Respiratory Failure ✅ Flashcards
When is respiratory failure said to be present?
When there is a major abnormality of gas exchange
What are the determinants of oxygenation?
- Mean airway pressure
- FiO2
What are the determinants of CO2 elimination?
- Tidal volume
- Rate
What is the PaO2 in adults?
> 8kPa
What pO2 is usually required to maintain saturations over 90% in a newborn?
5.3-8kPa
What does the PaO2 required to maintain saturations over 90% depend on in neonates?
- Proportion of fetal haemoglobin
- Arterial pH
Is it better to define respiratory failure in terms of arterial oxygen tension or oxygen saturations?
Arterial oxygen tension
Why is it better to define respiratory failure in terms of arterial oxygen tension in neonates?
Because the left shift of the oxyhemoglobin dissociation curve due to 70% HbF is eliminated by a 0.2 drop in pH
??? don’t understand this
What can hypoxaemia in newborn infants result from?
- Ventilation perfusion mismatch
- Extrapulmonary (right-to-left) shunts
- Tissue hypoxia
How can ventilation-perfusion mismatch be identified?
A good response to supplemental oxygen
Why is there a good response to supplemental oxygen in ventilation-perfusion mismatch?
Due to intrapulmonary shunting
What is the problem in ventilation-perfusion mismatch?
There is an increase in physiologic dead space
What causes ventilation-perfusion mismatch?
Parenchymal lung disease
Give 4 parenchymal lung diseases that can cause ventilation-perfusion mismatch
- Respiratory distress syndrome
- Pneumonia
- Meconium aspiration syndrome
- Bronchopulmonary dysplasia
Is there improvement with supplemental oxygen in respiratory failure caused by extrapulmonary shunts?
Relatively little
Give 2 examples of causes of extrapulmonary shunting leading to respiratory failure
- Pulmonary hypertension
- Cyanotic congenital heart disease
When does tissue hypoxia occur, in terms of oxygenation?
When oxygen transport is reduced below a critical level, i.e. below the metabolic demand
What happens when oxygen demands of tissues are not met?
Metabolism must be maintained anaerobically, or the tissue metabolic rate must be reduced
How can tissue oxygenation be evaluated?
- Mixed venous saturation
- Blood lactate levels
- Fractional oxygen extraction
What is the use of mixed venous saturations when evaluating tissue oxygenation?
It identifies global tissue hypoxia
What is the limitation of mixed venous saturation as a measure of tissue oxygenation?
Local tissue hypoxia can exist with normal mixed venous saturation
What is the limitation of blood lactate levels in the evaluation of tissue oxygenation?
It may be elevated in the absence of tissue hypoxia, e.g. sepsis
When does fractional oxygen extraction increase?
When there is compromised oxygen transport to organs and tissues
How can fractional oxygen extraction be measured?
Using near-infrared spectroscopy (NIRS)
Can regional tissue oxygen saturation be measured using NIRS methods?
Yes
How does fetal haemoglobin compare to adult haemoglobin?
- Higher oxygen affinity
- Lower p50
What is p50?
Oxygen tension at which 50% of haemoglobin is saturated at standard pH and temperature
What is the advantage of HbF having a higher affinity and lower p50 than adult Hb?
It favours oxygen uptake from placenta to fetus, as adequate transport of oxygen is achieved at a relatively low pO2
What is the disadvantage of the higher affinity and lower p50 of HbF?
Worsens oxygen delivery to the fetal tissue
What offsets the disadvantage of HbF having a higher affinity and lower p50 on the delivery of oxygen to fetal tissues?
Dissociation of oxygen from haemoglobin can occur with a relatively small decrease in oxygen tension at the tissue level
When does the p50 reach adult levels in a term infant?
By 4-6 months of age
What is the most frequently used indice to describe oxygenation at tissue level?
Oxygenation index
How is oxygenation index calculated?
( Mean airway pressure (cmH2O) x FiO2 x 100 ) / PaO2 (mmHg)
What is the most frequently used method for monitoring oxygen therapy?
Pulse oximetry
What is the limitation of pulse oximetry in preterm infants?
It cannot detect hyperoxia
What other blood gas abnormality might hypoxia be associated with?
Hypercarbia
What is hypercarbia defined as?
PaCO2 >6.5 kPa or >55mmHg
How can PaCO2 be calculated?
CO2 production / alveolar ventilation
How is alveolar ventilation calculated?
Alveolar ventilation (L/min) = (tidal volume - dead space) x frequency
When will respiratory failure associated with hypercarbia occur?
In situations associated with reduction in tidal volume and/or frequency
When will ventilatory failure resulting in hypercarbia occur?
In conditions associated with;
- Reduced central drive
- Impaired ventilatory muscle function
- Increased respiratory muscle workload
- Intrinsic (inadvertent) positive end expiratory pressure (PEEP)
- Diffusion abnormalities affecting alveolar capillary interface
