acidosis)
Chronic type 2 respiratory failure can develops over
several days to weeks to months
the kidneys excretes H2CO3 reabsorbs HCO3-, increasing HCO3- levels and slighlty lowering pH (compensation)
Respiratory acidosis
- CO2 + H2O
- H2CO3
- H+ + HCO3-
- HCO3- is an important buffer ensuring acid-base homeostasis
- type 2 respiratory failure results in hypercapnoae which leads to an increase in the amount of hydrogen ions in the blood leading to acidosis and pH drop
can be fatal
Management of type 1 respiratory failure: hypoxaemia
- treatment of the underlying condition
- correct hypoxaemia by giving oxygen and maintain O2 saturation between 94-98%
- intubation and ventilation
Management of Type 2 respiratory failure: hypoxaemia and hypercapnoae:
- treatment of underlying condition if possible
- administering controlled O2 aiming to keep saturation between 88-92%
- non-invasive intubation (NIV)
- intubation and ventilation
State the 4 types of hypoxia:
- ctotoxic or histotoxic hypoxia
- circulatory or stagnant hypoxia
- anaemia hypoxia
- hypoanaemic or hypoxic hypoxia
Cytotoxic or Histototoxic hypoxia
- reduced inability to utilise O2
- cyanide poisoning impairs mitochondrial cytochrome oxidase
Circulatory or Stagnant hypoxia
- reduced ability to deliver O2
- heart failure
Anaemic hypoxia
- reduced ability to deliver O2
- CO poisoning
Hypoanaemix or hypoxic hypoxia
- reduced ability to deliver O2
- PaO2 (reduced amount of oxygen in the blood)
Dead space
- anatomical dead space = upper respiratory tract upto 150 terminal bronchioles do not take part in gas exchange - 150ml
- alveolar dead space = alveoli that have lost blood supply do not participate in gas exchange
- physiological dead space = anatomical dead space + alveolar dead space
in healthy lungs physiological dead space =
anatomical dead space
Anatomical dead space function
humidifying air
