Respiratory failure Flashcards
What is respiratory failure?
- PaO2 < 8kPa
- subdivided into 2 main types according to PaCO2 level
- occurs when gas exchange is inadequate, resulting in hypoxia
What are the different types of respiratory failure?
- type I hypoxaemic
- type II hypercapnic
- type III perioperative - generally a subset of type I failure but is sometimes considered separately bc it is so common
- type IV - secondary to cardiovascular instability
What is type 1 respiratory failure?
- hypoxia
- PaO2 < 8kPa
- normal or low PaCO2
- ABG may reveal respiratory alkalosis
What is type 2 respiratory failure?
- ventilatory failure
- hypoxia (PaO2 < 8kPa)
- hypercapnia (PaCO2 > 6.5 kPa)
- ABG may reveal a respiratory acidosis or partially corrected resp acidosis if longstanding (eg. COPD)
What is type 3 (peri-operative) respiratory failure?
Residual anaesthesia effects, post-operative pain and abnormal abdominal mechanics contribute to decreasing functional residual capacity (FRC) and progressive collapse of dependant lung units (alveoli)
Causes of post-operative atelectasis incude;
- reduced FRC
- supine / obese / ascites
- anaesthesia
- upper abdominal incision
- airway secretions
What is type 4 (IV) (shock) respiratory failure?
Patients who are intubated and ventilated and in the process of resucitation for shock:
- cardiogenic
- hypovolaemic
- septic
In the normal lung, what is the ventilation: perfusion (V/Q) ratio?
- 1
- however, slightly lower at base of lung (where more perfusion than ventilation), and higher towards apex of lung (ventilation > perfusion)
- pulmonary system can adapt to small V/Q mismatches, eg. if a section of lung not ventilated -> hypoxic -> pulm vasculature constricts diverting blood to areas that are ventilated
- however, this can only partially overcome poor ventilation
- in global hypoxia eg. persistent alveolar hypoventilation, all the pulm vasculature will contract -> leads to pulm hypertension and -> cor pulmonale
What is an area with no ventilation (and thus a V/Q of 0) termed?
shunt
What is an area with no perfusion (and thus a V/Q of infinity) termed?
dead space
It is difficult to increase oxygen content by increasing blood flow, due to its non-linear saturation curve and poor solubility. It is easy to get rid of carbon dioxide due to its linear solution curve (does not become saturated).
What are the pathophysiological mechanisms that can lead to hypoxaemia?

What is the difference between acute and chronic respiratory failure?
- Acute respiratory failure is characterised by life-threatening derangements in ABGs and acid-base status. It develops over minutes to hours and pH is therefore more likely to be affected
- Chronic respiratory failure develops over several days or longer. It allows more time for renal compensation resulting in a greater increase in bicarb concentration. pH may only be slightly reduced as a consequence w/ manifestations that are less severe and not be as readily apparent
Describe the normal process of mechanical ventilation
- lungs have inherent elastic properties that cause them to collapse away from chest wall
- this generates a negative intrapleural pressure -> increases the more the lungs are stretched
- alveolar pressure = recoil + intrapleural pressure
- there is only flow of air if a pressure difference is present
- in inspiration, contraction of diaphragm decreases the IP pressure, transmitted to the alveoli. Palv is less than atmospheric -> air moves into lungs
- in expiration, resp muscles relax (IP pressure less negative), but recoil remains positive so Palv becomes positive -> air forced out of lung
- during quiet inspiration, the main muscle used = diaphragm
- during more vigorous respiration, intercostal and accessory muscles are utilised
- quiet expiration is passive + relies on recoil
- during forced expiration, abdominal muscles are used to help push the diaphragm back to its resting position
Describe the physiological control of breathing
- peripheral chemoreceptors respond to reduced PaO2, increased PaCO2 and increased [H<span>+</span>]
- they are located in carotid and aortic bodies, although carotid is most important in influencing ventilation
- innervated by CN IX, stimulated by PaO2 <8kPa
- central chemoreceptors respond to changes in [H+] in the CSF, which closely reflects PaCO2
- does not respond to peripheral increased [H+] as these ions cannot cross BBB
- PaCO2 is the most important determinant of ventilation, 75% of increased ventilation response to hypercapnia is due to central chemoreceptor activity
- O2 and CO2 control can alter the responses to changes in each other, eg. hypoxia reduces sensitivity to CO2
- in COPD, sensitivity to [H+] in the CSF may be lost, in these pts the resp drive may be lost; O2 treatment may therefore reduce respiratory drive
What are the acute causes of type 1 respiratory failure?
- asthma
- PE
- pneumonia
- parenchymal disease
- pulmonary oedema
- ARDS
- diseases of right-left shunt
What are the chronic causes of type I respiratory failure?
- emphysema
- kyphoscoliosis
- pulmonary alveolar fibrosis
What are clinical features of type 1 respiratory failure?
- features of underlying cause of the resp failure
- tachypnoea
- tachycardia
- sweating
- pulsus paradoxus
- use of accessory muscles of respiration
- intercostal recession
- inability to speak
- central cyanosis
if longstanding -> polycythaemia, pulmonary HTN, cor pulmonale
What is the pathophysiology of type 1 respiratory failure?
There are 3 mechanisms by which hypoxia can occur in the presence of normal/low PaCO2:
- V/Q mismatch (most common) - where areas of low ventilation relative to perfusion (low V/Q) contribute to hypoxaemia.
- Shunt -> extreme V/Q mismatch
- Diffusion defect
Due to any disease that impairs gas transport across the alveolar-capillary interface. Levels of CO2 in the blood can remain normal or reduce as ventilation increases to compensate for lack of oxygen. The remaining normal lung is sufficient to excrete the carbon dioxide being produced by tissue metabolism as CO2 has such a high diffusion coefficient.
Type 2 respiratory failure is caused by alveolar hypoventilation. What are the causes of this?
- pulmonary disease - LT asthma, COPD, pneumonia, pulm fibrosis, OSA
- reduced resp drive - sedative drugs, CNS tumour, trauma, raised ICP
- nerve - guillian-barre syndrome, motor neurone disease
- muscle - muscular dystrophy
- neuromuscular - cervical cord lesion, diaphragmatic paralysis, myaesthenia gravis
- other - fatigue due to tachypnoea in met acidosis, obesity, flail chest
What are the clinical features of type 2 respiratory failure?
Clinical features include those of the underlying lung disease as well as the hypoxia symptoms mentioned for T1RF, and now hypercapnic features also:
- headache (cerebral artery vasodilatation + inc ICP)
- peripheral vasodilatation
- tachycardia
- bounding pulse
- asterixis
- papilloedema
- confusion
- droswiness
- coma
What is the pathophysiology of type 2 respiratory failure?
- the conditions mentioned cause alveolar hypoventilation
- where alveolar ventilation is insufficient to excrete volume of CO2 being produced by tissue metabolism
- inadequate alveolar ventilation is due to:
- reduced ventilatory effort
- inability to overcome an increased resistance to ventilation
- failure to compensate for an increase in deadspace and/or CO2 production
- or a combination of these factors
- this results in decreased levels of oxygen and increased levels of carbon dioxide in the blood
- respiratory acidosis ensues
What investigations can/should be done for respiratory failure?
- pulse oximetry
- bloods -> FBC, U+Es, CRP, ABG
- imaging -> CXR
- ECG
- pulm function tests
Assess the RF according to ABC and treat the underlying cause.
What is the management of type 1 respiratory failure?
- give oxygen (35-60%) by facemask to correct hypoxia
- safe to administer as much oxygen as is required to return PaO2 to normal
- assisted ventilation if PaO2 still < 8 kPa, despite 60% oxygen
What is the management of type 2 respiratory failure?
- treat underlying cause
- give controlled oxygen therapy -> start at 24%
- recheck ABG/PaCO2 after 20 mins, if PaCO2 is steady or lower then increase oxygen concentration to 28%
- if however, PaCO2 risen by >1.5 kPa + pt still hypoxic -> consider resp stimulant eg. doxapram or provide NIPPV
- if this fails, consider intubation and ventilation if appropriate
The brain may be relatively insensitive to CO2 and respiration if primarily driven by hypoxia; caution should be exercised when giving O2. If 100% oxygen given and pt relies on hypoxic drive for ventilation, patient will begin to hypoventilate as they are receiving the oxygen needed to satisfy the hypoxia. The hypoventilation exacerbates the hypercapnia.
Small increases in PaCO2 can be tolerated but not if pH falls dramatically. The pH should not be allowed to fall below 7.25; under such circumstances, increased ventilation must be achieved either by use of resp stimulant or artifical ventilation.
What are the pathophysiological consequences of hypercapnia (and subsequent pH decrease)?
-
brain:
- alterations in brain biochem (GABA, glutamate, glutamine aspartate)
- increased cerebral blood flow
- raised ICP
-
circulation
- depression of cardiac contractility + fibrillation threshold
- coronary and systemic artery vasodilatation