Movement of air from environment to alveolus.
Exchange of gas b/w alveolus and arterial blood.
oxygenation (pulmonary function)
What are the steps involved with taking a breath? (5)
- 1. signal in resp center of brain
- 2. signal travels down cervical spinal nerves
- 3. connected with peripheral efferent nerves
- 4. synapse with neuromuscular junctions of diaphragm and intercostal muscles
- 5. muscles contract, thoracic cavity expands, creating negative intrathoracic pressure
What are factors affecting ventilation? (7)
- Tidal volume (VT), which is affected by dead space, lung compliance, airway resistance, neuromuscular function
- Respiratory rate (f, RR), which is controlled by the central ventilatory drive
The overall effectiveness of ventilation is determined by...
minute ventilation= RR x VT
How does blood gas analysis aid your assessment of ventilation?
- PCO2 value: PCO2 is primarily a function of ventilation
- increased= hypoventilation
- decreased= hyperventilation
Describe the difference b/w arterial and venous blood gas analysis for ventilation.
- arterial is gold standard, normal PCO2 is lower
- venous is easier, but normal PCO2 will be higher for a venous sample (has not returned CO2 to lungs for exhalation yet)
How is capnometry used to assess ventilation?
- measure end-tidal CO2 through a sensor attached to ET tube (infrared absorption technology)
- ETCO2 correlates to arterial CO2
- high ETCO2= hypoventilation
- low ETCO2= hyperventilation
What are the pros and cons of using capnometry to assess ventilation
- Pros: continuous assessment, primarily reflects ventilation
- Cons: MIGHT be affected with severely impaired perfusion
What are indications for capnometry? (3)
- monitoring anesthetized patients: low ETCO2- patient is too light or CVS collapse, high ETCO2- patient too deep or rebreathing of CO2
- patients requiring mechanical ventilation: monitor for over or under-ventilation
- monitoring effectiveness of compressions during CPR: ETCO2 >20mmHg associated with better success
What are 2 general causes of hypoventilation and 12 specific reasons?
- Neuromuscular: CNS dz, cervical spinal cord lesion, peripheral neuropathies (botulism), neuromuscular junction dz, myopathy
- Respiratory: airway obstruction, lower airway dz, parenchymal dz, pleural space dz, chest wall damage, abdominal distention
- Other: compensatory for metabolic alkalosis
What are causes of hyperventilation? (9)
- fever/ hyperthermia
- sepsis/ SIRS
- pain/ stress
- CNS dz
- compensatory for metabolic acidosis
Arterial oxygen pressure (PaO2) in arterioles that passed through/ by an alveolus should be about _____________.
Hemoglobin O2 saturation is determined by...
PaO2, which depends on amount of Hgb, O2 saturation of Hgb, temperature
What factors shift the Oxy-Hgb dissociation curve to the left? (4)
- decreased temp
- decreased CO2
- decreased 2,3-DPG
What factors shift the oxy-Hbg dissociation curve to the right? (4)
- increased temp
- increased CO2
- increased 2,3-DPG
Hypoxemia is defined as...
PaO2 < 80mmHg
What does a right shift of the oxy-Hgb dissociation curve indicate?
reduced affinity of Hgb for oxygen- releases it to the tissues easier
What does a left shift of the oxy-Hgb dissociation curve indicate?
increased affinity of O2 for Hgb
What are causes of hypoxemia? (5)
- V/Q mismatch
- diffusion impairment
- decreased inspired oxygen
Describe high versus low V/Q mismatch and causes of each.
- High: ventilation in excess of perfusion; caused by PTE, CVS collapse
- Low: perfusion in excess of ventilation; caused by pneumonia, pulmonary edema, contusions, PTE, pleural space disease
Hypoventilatory causes of hypoxemia are usually very responsive to ____________ due to ___________.
oxygen supplementation; increased diffusion gradient
What causes diffusion impairment, which leads to hypoxemia?
- scarring, thickening of alveolar wall, resulting in significant impediment to O2 diffusion
- chronic lung disease: infection, inflammation, congestion, idiopathic
What causes of hypoxemia are very responsive to supplemental O2? (3)
- diffusion impairment
- decreased inspired O2
What causes of hypoxemia are NOT responsive to O2 supplementation? (2)
- anatomical/ pulmonary shunts
- bad V/Q mismatch
Describe a shunt and causes of it.
- extreme form of V/Q mismatch with perfusion in complete absence of ventilation--> venous admixture with arterial blood
- anatomical: reverse PDA, tetralogy of fallot
- pulmonary: atelectasis, severe alveolar dz
How does pulse ox work?
- measures arterial oxygen Hgb saturation by measuring the absorption of 2 wavelengths of light through a pulsatile vascular bed
- normal SpO2= 97-98% (start to worry when <95%)
What are limitations of pulse oximetry? (4)
- late indicator of hypoxemia
- overestimates with hyperventilation (left shift of curve)
- measurement affected by motion, pigmentation, vasoconstriction/ hypothermia, tissue thickness, arrhythmias, abnormal Hgb
- does not tell you about anemia or icterus
The gold standard for assessing pulmonary function is...
arterial blood gas
What are the measured and calculated values on arterial blood gas?
- Measured: pH, PaO2, SaO2, PaCO2
- Calculated: A-a gradient, P/F ratio, rule of 120
PaO2 should be equal to...
- 5 x FiO2 (if on oxygen)
- 90-100mmHg (if breathing atmospheric air)
What is A-a gradient?
- quantifies ability of O2 to go from alveolus to arterial blood, taking ventilation into account
- hyperventilation will tend to increase PaO2
- hypoventilation will tend to decrease PaO2
- changes occur independently of pulmonary function
- **can only interpret when obtained at FiO2 of 21% (room air)
What is the equation for A-a gradient?
- A-a= (150- PaCO2/0.9) - PaO2
- Hypoxia d/t hypoventilation if A-a <10-20mmHg on room air
- Hypoxia d/t poor pulmonary function if A-a > 20mmHg
Describe the rule of 120.
- [simplified approach to A-a gradient]
- asserts that PaO2 + PaCO2 should be >120
- if sum is <120, pulmonary dysfunction exists
- if sum is >120, normal pulmonary function
- **again, only used if patient is breathing room air
What is the P/F ratio? What is the equation?
- it reflects the impact of FiO2 on PaO2
- P/F ratio= PaO2/ FiO2
- P/F= 400-500 is normal
- P/F= 300-400 mild pulmonary dysfunction
- P/F <300 acute respiratory distress syndrome
What are the limitations of using P/F ratio for assessing pulmonary function? (2)
- need to know exact FiO2
- does not take into account the effects of ventilation