Equipment & Monitoring
Anesthesia machine, capnography, evoked potentials, TEE, pulse oximetry, vaporizers, ventilator. ← All topics
Q1. Fail-safe valve threshold
The fail-safe valve on the anesthesia machine activates when oxygen pressure drops below:
A. 50 psi
B. 25 psi
C. 16 psi
D. 10 psi
E. 5 psi
Show answer
Answer: B. The fail-safe valve activates when oxygen supply pressure drops below 25 psi, automatically shutting off or proportionally reducing nitrous oxide and other gas flows to prevent delivery of a hypoxic mixture. This threshold is set above the second-stage regulator pressure of 16 psi but below the first-stage regulator output of 45 psi. The 50 psi distractor may be confused with pipeline supply pressure. It is critical to understand that the fail-safe valve only senses pressure, not oxygen concentration, so it cannot detect problems like pipeline crossover where an incorrect gas is supplied at normal pressure. A separate oxygen analyzer with low-concentration alarm is required to detect such crossover situations and ensure patient safety.
Q2. E-cylinder pressure and volume — oxygen
A full E-cylinder of oxygen contains approximately:
A. 350 L at 1900 psi
B. 660 L at 2200 psi
C. 1500 L at 2000 psi
D. 1900 L at 750 psi
E. 200 L at 1000 psi
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Answer: B. A full oxygen E-cylinder contains 660 L at 2200 psi. Oxygen exists only in the gas phase in the cylinder, so pressure drops linearly with volume as the cylinder empties. To estimate remaining minutes of oxygen, use the formula: (gauge pressure in psi / 2200) × 660 / flow rate in L/min. This linear relationship contrasts with nitrous oxide E-cylinders, which contain 1590 L at 745 psi and maintain constant pressure until approximately 16% of contents remain, because liquid nitrous oxide is present and vaporizes to maintain pressure until nearly depleted.
Q3. Nitrous oxide cylinder pressure
A nitrous oxide E-cylinder shows 745 psi. The most accurate statement is:
A. The cylinder is full and ready for use in the operating room
B. The cylinder is approximately half-full based on the pressure reading
C. The cylinder still contains liquid N₂O; pressure drops only below 16% capacity
D. The cylinder pressure gauge is malfunctioning and reading incorrectly
E. The fail-safe valve should engage due to the low pressure reading
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Answer: C. Nitrous oxide is stored as liquid at room temperature because its boiling point is −88°C. The pressure gauge reads the vapor pressure above the liquid, which remains constant at 745 psi (at 20°C) as long as any liquid is present. The gauge pressure will not begin to drop until all liquid has vaporized, which occurs when approximately 16% of the original contents remain (about 253 liters in an E-cylinder). Because the pressure gauge does not reflect how much nitrous oxide remains until this point, the cylinder must be weighed to obtain an accurate estimate of the volume remaining. A full E-cylinder contains approximately 1590 liters of nitrous oxide.
Q4. Capnography waveform — obstruction
A capnograph waveform with a slow, blunted upstroke and absent plateau most likely represents:
A. Normal capnogram with square waveform and distinct plateau phase
B. Severe airway obstruction from asthma, bronchospasm, or kinked ETT
C. Esophageal intubation with absent or minimal CO2 detection
D. Cardiac arrest with sudden loss of pulmonary blood flow
E. Hyperventilation with decreased ETCO2 but preserved waveform morphology
Show answer
Answer: B. A sloping or shark-fin capnogram with a slow, blunted upstroke and absent plateau indicates airflow obstruction. This pattern is classically seen with asthma, COPD exacerbation, bronchospasm, kinked endotracheal tube, or partial bronchial obstruction. The prolonged upstroke reflects delayed and uneven emptying of alveoli due to increased airway resistance. This pattern can also be seen with a leaky circuit at low fresh gas flow. In contrast, a normal capnogram displays a square waveform with a rapid upstroke and distinct plateau (phase II and III). Esophageal intubation shows absent or rapidly declining CO2. Cardiac arrest produces sudden loss of the waveform due to absent pulmonary blood flow. Hyperventilation decreases ETCO2 values but preserves normal waveform morphology with an intact plateau.
Q5. Sudden EtCO₂ drop to zero
A sudden drop of EtCO₂ from 35 to 0 mmHg is most likely due to:
A. Sudden hyperventilation reducing alveolar CO₂ concentration and eliminating exhaled carbon dioxide
B. Disconnection or accidental extubation preventing exhaled gas from reaching the capnography sampling site
C. Massive pulmonary embolism causing complete cessation of pulmonary blood flow and gas exchange
D. Cardiac arrest with ongoing chest compressions maintaining minimal circulation but inadequate CO₂ delivery
E. Severe bronchospasm creating complete airway obstruction and preventing movement of exhaled gases distally
Show answer
Answer: B. An EtCO₂ reading that drops suddenly to zero indicates no gas movement at the sampling line. This occurs with circuit disconnection, accidental extubation, esophageal intubation, complete airway obstruction, or sampling line failure. In contrast, a sudden decrease in EtCO₂ that does not reach zero suggests decreased cardiac output from causes such as pulmonary embolism, cardiac arrest, or ventilation-perfusion mismatch. Hyperventilation decreases EtCO₂ but rarely to zero. Massive PE and cardiac arrest typically show a sudden drop to low values (5-15 mmHg) rather than zero because some CO₂ continues to be delivered to the lungs. Bronchospasm severe enough to prevent all gas flow would also prevent ventilation entirely, which is uncommon, and the capnograph would show absent waveforms rather than a reading of zero with continued ventilator cycling.
Q6. Capnography — rebreathing
A capnograph shows a sustained baseline above zero between breaths. This indicates:
A. Hyperventilation causing increased alveolar CO₂ washout and altered waveform morphology
B. Rebreathing from incompetent expiratory valve or exhausted CO₂ absorbent in circuit
C. Bronchospasm producing prolonged expiratory phase with characteristic upsloping plateau morphology
D. Hypothermia reducing metabolic CO₂ production and decreasing end-tidal values overall
E. Tachypnea preventing complete exhalation and causing respiratory cycle overlap artifact
Show answer
Answer: B. A sustained baseline above zero between breaths on capnography indicates rebreathing of CO₂. The inspiratory baseline should normally be zero, as the patient should be inhaling fresh gas without CO₂. When the baseline is elevated, it means the patient is rebreathing exhaled gas containing CO₂. Common causes include an incompetent (stuck-open) expiratory valve that allows exhaled gas to return to the inspiratory limb, exhausted soda lime (CO₂ absorbent) that can no longer remove CO₂ from the circuit, or inadequate fresh gas flow in a Mapleson circuit. Management includes replacing the CO₂ absorbent, checking the integrity of the unidirectional valves, and ensuring adequate fresh gas flow. The other options produce different capnographic patterns: hyperventilation lowers end-tidal CO₂ but maintains a zero baseline, bronchospasm creates an upsloping alveolar plateau, hypothermia decreases overall CO₂ production and end-tidal values, and tachypnea may prevent complete return to baseline but does not cause sustained elevation from rebreathing.
Q7. EtCO₂ during cardiac arrest
During CPR, an EtCO₂ <10 mmHg despite adequate ventilation suggests:
A. Hyperventilation causing excessive CO₂ elimination and decreased alveolar partial pressure
B. Poor cardiac output from compressions indicating inadequate chest compression quality or futility
C. Esophageal intubation with ventilation of the gastrointestinal tract rather than lungs
D. Pneumothorax causing decreased pulmonary blood flow and impaired ventilation-perfusion matching
E. Bronchospasm resulting in air trapping and incomplete alveolar gas exchange
Show answer
Answer: B. EtCO₂ during CPR reflects pulmonary blood flow and therefore cardiac output generated by chest compressions. An EtCO₂ less than 10 mmHg despite adequate ventilation indicates poor perfusion, which suggests either inadequate compression quality or a prolonged arrest with low chance of return of spontaneous circulation (ROSC). The AHA recommends EtCO₂ monitoring to guide compression quality during resuscitation. A sudden rise in EtCO₂ during CPR typically signals return of spontaneous circulation. While hyperventilation can lower EtCO₂, it would not typically drop below 10 mmHg with adequate compressions. Esophageal intubation would show EtCO₂ near zero from the start. Pneumothorax and bronchospasm would be suggested by other clinical findings and would not specifically correlate with this EtCO₂ threshold during adequate ventilation.
Q8. Capnography in obstructed airway after intubation
A patient post-intubation has a capnogram that returns to baseline but rises again before the next inspiration with a "second peak." The most likely cause is:
A. Severe asthma causing delayed emptying of obstructed alveolar units that exhale sequentially after normal units
B. Single-lung transplant with native diseased lung emptying slowly and exhaling separately after the transplanted lung
C. Bronchospasm causing delayed emptying of constricted alveolar units that exhale sequentially after normal units
D. Esophageal intubation causing gastric CO2 washout followed by secondary peak from residual gas in stomach
E. Ventilator malfunction with rebreathing valve failure allowing expired CO2 to re-enter circuit during expiratory phase
Show answer
Answer: B. The biphasic capnogram with a second peak before the next inspiration is a recognized pattern after single-lung transplant for COPD. The native diseased lung has severe airflow obstruction and empties much more slowly than the transplanted healthy lung, creating two distinct exhalation peaks—one from the transplanted lung followed by a second peak from the native lung. This creates the characteristic double-hump appearance on capnography. While severe asthma and bronchospasm can cause delayed alveolar emptying, they typically produce a sloped plateau phase rather than a distinct second peak. Esophageal intubation shows rapidly diminishing CO2 after a few breaths. Ventilator malfunction with rebreathing would show failure to return to baseline rather than a true second peak.
Q9. Pulse oximetry wavelengths
The pulse oximeter measures oxygen saturation by comparing absorbance at:
A. 500 nm (green) and 700 nm (red)
B. 660 nm (red) and 940 nm (infrared)
C. 800 nm (near-infrared) and 900 nm (near-infrared)
D. 400 nm (violet) and 600 nm (orange)
E. 1000 nm (infrared) and 1500 nm (infrared)
Show answer
Answer: B. Pulse oximeters use two wavelengths: red light at 660 nm (where deoxyhemoglobin absorbs more) and infrared light at 940 nm (where oxyhemoglobin absorbs more). The Beer-Lambert law is applied to the pulsatile component of arterial blood to calculate oxygen saturation. Important limitations include carboxyhemoglobin, which reads as 100% because it has similar absorbance to oxyhemoglobin at 660 nm, and methemoglobin, which reads near 85% regardless of true saturation. Dyes such as fluorescein, methylene blue, and indigo carmine can cause transient drops in SpO₂ readings.
Q10. Pulse ox in methemoglobinemia
In severe methemoglobinemia, the pulse oximeter typically reads:
A. Reads 100% due to absorption at 660 nm wavelength
B. Trends toward 85% regardless of true arterial saturation
C. Reads below 50% due to absorption at 940 nm
D. Accurately reflects true saturation with standard two-wavelength technology
E. Reads zero due to complete loss of signal
Show answer
Answer: B. In severe methemoglobinemia, the pulse oximeter typically trends toward 85% regardless of the true arterial oxygen saturation. This occurs because methemoglobin absorbs light at both 660 nm (red) and 940 nm (infrared) wavelengths used by standard pulse oximeters. When absorption is equal at both wavelengths, the ratio of absorbances approaches 1:1, which corresponds to an SpO₂ reading of approximately 85%. This is in contrast to carboxyhemoglobinemia, where the pulse oximeter reads falsely high due to spectral overlap between carboxyhemoglobin and oxyhemoglobin at 660 nm. Accurate measurement of methemoglobin requires co-oximetry, which uses four or more wavelengths to distinguish between oxyhemoglobin, deoxyhemoglobin, methemoglobin, and carboxyhemoglobin.
Q11. CVP waveform a-wave abnormalities
Cannon a-waves on the CVP tracing indicate:
A. Tricuspid stenosis causing increased resistance to atrial emptying during contraction
B. Atrial fibrillation causing chaotic atrial activity without organized mechanical contraction
C. AV dissociation causing atrial contraction against a closed tricuspid valve
D. Tricuspid regurgitation causing retrograde flow from ventricle back into atrium
E. Right bundle branch block causing delayed right ventricular depolarization and contraction
Show answer
Answer: C. Cannon a-waves occur when the atrium contracts against a closed tricuspid valve due to AV dissociation. This is seen in complete heart block, junctional rhythms, and ventricular pacing where atrial and ventricular contractions are not coordinated. The atrium attempts to eject blood into the ventricle while the tricuspid valve is closed, producing an exaggerated pressure wave in the CVP tracing. Other important CVP waveform findings include: absent a-waves in atrial fibrillation (no organized atrial contraction); large a-waves in tricuspid stenosis or right ventricular hypertrophy (increased resistance to atrial emptying); large v-waves in tricuspid regurgitation (retrograde flow during ventricular systole); and prominent x and y descents in pericardial constriction (rapid early filling with restricted late filling).
Q12. Arterial line transduction errors
An arterial line shows a damped waveform with a low pressure reading. The most common cause is:
A. Tubing that exceeds standard length causing excessive resonance in the system
B. Air bubble in tubing, partial catheter occlusion, or kinked tubing components
C. Transducer positioned significantly above or below the phlebostatic axis reference point
D. Patient core temperature below 35 degrees Celsius affecting vascular tone
E. Flush solution bag pressurized beyond 300 mmHg creating excessive flow
Show answer
Answer: B. Overdamping produces a damped waveform with low systolic pressure readings and is most commonly caused by air bubbles in the tubing, partial occlusion of the catheter by clot or fibrin, or kinked tubing. Overdamping underestimates systolic pressure, overestimates diastolic pressure, but typically provides an accurate mean arterial pressure (MAP). In contrast, underdamping is caused by excessively long tubing or excessive catheter movement and overestimates systolic pressure. Soft compliant tubing can also contribute to overdamping. The square wave test (fast-flush test) is used to assess damping characteristics of the arterial line system.
Q13. SSEP sensitivity to volatile anesthetics
Compared to MEPs, somatosensory evoked potentials (SSEPs) are:
A. More sensitive to volatile anesthetics than motor evoked potentials
B. Less sensitive to volatile anesthetics than motor evoked potentials
C. Equally sensitive to volatile anesthetics as motor evoked potentials
D. Not affected by volatile anesthetics unlike motor evoked potentials
E. Enhanced by volatile anesthetics unlike motor evoked potentials
Show answer
Answer: B. SSEPs are less sensitive to volatile anesthetics than MEPs. The hierarchy of volatile sensitivity from lowest to highest is: BAEPs < SSEPs < MEPs < VEPs. SSEPs remain interpretable at approximately 0.5–1 MAC volatile anesthetics when combined with propofol. In contrast, MEPs require total intravenous anesthesia (TIVA) with propofol-based techniques and no volatile agents, as volatiles severely suppress MEP amplitude and reliability. Ketamine and etomidate actually increase evoked potential amplitudes and can be useful adjuncts for neuromonitoring.
Q14. SSEP significant change
A 50% drop in SSEP amplitude or 10% increase in latency is clinically significant during:
A. Lumbar spine surgery and other procedures involving the lower spinal cord
B. Carotid endarterectomy and other procedures involving cerebral perfusion monitoring
C. Scoliosis surgery and other procedures involving spinal cord monitoring
D. Coronary artery bypass and other procedures involving cardiopulmonary bypass
E. Neck dissection and other procedures involving cervical vascular structures
Show answer
Answer: C. A 50% drop in SSEP amplitude or 10% increase in latency represents the standard warning threshold for spinal cord injury during scoliosis surgery, thoracoabdominal aortic aneurysm repair, and other spine procedures where spinal cord integrity is at risk. These criteria are used to alert the surgical team to potential ischemia or mechanical injury to the spinal cord. Motor evoked potentials (MEPs) are often preferred for monitoring anterior spinal artery territory because they assess anterior cord motor function more directly than SSEPs, which primarily monitor the posterior columns. The wake-up test can serve as a backup method to confirm motor function when neuromonitoring changes occur.
Q15. BAEP resistance
Brainstem auditory evoked potentials (BAEPs) are useful for posterior fossa surgery because:
A. They are easily abolished by volatile anesthetics at standard MAC concentrations
B. They are most resistant to volatile anesthetics of all evoked potential modalities
C. They require nitrous oxide administration for adequate signal amplitude and interpretation
D. They are independent of cranial nerve VIII and monitor only brainstem
E. They only monitor frontal cortex activity and are unaffected by brainstem
Show answer
Answer: B. BAEPs are most resistant to volatile anesthetic effects compared to other evoked potential modalities such as somatosensory evoked potentials (SSEPs) or motor evoked potentials (MEPs). This makes them particularly useful for monitoring during posterior fossa surgery and acoustic neuroma resection. BAEPs monitor the integrity of the auditory pathway from cranial nerve VIII through the brainstem to the auditory cortex. Unlike SSEPs and MEPs, BAEPs can be reliably monitored even at higher concentrations of volatile anesthetics, though total intravenous anesthesia may still be preferred for optimal signal quality. They do not require nitrous oxide for amplification and are directly dependent on CN VIII function.
Q16. NIM (recurrent laryngeal) monitoring
During thyroidectomy, recurrent laryngeal nerve monitoring requires:
A. Specialized NIM ETT with electrodes contacting vocal cords; avoid neuromuscular blockade or use partial reversal
B. Standard ETT with external surface electrodes placed on neck; avoid neuromuscular blockade or use partial reversal
C. Specialized NIM ETT with electrodes contacting vocal cords; use only sugammadex for neuromuscular blockade reversal
D. Standard ETT placed while awake via fiberoptic technique; avoid all neuromuscular blockade throughout the case
E. Specialized NIM ETT with electrodes contacting vocal cords; avoid all opioids to preserve spontaneous ventilation
Show answer
Answer: A. Recurrent laryngeal nerve monitoring during thyroidectomy requires a specialized NIM (nerve integrity monitor) endotracheal tube that has embedded electrodes positioned to contact the vocal cords. These electrodes detect electromyographic signals from the vocalis muscle when the recurrent laryngeal nerve is stimulated by the surgeon. Neuromuscular blockade must be avoided after intubation, or if used, must be partially reversed before monitoring begins, because paralysis prevents EMG signal generation. The anesthesiologist should notify the surgeon if a paralytic agent has been administered. Standard ETTs lack the necessary electrodes. Sugammadex-only reversal is not required; any adequate reversal method works. Awake intubation is unnecessary. Opioids do not interfere with nerve monitoring and need not be avoided.
Q17. Cerebral oximetry NIRS
Near-infrared spectroscopy (cerebral oximetry) measures:
A. Cerebral blood flow in the anterior circulation territory
B. Regional cerebral tissue oxygen saturation, primarily venous-weighted
C. Intracranial pressure via transmission of infrared light waves
D. Cerebral metabolic rate of oxygen consumption in real time
E. Hemoglobin concentration without differentiating oxygenation status
Show answer
Answer: B. Near-infrared spectroscopy (NIRS) uses approximately 700–900 nm wavelength light to measure regional cerebral tissue oxygen saturation (rSO₂), which reflects a mixed venous-arterial blood pool that is approximately 75% venous and 25% arterial. Normal rSO₂ values range from 60–75%. NIRS is used for trending during carotid endarterectomy, beach-chair position shoulder surgery, and cardiopulmonary bypass. A decrease of more than 20% from baseline or an absolute value below 50% suggests cerebral ischemia and warrants intervention. NIRS does not directly measure cerebral blood flow, intracranial pressure, cerebral metabolic rate, or isolated hemoglobin concentration.
Q18. TEE absolute contraindications
Absolute contraindications to transesophageal echocardiography include:
A. Esophageal stricture, perforation, scleroderma, recent upper GI surgery
B. Hiatal hernia, mild dysphagia, therapeutic anticoagulation, prior chest radiation
C. Mild dysphagia, asymptomatic hiatal hernia, coagulopathy, severe cervical arthritis
D. Therapeutic anticoagulation, asymptomatic hiatal hernia, coagulopathy, recent CABG
E. Recent CABG, prior chest radiation, severe cervical arthritis, mild dysphagia
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Answer: A. Absolute contraindications to transesophageal echocardiography include perforated viscus, esophageal stricture, esophageal trauma, esophageal tumor, scleroderma, Mallory-Weiss tear, Zenker diverticulum, active upper GI bleeding, recent upper GI surgery, and esophagectomy. These conditions carry unacceptable risk of esophageal perforation or worsening of existing pathology. Relative contraindications include severe cervical arthritis, prior chest radiation, symptomatic hiatal hernia, and coagulopathy. Recent CABG, therapeutic anticoagulation, mild dysphagia, and asymptomatic hiatal hernia are not absolute contraindications and TEE can be performed with appropriate precautions when clinically indicated.
Q19. TEE LV function — sphericity index
The sphericity index (LV long-axis / short-axis) is normally:
A. Less than 1.0, indicating a globular ventricle
B. Equal to 1.0, indicating a perfectly spherical ventricle
C. Greater than 1.5, indicating a normal elliptical ventricle
D. Greater than 3.0, indicating an excessively elongated ventricle
E. Negative value, indicating inverted long-axis to short-axis ratio
Show answer
Answer: C. The normal sphericity index is greater than 1.5, reflecting the normal elliptical shape of the left ventricle where the long axis exceeds the short axis. In dilated cardiomyopathy, the ventricle becomes more globular and the sphericity index decreases toward 1.0. The sphericity index is an important predictor of left ventricular remodeling and clinical outcome in heart failure patients.
Q20. E/A ratio interpretation
A mitral inflow E/A ratio >2 indicates:
A. Normal diastolic function with preserved left ventricular relaxation
B. Impaired relaxation consistent with grade 1 diastolic dysfunction
C. Pseudonormal filling pattern consistent with grade 2 dysfunction
D. Restrictive filling pattern consistent with severe diastolic dysfunction
E. Atrial fibrillation with loss of organized atrial contraction
Show answer
Answer: D. Diastolic function is graded by mitral inflow patterns. Normal function shows E > A. Grade 1 (impaired relaxation) shows E < A with prolonged deceleration time. Grade 2 (pseudonormal) shows E ≈ A and can be unmasked by Valsalva maneuver, which reduces preload and reveals the underlying relaxation abnormality. Grade 3 (restrictive filling) shows E >> A with an E/A ratio greater than 2, indicating severe diastolic dysfunction with elevated filling pressures and reduced ventricular compliance. Tissue Doppler e' velocity less than 8 cm/s confirms diastolic dysfunction independent of preload conditions and helps distinguish pseudonormal from truly normal patterns.
Q21. Train-of-four
For appropriate extubation from neuromuscular blockade, TOF ratio should be:
A. ≥0.5 measured by quantitative neuromuscular monitor
B. ≥0.7 measured by quantitative neuromuscular monitor
C. ≥0.9 measured by quantitative neuromuscular monitor
D. ≥0.8 measured by quantitative neuromuscular monitor
E. ≥0.6 measured by quantitative neuromuscular monitor
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Answer: C. TOF ratio ≥0.9 indicates adequate recovery from neuromuscular blockade and is the appropriate threshold for safe extubation. Clinical signs such as sustained head lift for 5 seconds, hand grip strength, and eye opening are all unreliable indicators of adequate recovery. Quantitative neuromuscular monitoring is recommended to accurately measure the TOF ratio. The presence of 4 visible twitches on qualitative assessment does not guarantee adequate recovery, as residual weakness can persist at lower TOF ratios. Sugammadex achieves TOF >0.9 more reliably than neostigmine. TOF ratios below 0.9 are associated with increased risk of postoperative pulmonary complications and residual neuromuscular blockade.
Q22. Hagen-Poiseuille and gas flow
In laminar flow, gas flow rate is most affected by:
A. Length of the tubing through which gas travels
B. Pressure gradient across the length of tubing
C. Viscosity of the gas flowing through tubing
D. Radius of the tubing raised to fourth power
E. Density of the gas flowing through tubing
Show answer
Answer: D. According to the Hagen-Poiseuille equation for laminar flow (Q = πr⁴ΔP / 8μL), gas flow rate is most affected by the radius of the tubing, which is raised to the fourth power. This means that doubling the radius increases flow by 16-fold (2⁴ = 16). This relationship is clinically critical in pediatric airway management, where small endotracheal tubes dramatically reduce flow. While pressure gradient, viscosity, and length also affect flow, their relationships are linear or inverse linear, making them far less influential than radius. In contrast, turbulent flow (Reynolds number >4000) depends on gas density rather than viscosity, which is why heliox is useful in upper airway obstruction where turbulent flow predominates.
Q23. Mapleson circuits
Which Mapleson circuit is most efficient for spontaneous ventilation?
A. Mapleson A, requiring fresh gas flow equal to minute ventilation to prevent rebreathing
B. Mapleson B, requiring fresh gas flow equal to minute ventilation to prevent rebreathing
C. Mapleson C, requiring fresh gas flow equal to minute ventilation to prevent rebreathing
D. Mapleson D, requiring fresh gas flow equal to minute ventilation to prevent rebreathing
E. Mapleson E, requiring fresh gas flow equal to minute ventilation to prevent rebreathing
Show answer
Answer: A. The Mapleson A circuit is most efficient for spontaneous ventilation. The mnemonic for efficiency during spontaneous ventilation is "All Dogs Can Bite" (A > D > C > B), while for controlled ventilation the order reverses: "Dead Bodies Can't Argue" (D > B > C > A). The Mapleson A requires fresh gas flow equal to minute ventilation to prevent rebreathing during spontaneous ventilation, making it the most efficient. In contrast, during controlled ventilation, the Mapleson D becomes most efficient. For controlled ventilation with any Mapleson circuit, fresh gas flow must be approximately 2 times minute ventilation to prevent rebreathing, compared to only 1 times minute ventilation needed for spontaneous breathing with the Mapleson A.
Q24. Vaporizer altitude
A desflurane vaporizer set at 6% delivers what concentration at altitude (atmospheric pressure 380 mmHg, half of sea level)?
A. 3% concentration because the vaporizer output is reduced proportionally to atmospheric pressure
B. 6% concentration because the heated electronic vaporizer delivers a fixed percentage regardless of altitude
C. 12% concentration because the vaporizer compensates by doubling output to maintain anesthetic effect
D. Cannot be calculated without knowing the exact temperature and carrier gas flow rate
E. 6% partial pressure equivalent because the vaporizer adjusts output to maintain constant anesthetic effect
Show answer
Answer: B. Desflurane uses a heated electronic vaporizer that operates at 39°C with 2 atm internal pressure. Unlike variable-bypass vaporizers used for sevoflurane and isoflurane, the desflurane vaporizer delivers a fixed concentration (percentage) regardless of altitude. At altitude, if set to 6%, it delivers 6% concentration. However, because atmospheric pressure is halved at this altitude (380 mmHg versus 760 mmHg at sea level), the partial pressure of desflurane is also halved, resulting in less anesthetic effect despite the same percentage. In contrast, variable-bypass vaporizers (sevoflurane, isoflurane) deliver a fixed partial pressure, so at altitude the percentage delivered increases but the anesthetic effect remains unchanged.
Q25. PIP vs Plateau pressure interpretation
A ventilated patient develops elevated peak inspiratory pressure but unchanged plateau pressure. The most likely cause is:
A. Decreased lung compliance from pneumonia or atelectasis
B. Increased airway resistance from bronchospasm or kinked ETT
C. Pneumothorax causing decreased compliance on affected side
D. Volume overload leading to pulmonary edema and stiff lungs
E. Auto-PEEP from inadequate expiratory time or air trapping
Show answer
Answer: B. Elevated peak inspiratory pressure with unchanged plateau pressure indicates increased airway resistance. Common causes include bronchospasm, kinked endotracheal tube, and mucus plugging. The plateau pressure reflects alveolar pressure and lung compliance, while the difference between PIP and plateau reflects resistance in the airways. In contrast, elevated PIP with elevated plateau pressure indicates decreased lung compliance from conditions such as atelectasis, pneumonia, pulmonary edema, pneumothorax, or mainstem bronchus intubation. Plateau pressures above 30 cm H₂O are associated with increased risk of ventilator-induced lung injury.
Q26. Core temperature monitoring sites
The gold standard for core temperature is:
A. Tympanic membrane temperature measurement
B. Esophageal distal probe temperature measurement
C. Pulmonary artery catheter temperature measurement
D. Nasopharyngeal posterior probe temperature measurement
E. Rectal thermistor probe temperature measurement
Show answer
Answer: C. Pulmonary artery catheter temperature is the gold standard for core temperature monitoring because it directly measures blood temperature in the central circulation. Good alternatives that closely approximate core temperature include tympanic membrane (reflects carotid artery temperature), nasopharyngeal (when placed posteriorly), esophageal (when placed in distal third), and oropharyngeal sites. Less reliable sites include bladder temperature (only accurate at high urine output rates), axillary temperature (affected by peripheral vasoconstriction), and rectal temperature (affected by insulating stool and heat production from enteric organisms). The main cause of perioperative heat loss is radiation, accounting for approximately 60% of heat loss in the operating room.
Q27. Bispectral index range
The target BIS (Bispectral Index) range for general anesthesia is:
A. 0–20, representing deep burst suppression or isoelectric activity
B. 40–60, representing adequate depth for general anesthesia
C. 60–80, representing light sedation or emergence from anesthesia
D. 80–100, representing awake or minimally sedated states
E. >90, representing fully awake with normal cortical activity
Show answer
Answer: B. The target BIS range for general anesthesia is 40–60, which represents adequate anesthetic depth. A BIS of 0 indicates an isoelectric EEG, while 100 represents a fully awake state. Titrating anesthesia to the 40–60 range reduces the risk of intraoperative awareness, especially in high-risk scenarios such as total intravenous anesthesia (TIVA), neuromuscular blockade, or paralyzed trauma patients. BIS monitoring can be affected by several factors: hypothermia and hypoglycemia can lower readings, while electrocautery can cause artifact. Neuromuscular blocking drugs may produce falsely high readings if the patient is inadequately anesthetized but paralyzed. BIS does not reliably trend with certain anesthetic agents including ketamine, dexmedetomidine, and nitrous oxide.
Q28. Stroke volume variation
Stroke volume variation (SVV) is a reliable predictor of fluid responsiveness in patients who are:
A. Spontaneously breathing with regular sinus rhythm and closed chest
B. Closed chest, regular sinus rhythm, mechanically ventilated with TV ≥8 mL/kg, no high PEEP
C. In atrial fibrillation with mechanical ventilation and closed chest
D. On VV-ECMO with mechanical ventilation and regular sinus rhythm
E. With open chest, regular sinus rhythm, and mechanical ventilation
Show answer
Answer: B. SVV and PPV are reliable predictors of fluid responsiveness only when specific conditions are met: closed-chest mechanical ventilation, regular cardiac rhythm, tidal volume ≥8 mL/kg, absence of excessive PEEP, and no right ventricular failure. SVV >13% or PPV >12% predicts fluid responsiveness in these patients. Spontaneous breathing eliminates the consistent intrathoracic pressure changes needed for accurate measurement. Cardiac arrhythmias like atrial fibrillation cause beat-to-beat variability that confounds interpretation. Open chest conditions eliminate the thoracic compliance changes that drive the variation. VV-ECMO can alter intrathoracic pressures and venous return in ways that make SVV unreliable. For patients who are spontaneously breathing or have arrhythmias, passive leg raise is a validated alternative method to assess fluid responsiveness.