Blood Gases and function tests and ventilation Flashcards
What is the pCO2 equation?
PCO2 = 0.863 x (CO2 production/alveolar ventilation)
What is important to remember about assessing alveolar ventilation?
Cannot assess it just by looking at a patient
What are the physiologic processes that can cause hypoventilation?
Reduced total ventilation
Normal total but increased dead space ventilation
What can cause alveolar hyperventilation?
Increased central respiratory drive - from pain or anxiety, compensation for metabolic acidosis
What is a compensated respiratory acidosis?
Near normal ph with high bicarbonate and pCO2
What determines if the lungs are doing a good job getting oxygen from atmosphere to blood?
The A-a gradient
Difference between oxygen in alveoli and oxygen in arterial blood
What is an elevated A-a gradient?
Anything over 15 mm Hg
Increases with age
How can high A-a gradient due to diffusion block be recognized?
Only reduces arterial blood ox during things like exercise, not at rest
Happens in fibrotic lung diseases
How can high A-a gradient due to v:q mismatch be recognized?
Reduced ventilation but normal perfusion
Most common cause
How can high A-a gradient due to a shunt be recognized?
pO2 not increased even if patient breaths 100% oxygen
R to L shunt suspected if paO2 <95% while patient breathing 100% oxygen
What is suspected if paO2 is not affected but SaO2 is decreased?
Carbon monoxide poisoning
When can SpO2 differ from SaO2?
When SaO2 <85% - ignore SpO2
Can be normal in CO poisoning
No info about alveolar ventilation or acid base balance
Inaccurate in patient with poor perfusion of extremities (shock)
How can cyanide poisoning be recognized?
Adequately oxygenated blood is reaching the tissue but tissue cannot utilize it
Consequences of tissue hypoxia but all stats are normal
What are the five causes of tissue hypoxia?
Reduced pAO2
Reduced paO2 with normal pAO2
Reduced CaO2 with normal paO2 (anemia, CO poisoning)
Reduced tissue perfusion (hypotension, arterial occlusion)
Reduced tissue oxygen utilization (cyanide poisoning)
How is the A-a gradient calculated?
A-a = (150 - 1.25(paCO2)) - pAO2
How can bicarbonate be determined to assess renal compensation ?
Henderson hasselbach equation
PH = 6.1 + log(HCO3-/0.03paCO2)
What are the five common indications for PFTs?
Assessment of patient with pulm complaints
Determine pattern of respiratory impairment
Serial evaluations
Pre-OP assessment
Occupational or environmental reasons
What are four categories of information obtained from PFTs?
Lung volumes and capacities
Flow rates
Diffusing capacity
Maximal inspiratory pressure and maximum expiratory pressure
What is total lung capacity?
Total volume of gas within lungs after maximal inspiration
What is residual volume?
Volume of gas remaining in lungs after maximal expiration
What is vital capacity?
Volume of gas expired when inspiring all the way to TLC and exhaling down to RV
Largest volume of air that can be moved into or out of the lungs
Made up of IRV, TV, ERV
What is functional residual capacity?
Volume of gas in lungs at resting state (during tidal breathing)
How does spirometry work?
Deep inspiratory breath to TLC then forced exhalation to RV
Measure how fast and how long
What values can you obtain from spirometry and which can’t you?
Can get FEV1 and FVC
Cannot get RV, TLC, or FRC
Does provide inspiratory capacity
What is normal FEV1/FVC?
70-80%
What are possible methods of measuring lung volumes?
Body plethysmography (patient sits in box and performs maneuvers, uses boyles law) Dilutional lung volumes using helium dilution or nitrogen washout - severe COPD may cause falsely low value, use other method These measure FRC
How can different lung volumes be calculated if FRC is obtained?
FRC - ERV = RV
IC + FRC = TLC
VC + RV = TLC
TLC is important for diagnosing restrictive lung diseases
What are the gold guidelines for defining airflow OBSTRUCTION?
FEV1/FVC less than 70%
Other than a fixed ratio, what value can be used to determine if obstruction is present?
Lower limits of normal
Obstructive if ratio below the 5th percentile of predicted
What is FEF 25-75%?
Expired airflow rate between the 25 and 75% points on a forced expiratory Spirogram
Average flow rate over middle half of expiration
Indicates status of medium to small airways
Can detect early stages of obstruction when ratio still normal
What is a flow volume loop?
Forced expiration and inspiration curves create closed loop Y axis is flow X axis is volume Above x axis is expiratory flow Below x axis is inspiratory flow
What is the pattern of restrictive lung diseases on PFTs?
FEV1 and FVC both decreased Ratio is normal or increased TLC is decreased Convex pattern on flow volume loop Low DLCO suggests fibrosis
What is the differential diagnosis of restrictive lung disease?
PAINT pleural diseases Alveolar diseases Interstitial lung diseases Neuromuscular diseases Thoracolumbar
What is the DLCO?
Diffusing capacity of lung for carbon monoxide
Rate at which CO is absorbed from alveolar gas by pulmonary capillaries
What are the disease processes that lead to a reduced diffusing capacity?
Respiratory: Emphysema, Interstitial lung disease, Pulmonary vascular disease (isolated reduced, other stats normal)
Anemia (reduces) and polycythemia (increases) affect hemoglobin availability
What do MIP and MEP help measure?
Strength of respiratory muscles
What is the obstructive pattern on PFTs?
Reduced ratio and reduced FEF 25-75
Scooped out appearance of expiratory limb on flow volume loop
Presence of bronchodilator response in asthma
Elevated TLC and RV
Reduced DLCO
FEV1 <50% indicates severe
When is mechanical ventilation indicated?
Acute setting for critically ill patients
Respiratory failure - oxygenation or ventilation
Controlled setting for support while patient under anesthesia for operation
Supportive not curative - must fix underlying disease process
What is the mode of ventilators?
Pattern of cycling used to drive gas flow from ventilator to patient - volume or pressure regulated
Most deliver fixed volume
When is controlled mechanical ventilation (CMV) used?
Ventilator provides full support and doesn’t allow patient to support own ventilation - cannot take spontaneous breaths
Effective under general anesthesia, comatose, unable to make any inspiratory effort
Not used in patients capable of spontaneous breathing
When is assist control ventilation (AC) used?
Ventilator assists patients breath to provide full tidal volume breath
Back up rate and full tidal volume breath if patient does not initiate a spontaneous breath
Awake, moderately sedated (post OP), or fully paralyzed patients
When is synchronized intermittent mandatory ventilation (SIMV) used?
Ventilator delivers preset number of breaths per minute at specific tidal volume
Patient can spontaneously breath - not assisted
When is pressure support ventilation (PS) used?
Patient determines inflation volume and respiratory cycle - requires spontaneously breathing patient
Pre selected pressure assists during inspiration
When is pressure control ventilation (PC) used?
Very little participation by patient
Pressures and breathing times set by clinician
What is a normal tidal volume setting?
8-10
Lower in ARDS to protect lung from high pressures during inspiration (leads to volutrauma, barotrauma, atelectotrauma)
What is the positive end expiratory pressure (PEEP) when using ventilators?
Collapse of airspaces at end of expiration common in ventilators - causes atelectasis
PEEP reduces alveoli closing/opening and can improve oxygenation
What is CPAP?
Continuous positive airway pressure
Patient breathing spontaneously
No support provided - splints open airways
Set pressure delivered during inspiration and expiration
Used mostly in obstructive sleep apnea and cardiogenic pulm edema
What is BiPAP?
Patient breathing spontaneously
Provides two pressures - inspiratory positive airway and expiratory positive airway
Primarily in COPD when carbon dioxide elimination is needed
What is extracorporeal membranous oxygenation (ECMO)?
Supports gas exchange
Removes blood from patient and circulates it through artificial lung
When is ECMO used?
In ARDS
Mostly in neonatal population
Limited use in adult respiratory failure
