Flashcards in Thoracic Surgery, Pulmonary Physiology, Obesity, and Hemoglobin Deck (58)
What are important considerations in the preoperative evaluation of a patient for thoracic surgery?
- Focus on the extent and severity of pulmonary disease and cardiovascular involvement.
- Inquire about: dyspnea, cough, cigarette smoking, exercise tolerance, risk factors for acute lung injury.
What are pack years?
The #of packs smoked/day x the # of years
On physical examination of the thoracic surgery patient, what signs are you looking for?
Presence of cyanosis and clubbing, breathing pattern, type of breath sounds (wet- crackles, dry- wheezes).
Look for midline trachea- if displaced, consider difficult intubation.
T or F: one important factor in the evaluation of a thoracic surgery patient is the presence of an increase in PVR 2/2 to a fixed reduction in the x-sectional area of the pulmonary vascular bed.
True- pulmonary circulation is normally a low-pressure, high-compliance system capable of handling an increase in blood flow by recruitment of normally underperfused vessels. This acts as a compensatory mechanism that normally prevents an increase in pulmonary arterial pressure.
In COPD, there is distention of the pulmonary capillary bed with decreased ability to tolerate an increase in blood flow (decreased compliance). These patients demonstrate an increase in PVR when cardiac output increases because of a decreased ability to compensate for an increase in pulmonary blood flow --> results in pulmonary HTN.
T or F: An increase in PVR is of significance in the management of the patient during anesthesia because factors such as acidosis, sepsis, hypoxia, and application of PEEP, all further increase the PVR and increase the likelihood of right ventricular failure.
Common features on ABG analysis of COPD patients.
Hypoventilation and CO2 retention.
BLUE BLOATERS: cyanotic, hypercarbic, hypoxemic, overweight, in a state of chronic respiratory failure with decreased ventilatory response to CO2. In these patients, the high PaCO2 increases CSF HCO3 concentrations, and the medullary chemoreceptors become reset to a higher level of CO2. Sensitivity to CO2 is decreased. These patients hypoventilate when given high oxygen concentrations to breathe because of a decreased hypoxic drive.
PINK PUFFERS: pts with emphysema, thin, dypneic, pink, normal ABG values. Have increase in minute ventilation to maintain normal PaCO2, which explains increase in work of breathing and dyspnea. Preoperative PaO2 correlates with intraoperative PaO2 during one lung ventilation.
What are three goals in performing PFTs in a patient scheduled for lung resection?
1- identify the patient at risk for increased postoperative morbidity and mortality.
2- identify patient who will need short-term or long-term postoperative ventilatory support.
3- evaluate the beneficial effect and reversibility of airway obstruction with use of bronchodilators. (PFTs are usually performed before and after bronchodilator therapy to assess the reversibility of airway obstruction --> a 12% improvement in PFTs may be considered a positive response to bronchodilator therapy and indicates that this therapy should be initiated before surgery. )
What is FEV1?
Forced expired volume in 1 second.
When referring to FEV1, indicate the % of predicted value, rather than just using the actual results in liters.
The % of predicted value takes into account the age and size of the patient, and the number may have a different implication in another patient.
Why is the ratio of FEV1/FVC useful?
Allows to differentiate between restrictive and obstructive lung disease.
Restrictive lung disease: normal FEV1/FVC
Obstructive lung disease: decreased FEV1/FVC because FEV1 is markedly decreased
What is DLCO? Why is it important?
The ability of the lung to perform gas exchange is reflected by the diffusing capacity for CO.
DLCO is impaired in disorders such as interstitial lung disease, which affects the alveolar-capillary site.
PREDICTED POSTOPERATIVE DIFFUSING CAPACITY PERCENT IS THE STRONGEST SINGLE PREDICTOR OF RISK OF COMPLICATIONS AND MORTALITY AFTER LUNG RESECTION.
High-risk patients, or patients unlikely to recover from pneumonectomy are cited as:
PaCO2 > 45, PaO2 < 50 on room air
FEV < 25%
FEV1 < 2L preoperatively or < 0.8L or < 40% of predicted postoperatively
FEV1/FVC < 50% of predicted
maximum breathing capacity < 50% predicted
maximum VO2 < 10 mL/kg/min
What is important in the preoperative care of a COPD patient?
- optimize lung function
- address bronchospasm (with bronchodilators), infections (with antibiotics), smoking cessation
- Patients who smoke should be advised to stop smoking at least 2 months prior to an elective operation to decrease the risk of postoperative pulmonary complications (PPCs)
What is a hallmark feature in advanced emphysema?
dynamic hyperinflation, intrinsic PEEP
What concerns must you have during mechanical ventilation in an emphysematous patient?
Be concerned for worsening intrinsic PEEP leading to respiratory and hemodynamic consequences such as barotrauma, reduced CO, and impaired gas exchange.
How does positive pressure ventilation affect the cardiopulmonary system?
1- increased mean intrathoracic pressure reduces venous return and therefore reduces CO
2- hyperexpanded lungs may cause tamponade of the heart, especially the thin-walled right ventricle.
3- Increased PVR and shunting of blood to nonventilated lung causes hypoxia
4- Dilation of the RV reduces LV diastolic compliance
All of the above reduce oxygen delivery and cause mismatch between supply and demand.
What are reasonable maneuvers to minimize intrinsic PEEP?
1- low tidal volume
2- reduce respiratory rate
3- low I:E ratio (such as 1:3 and 1:4)
4- increase inspiratory flow to deliver tidal volume in a short time to reduce the I:E ratio an dincrease expiratory time.
What are three causes for hyperventilation?
1- arterial hypoxemia
2- metabolic acidosis
3- central etiologies (eg intracranial hypertension, hepatic cirrhosis, anxiety, pharmacologic agents)
T or F: During spontaneous ventilation, the ratio of alveolar ventilation to dead space ventilation is 2:1. The alveolar-to-dead space ventilation ratio during positive-pressure ventilation is 1:1. Thus, minute ventilation during mechanical ventilatory support must be greater than that during spontaneous ventilation to achieve the same PaCO2
T or F: PaCO2 ≥ PETCO2 unless the patient inspires or receives exogenous CO2.
True- the difference between PaCO2 and PETCO2 is due to dead space ventilation. The most common reason for an acute increase in dead space ventilation is decreased cardiac output --> you will see your end-tidal CO2 dramatically decrease in hypotension.
T or F: When FRC is reduced, lung compliance falls and results in tachypnea, and venous admixture increases, creating arterial hypoxemia
T or F: There is no compelling evidence that defines rules or parameters for ordering preoperative pulmonary function tests.
True- rather, they should be obtained to ascertain the presence of reversible pulmonary dysfunction (bronchospasm) or to define the severity of advanced pulmonary disease.
T or F: During ventilation, the predominant changes in thoracic diameter occur in the anteroposterior direction in the upper thoracic region and in the lateral or transverse direction in the lower thorax.
T or F: The ventilatory muscles include the diaphragm, intercostal muscles, abdominal muscles, cervical strap muscles, SCM muscles, and the large back and intervertebral muscles of the shoulder girdle.
T or F: Normally, at rest, inspiration requires work while exhalation is passive. As work of breathing increases, abdominal muscles assist with rib depression and increase intra-abdominal pressure to facilitate forced exhalation causing the “stitch,” or rib pain that athletes experience when they actively exhale.
Which muscles of inspiration become the primary inspiratory muscles in the setting of cervical spinal cord transection?
The cervical strap muscles
What is FRC?
FRC refers to the volume of gas remaining in the lungs at the end of a passive end expiration. It reflects the opposing and equal forces between the natural tendency of the lungs to collapse and the chest wall musculature to remain expanded.
Normally 35 ml/kg
Composed of expiratory reserve volume and residual volume.
What are 5 independent predictors of difficult bag mask ventilation?
1- age > 55 years
2- BMI > 26
3- lack of teeth
4- presence of a beard
5- history of snoring
T or F: proper positioning of the morbidly obese individuals requires supporting the upper back and head so that a horizontal line exists between the sternal notch and the external auditory meatus.
What happens to the following in the setting of obesity?
- blood volume
- cardiac output
- body water
- lean body mass
- muscle mass
- volume of distribution for lipophilic drugs
- protein binding
- renal clearance
- hepatic clearance
- pseudocholinesterase activity
- Increased blood volume
- increased cardiac output
- decreased body water
- increased lean body mass
- increased muscle mass
- increased volume of distribution for lipophilic drugs
- altered protein bidning
- increased renal clearance
- normal hepatic clearnace
- increased pseudocholinesterase activity
What happens to pulmonary function in the obese patient? Address the following:
- FEV1/FVC ratio
- Inspiratory capacity
- FRC decreases (the first and most noticeable change, mostly due to a reduction in expiratory reserve volume and chest compliance) --> leads to V/Q mismatch and shunting which worsens in the supine position. These patients require more time to preoxygenate and denitrogenate their lungs and are more likely to have rapid desaturations on induction of anesthesia.
- Decreased FEV1/FVC ratio
- Increased inspiratory capacity
- DLCO and gas exchange is preserved to normal values
What are the effects of hypercapnia?
- causes a shift in the oxy-Hb curve to the right, facilitating O2 release at the tissue level
- incidence of oculocardiac reflex is increased in the presence of hypercapnia
- increased CBF and a rise in ICP and IOP
- Increase in QT interval
- hypercapnia results in bronchodilation but is a potent pulmonary vasoconstrictor.
- hypercapnia causes sympathetic stimulation.
What are the effects of volatile anesthetics on spontaneous ventilation? Address the following:
- respiratory rate
- tidal volume
- Ratio of dead-space ventilation to alveolar ventilation
- minute ventilation
- hypoxic respiratory drive
- hypercarbic respiratory drive
- respiratory rate increased
- tidal volume decreased
- FRC decreased
- ratio of dead-space ventilation to alveolar ventilation increased
- PaCO2 increased
- minute ventilation preserved
- blunting of hypoxic respiratory drive
- blunting of hypercarbic respiratory drive
T or F: Induction of anesthesia and placement of an ETT is associated with a decrease in FRC of approximately 15-20%. This decrease in FRC will promote atelectasis and hypoxemia if FRC decreases below closing capacity.
Hysteresis describes the phenomenon that the compliance of the lung is greater during deflation than during inflation. Hysteresis means that more than expected pressure is required during inflation, yet less than expected recoil pressure is present during deflation of the lungs.
Hysteresis is largely due to the effects of surface tension which increases the energy needed to recruit alveoli during inspiration.
Describe Laplace's law.
Describes the relationship between surface tension and pressure within a sphere.
Pressure in a sphere = (2 x wall tension)/ radius of the sphere
Describe Reynold's number.
A dimensionless value that describes the nature of gas flow through a straight cylinder. When the number is < 2000, flow is predominantly laminar. When the number is > 4000, flow is turbulent. Between 2000-4000, both types of flow exist.
A lower Reynold's number promotes laminar flow. This is why helium can be combined with oxygen to treat upper airway obstruction or stridor.
T or F: small airways contribute to approximately 20% of the total airway resistance.
True- explained by the large number of small airways. At each new generation of airway branching, the radius of the airway decreases, which increases the resistance. Also, at each airway branching there is an exponential increase i the number of small airways that exist in parallel, and total airway resistance is the sum of the individual resistances in large, medium and small airways.
What is hypoxic pulmonary vasoconstriction?
Occurs in small pulmonary arterial vessels in response to a decrease of both pulmonary arterial PO2 and alveolar PO2.
*Both pulmonary arterial PO2 and alveolar PO2 stimulate HPV, but there is a larger influence from decreasing alveolar PO2. An increase in pulmonary or alveolar PCO2 can also increase HPV but to a lesser extent than pulmonary or alveolar PaO2.*
A reflex that occurs when there is an increase in pulmonary vascular resistance in areas of atelectasis with a resultant shift of blood flow away from poorly to well-ventilated regions, thus decreasing shunt fraction and maintaining oxygenation. HPV occurs in response to a PaO2
T or F: airway resistance is regulated by neurohumoral agents and autonomic neural input via their effects of the smooth muscle surrounding the airway.
True- stimulation of the parasympathetic nervous system causes airway smooth muscle constriction.
Various neurohumoral agents will also cause smooth muscle constriction, including histamine, acetylcholine, thromboxane A2, serotonin, prostacycline, and leukotrienes.
cAMP has a bronchodilating effect.
What is an A-a gradient? Why is it important?
The A-a gradient is a measure of the difference between the alveolar concentration of oxygen and the arterial concentration of oxygen. It is useful when attempting to determine the cause of hypoxemia.
A-a gradient = PAO2- PaO2
What is the normal A-a gradient in a healthy young adult?
A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg.
How do you calculate the A-a gradient?
First calculate the PAO2 using the alveolar gas equation:
PAO2 = FiO2 (Patm - PH2O) - PaCO2/RQ
A-a gradient = PAO2 - PaO2
What does an abnormally elevated A-a gradient suggest?
An abnormally increased A–a gradient suggests a defect in diffusion, V/Q (ventilation/perfusion ratio) mismatch, or right-to-left shunt.
Describe the Bohr effect.
Refers to the changes in hemoglobin's affinity for oxygen with changes in PCO2 and pH. The affinity for oxygen increases with a decrease in PaCO2 or an increase in pH.
What shifts the hemaglobin dissociation curve to the right? To the left?
Left: Decreased PaCO2, increased pH, decreased temp, decreased 2,3-BPG
Right: increased PaCO2, decreased pH, increased temp, increased 2,3-BPG
What is 2,3-BPG?
When 2,3-BPG binds to deoxyhemoglobin, it acts to stabilize the low oxygen affinity state (T state) of the oxygen carrier
How do you calculate the oxygen content of blood?
Total oxygen content = oxygen bound by Hgb + dissolved oxygen
Oxygen bound by Hgb = oxygen biding capacity for Hgb * Hct * %O2 saturation/100
Dissolved Oxygen content = PaO2 * 0.00304
Total oxygen content = (1.34 mL O2/gHgb) * (Hct)*(%oxygen saturation/100) + (PaO2 * 0.00304 mL O2/dL blood)
T or F: the result of lower partial pressure of oxygen at altitude is hypoxemia. Hypoxemia from any cause will result in hypoxic vasconstriction, which will increase pulmonary artery pressure.
Describe West Zone 1.
Zone 1 is the apex of the lung. Alveolar pressure exceeds pulmonary venous and arterial pressure.
In this area, pulmonary capillaries are flattened by the larger alveolar pressures, and no pulmonary blood flow exists. No perfusion!
Describe West Zone 2.
Arterial pressure is greater than alveolar pressure, which is greater than venous pressure. It is the arterial-to-alveolar pressure difference that determines blood flow to this zone of the lung.
Describe West Zone 3.
Venous pressure > alveolar pressure , so blood flow is determined by the arterial venous pressure gradient.
In a patient with normal respiratory mechanics, maximum inspiratory pressure and vital capacity should be what?
Maximal inspiratory pressure should be less (more negative) than -20 cm H2O or vital capacity should be greater than 8 cc/kg.
What is the definition of hypoxemia?
PaO2 < 80 mm Hg on room air.
Criteria for extubation:
1- RSBI < 100
2- ability to generate negative inspiratory force (> -20)
3- tidal volume (5 cc/kg)
4- vital capacity 10-15 cc/kg
5- RR < 25 breaths/min
What is lung compliance?
Refers to how easy it is to inflate a lung. A lung that inflates easily has high compliance. A lung that does not inflate easily has low compliance.
Compliance = change in volume / change in transpulmonary pressure
How do the following affect FRC?
- body size
- diaphragmatic paralysis
- greater the height, greater the FRC
- lower FRC with age
- women have lower FRC than men
- diaphragmatic paralysis decreases FRC
- Posture (FRC greatest standing > sitting > prone > lateral > supine)
- increased intraabdominal pressure decreases FRC
What are changes seen with cessation of smoking at 48-72 hours, 2-4 weeks, and 8 weeks?
48-72 hours: paradoxically, patients demonstrate increased secretions and more reactive airways. Decreased carboxyhemoglobin levels and increased tissue oxygenation by right shift of oxyhemoglobin curve.
2-4 weeks: decreased secretions, decreased airway reactivity.
8 weeks: decrease in overall post-op morbidity and mortality.