Respiratory Physiology & Impaired Gas Exchange Flashcards
What is dead space?
Where gas exchange is not being undertaken
What are the two types of dead space?
Anatomical: In conducting airways, e.g. trachea
Alveolar: Alveoli that aren’t being exposed to blood flow so don’t contribute to gas exchange
What makes up the majority of dead space?
Anatomical, only very small amount of alveolar dead space
What is reduced alveolar ventilation associated with?
- Rapid shallow breathing
- Increased physiological dead space
True or false: The distribution of air in the lungs is not equal
True
Why is the distribution of ventilation uneven within the lung?
- Varying pleural pressures from the bases to the apices (interaction with lung compliance)
- Patterns of respiratory activation
When you take a breath in, where does most of the air go?
The base of the lungs, i.e. it is not evenly distributed
Why is the intrapleural pressure less negative at the base than the apex?
Because of the weight of the lungs
In what area of the lungs do the alveoli expand the most?
At the apex, because of the differences in pleural pressure
What is the relationship between volume and lung compliance?
At higher volumes the lung becomes stiffer
How is blood flow distributed across the lungs?
More blood flow at the base, decreases as it gets higher due to gravity and pressures from the surrounding alveoli
What is the relationship between ventilation and perfusion and what does it mean?
- Ratio of air reaching the alveoli to blood reaching the alveoli via capillaries
- Greater at the apex
- Blood coming from different parts of the lungs have different levels of oxygenation
What 3 main respiratory problems can physio treat?
- Shortness of breath (dyspnoea)
- Difficulty clearing sputum
- Lung collapse (atelectasis)
What two factors determine oxygen delivery to the tissues?
- Content of oxygen in arterial blood (CaO2)
- Cardiac output (CO)
Delivered O2 = CO x CaO2
What are the two ways oxygen is delivered to tissues?
- Bound to haemoglobin (Hb) in red blood cells
- Dissolved in plasma
What is hypoxia?
Inadequate oxygen delivery to tissues
What are the signs of severe generalised hypoxia?
- Reduced level of consciousness (LOC)
- Seizures
- Coma
- Death
What are the signs of hypoxia?
- Cyanosis (blueish discolouration)
- Tachycardia & hypertension
- Shortness of breath & tachypnea
- Restlessness & confusion
- Sweating
- Pallor
What are the causes of hypoxia?
- Reduced CO or perfusion
- Dysoxia (disruption of cell enzymes)
- Haemoglobin deficiency
- Hypoxaemia (decreased partial pressure of oxygen in arterial blood, PaO2)
What are the normal haemoglobin ranges?
Females: 11.5-16 g/100mLs
Males: 13-18 g/100mLs
How is hypoxaemia defined?
Decreased partial pressure of oxygen in arterial blood PaO2 < 80mmHg
What is hypoxaemic respiratory failure?
PaO2 < 60mmHg
What is the normal PaO2 in arterial blood?
80-100mmHg
What is the effect of ageing on PaO2?
PaO2 decreases as a normal function of age
How does PaO2 and PaCO2 change at gas exchange?
Venous PaO2: 40mmHg
Arterial PaO2: 100mmHg
Venous PaCO2: 46mmHg
Arterial PaCO2: 40mmHg
During inspiration, why does PaO2 decrease between the atmosphere and the alveoli?
Due to moisture in the airways
What is the normal arterial percent O2 saturation (SaO2)?
95-100%
What happens at PaO2 of 60mmHg?
SaO2 is approx 90% Oxygen content (CaO2) falls rapidly below SaO2 of 90%
What happens with decreased affinity of O2 for Hb?
- Right shifted curve
- Favours O2 unloading
- Acute acidosis
- High CO2
- Increased temp
- Abnormal Hb
What happens with increased affinity of O2 for Hb?
- Left shifted curve
- Favours O2 loading
- Acute alkalosis
- Decreased PCO2
- Decreased temp
What is FiO2?
Fraction of inspired oxygen, normal level is 0.21 (21% or room air)
How is oxygenation assessed in clinical practice?
- Clinical examination
- Arterial blood gases
- Pulse oximetry
What are pulse oximeters?
Non-invasive measure of oxyhaemoglobin saturation (SpO2) and heart rate
Normal values same as SaO2
What can give a pulse oximeter a false high?
- HbCO
- Nail polish
- Dark skin pigmentation
What are arterial blood gases (ABGs)?
Gold standard measure of the integrity of the respiratory system
What do ABGs measure?
- pH
- PaCO2
- PaO2
- HCO3 (bicarbonate)
- BE (base excess)
- SaO2 (O2 haemoglobin saturation)
What are the normal ABG values?
- pH 7.35-7.45
- PaCO2 35-45mmHg
- PaO2 80-100mmHg
- HCO3 22-28mmol/L
- BE -3 to +3
- SaO2 95-100%
What does enzymatic activity rely on?
Closely regulated hydrogen ion activity
What is the role of the kidneys in acid elimination?
- Excrete the non carbonic acid that is produced by metabolism
- Reabsorb bicarbonate to maintain acid-base balance
- Regulate pH by changing HCO3 levels
What is respiratory acidosis and when can it occur?
Hypoventilation (shallow breaths)
- Overdose of sedative
- Depression of respiratory drive during sleep
- Respiratory fatigue
What is respiratory alkalosis and when can it occur?
Hyperventilation
- Emotional stress/anxiety
- Fever
- Pain
- Hypoxia at altitude
- Asthma
What is metabolic acidosis and when can it occur?
Accumulation of fixed acids
- Renal failure
- Ingestion of toxins
- Uncontrolled diabetes
- Severe diarrhea
What is metabolic alkalosis and when can it occur?
Accumulation of bicarbonate
- Vomiting
- Nasogastric drainage
- Diuretics
What are the physiological causes of hypoxaemia?
- Ventilation/perfusion mismatch
- Hypoventilation
- Diffusion impairment
What is an anatomical shunt?
When blood passes into the arterial system without going through the ventilated areas of the lung (V/Q = 0)
What is V/Q mismatch the most common cause of?
Hypoxaemia
What conditions cause V/Q mismatch?
- COPD
- Pneumonia
- Asthma
- Ageing
- Pulmonary oedema
What are examples of when a shunt occurs?
- Atelectasis (collapse)
- Pulmonary oedema
- Pneumonia
When does hypercapnic respiratory failure occur?
PaCO2 > 50mmHg
How is CO2 transported?
- Dissolved in physical solution (8%)
- Chemically combined with protein (12%)
- Ionised as bicarbonate (80%)
What is hypercapnia?
Elevated CO2
What are the physiological causes of hypercapnia?
- Decreased minute ventilation (MV)
- Ventilation/perfusion mismatch
- Increase CO2 production in the setting of fixed ventilation
What are the clinical signs of hypercapnia?
- Altered mental state (lethargy, drowsiness)
- Headache
- Tachycardia
- Hypertension
- Sweating
- Vasodilation
- Dyspnea
- Muscle tremor
Where does equilibration not occur?
Between the PO2 in the alveolar gas and the pulmonary capillary blood
What are examples of diffusion impairment?
- Diffuse interstitial fibrosis
- Asbestosis
- Sarcoidosis
How is impaired gas exchange managed?
- Treat underlying cause
- Manage complications
- Correct acid/base balance
- Provide O2
What are the indications for home O2?
- Nocturnal O2 (SpO2 <88%)
- Terminally ill
- Role in exercises as supplement
- Arterial PaO2 < 55mmHg on room air, rest & awake
- Daytime PzO2 55-59mmHg, hypoxic organ damage/heart failure
What are the adverse effects of O2?
- O2 toxicity (high levels of O2 for long periods)
- Depression hypoxic drive
- Absorption atelectasis
- Impaired mucus clearance (drying of mucus)
- Retrolental fibroplasia (blindness)
- Fire
