Week 8 - Anesthesia for Thoracic Surgery Flashcards
Define Total Lung Capacity (TLC). What is the typical volume?
The gas volume in the lung after a MAXIMUM inspiration
-typical volume = 6-8L
What increases and decreases total lung capacity?
- Can be increased in pts with COPD due to hyperinflation or destruction of alveolar wall and resultant loss of elastic tissue
- Can be decreased in restrictive disorders such as pulmonary fibrosis – decrease is in proportion to the severity of the fibrotic process
Define Residual Volume (RV) of the lungs
The volume of gas remaining in the lung after a MAXIMUM expiratory effort
-typical volume is 2 - 2.5 L
Define Vital Capacity (VC) of the lung
The maximum volume that can be inspired and expired
- difference between TLC and RV
- typical volume is 4-6 L
Define Expiratory Reserve Volume (ERV) of the lungs
Volume of gas that can be expelled beyond the tidal exhalation with a forced exhalation maneuver
Define Functional Residual Capacity (FRC) of the lungs
Volume of gas remaining in the lung after an ordinary expiration
- typical FRC is 3-4 L
- most important for anesthesia
Why do we care so much about FRC in anesthesia?
Preoxygenation
-the higher the concentration of O2 in the FRC the greater amount of apnea time we have before desaturation occurs
What 4 factors is FRC dependent on?
- Age
- Height
- Sex
- Weight
- FRC goes UP with age (loss of elastic lung recoil) and height (more space and volume is greater)
- FRC goes DOWN with Female sex (smaller volumes) and Weight (external pressures on thoracic cavity)
How does the FRC change in pts with COPD or restrictive lung disease?
COPD: FRC is increased – increase in FRC is accelerated in pts with COPD over the normal aging increase due to chronic air trapping and more severe loss of elastic tissue
Restrictive Lung Disease: FRC is decreased – i.e. pulmonary fibrosis
What does Bordy’s equation measure? How do you calculate it?
Oxygen consumption
weight (kg)^3/4 x 10
*for the average 70kg adult, this results in an oxygen consumption of approx 240 mL/min
How can you calculate CO2 production?
CO2 production per min can be calculated as:
oxygen consumption x 0.8
During periods of apnea, PaCO2 will rise _____ mmHg during the 1st min and _______ mmHg each subsequent minute.
During periods of apnea, PaCO2 will rise 6 mmHg during the 1st min and 3 mmHg each subsequent minute.
What is pulmonary perfusion largely dependent on?
Gravity
-West’s 4 lung zones
What are the four main causes of Hypoxemia?
- Hypoventilation
- V/Q Mismatch
- Impaired Diffusion
- Right-to-Left Shunt
What is Hypoxic Pulmonary Vasoconstriction?
A compensatory mechanism which reduces blood flow in hypoxic lung regions
- major stimulus for this is low alveolar oxygen tension (can be caused by hypoventilation or delivery of gas with a low PO2)
- has a rapid onset over the first 30 min and then a slower increase to a maximal response at approx 2 hrs
What is the benefit of hypoxic pulmonary vasoconstriction?
Decrease in blood flow to hypoxic areas functions to correct right-left shunt (redirects flow)
Decrease in shunt will maximize oxygenation of pulmonary circulation
What can Hypoxic Pulmonary Vasoconstriction be inhibited by?
- Inhaled anesthetics greater than 1 MAC (Iso shows the least effect, Des shows greatest effect) – relatively weak inhibitors
- Nitro vasodilators
- Extremes of acidosis/alkalosis
- Hypothermia
- Calcium Channel Blockers
*we do NOT want to inhibit
What effects does anesthesia have on respiratory function?
- FRC is decreased by 0.8-1L from supine positioning and 0.4-0.5L from induction of general anesthesia
- Compliance is decreased
- Overall resistance is increased
What are indications for thoracic surgery?
- Thoracic malignancies
- Chest trauma
- Mediastinal tumors
- Esophageal disease
- Tracheal resections
- Lung transplantation
What is the role of the anesthetist in the preop process for thoracic surgery?
- Interpret given info
- Assess risk
- Develop a plan to minimize the risk of periop complications
*preop testing to be evaluated includes Pulmonary Function Tests, Lung parenchymal function, and cardiopulmonary interactions
What portion of Pulmonary Function Tests should you focus on? What are normal values?
Focus on FEV1, FVC, and FEV1/FVC ratio
FEV1 = 4L
FVC = 5L
FEV1/FVC ratio = 0.8
*should be remembered the “normal” values will decrease with age, so analysis of how the pt performed in their PFT compared to their predicted values given their demographics must be considered
What does the FEV1/FVC ratio help determine?
Helps determine whether we have a normal, obstructive, or restrictive respiratory pattern
What FEV1/FVC ratio represents an obstructive defect and a restrictive defect?
Obstructive: ratio is LESS than the lower limit of normal
Restrictive: ratio is GREATER than the lower limit of normal and the FVC is decreased
What lung parenchymal testing values are used as cut-offs for candidacy for pulmonary resection?
PaO2 of <60 mmHg or PaCO2 of >45 mmHg