Capnography — MCQs
A patient is undergoing MRND for laryngeal malignancy; while dissecting the venous tributaries the surgeon elevated the internal jugular vein for ligation. Suddenly the patient's EtCO2 dropped from 38 mmHg to 12 mmHg and the patient developed hypotension along with cardiac arrhythmia. Which of the following is the most likely cause?
Curare notch seen in capnograph is due to
Which of the following conditions does not cause a rise in end-tidal CO2 during thyroid surgery?
You are in the operating room and notice the tracing in yellow colour on this device. What does it indicate?
Endotracheal tube in the esophagus is best assessed by:
Capnography is useful for
What causes sudden decreased end tidal CO2 in GA?
Modern monitors to measure ETCO2 make use of
Which of the following tubes is used in surgery for cleft palate repair?
Bains' circuit is which type of Mapleson circuit?
Capnography Indian Medical PG Practice Questions and MCQs
Question 1: A patient is undergoing MRND for laryngeal malignancy; while dissecting the venous tributaries the surgeon elevated the internal jugular vein for ligation. Suddenly the patient's EtCO2 dropped from 38 mmHg to 12 mmHg and the patient developed hypotension along with cardiac arrhythmia. Which of the following is the most likely cause?
- A. Sympathetic overactivity
- B. Vagal stimulation
- C. Venous air embolism (Correct Answer)
- D. Carotid body stimulation
Explanation: ***Venous air embolism*** - A sudden drop in **EtCO2**, **hypotension**, and cardiac arrhythmia during neck dissection, especially with manipulation of the internal jugular vein, strongly indicates venous air embolism. - This occurs when air enters an open vein, travels to the right heart, and obstructs pulmonary blood flow, leading to increased **dead space** and decreased **CO2 exhalation**. *Sympathetic overactivity* - This would typically lead to **tachycardia** and **hypertension**, not bradycardia and hypotension as suggested by the cardiac arrhythmia and drop in blood pressure. - While sympathetic stimulation can cause arrhythmias, the combination with **hypotension** and a precipitous **EtCO2 drop** points away from this as the primary cause. *Vagal stimulation* - Vagal stimulation would primarily cause **bradycardia** and **hypotension** due to parasympathetic effects on the heart, but it would not explain a sudden, profound drop in **EtCO2**. - The drop in **EtCO2** is a key indicator of impaired pulmonary perfusion or CO2 transport, which is not a direct result of vagal stimulation. *Carotid body stimulation* - Stimulation of the carotid body (chemoreceptors) primarily affects **respiration** and can cause **bradycardia** and **hypotension** through a chemoreceptor reflex. - However, it does not explain the dramatic drop in **EtCO2** observed, which is more indicative of a circulatory or pulmonary event.
Question 2: Curare notch seen in capnograph is due to
- A. Spontaneous breathing (Correct Answer)
- B. Carbon dioxide rebreathing
- C. Valve malfunction
- D. Bronchospasm
Explanation: ***Spontaneous breathing*** - The "Curare notch" a dip in the expiratory plateau of the **capnograph waveform**, occurs when a patient begins to initiate **spontaneous breaths** against the ventilator. - This signifies that the **neuromuscular blockade** from paralytic agents (like curare derivatives) is wearing off. *Carbon dioxide rebreathing* - **CO2 rebreathing** would typically elevate the baseline of the capnogram and potentially prolong the expiratory plateau, but it would not create a characteristic notch. - This is usually caused by insufficient fresh gas flow or a faulty absorbent, leading to the patient inhaling previously exhaled CO2. *Valve malfunction* - A **valve malfunction**, such as a stuck expiratory valve, would typically cause a persistent elevation of the expiratory CO2 plateau or an inability to exhale, rather than a transient notch. - This would indicate a problem with the mechanical ventilation circuit. *Bronchospasm* - **Bronchospasm** typically results in a **sloping or shark fin-shaped expiratory plateau** on the capnogram due to prolonged expiration and uneven gas emptying from the lungs. - It does not produce a distinct "notch" as seen with the return of spontaneous breathing.
Question 3: Which of the following conditions does not cause a rise in end-tidal CO2 during thyroid surgery?
- A. Anaphylaxis (Correct Answer)
- B. Thyroid storm
- C. Neuroleptic malignant syndrome
- D. Malignant hyperthermia
Explanation: **Anaphylaxis** - During anaphylaxis, there is often **bronchospasm** and **hypoventilation**, leading to a *decrease* or no change in end-tidal CO2 due to reduced gas exchange. - While it can cause cardiovascular collapse, the primary respiratory effect that impacts ETCO2 measurement is typically decreased CO2 excretion rather than increased production. *Malignant hyperthermia* - This condition is characterized by a rapid, uncontrolled increase in **metabolism** and **muscle rigidity**, resulting in massive **CO2 production**. - The increased CO2 production overwhelms the ventilatory capacity, leading to a marked and rapid rise in **end-tidal CO2** despite increased minute ventilation. *Thyroid storm* - Thyroid storm causes a hypermetabolic state with increased **cellular oxygen consumption** and **CO2 production**. - The body's significantly elevated metabolic rate leads to higher CO2 levels that can be reflected in an elevated **end-tidal CO2**. *Neuroleptic malignant syndrome* - This syndrome involves severe **muscle rigidity** and a hypermetabolic state similar to malignant hyperthermia, although with a different etiology. - The uncontrolled muscle activity and enhanced cellular metabolism result in increased **CO2 production** and, consequently, a rise in **end-tidal CO2**.
Question 4: You are in the operating room and notice the tracing in yellow colour on this device. What does it indicate?
- A. O2 pressure in exhaled air
- B. Capnography (Correct Answer)
- C. O2 pressure in inhaled air
- D. Airway pressure
Explanation: ***Capnography*** - The yellow tracing displays a waveform that is characteristic of a **capnogram**, which measures the concentration of **carbon dioxide (CO2)** in the expired breath over time. - The rectangular shape with a sudden rise, plateau, and rapid fall is typical of the **CO2 waveform** during a respiratory cycle. *O2 pressure in exhaled air* - While oxygen levels can be monitored, the characteristic waveform shown with its distinct plateau phase is specific to **carbon dioxide** measurement. - Oxygen monitoring provides different types of waveforms or numerical values, such as **pulsus oximetry**, which shows oxygen saturation. *O2 pressure in inhaled air* - Monitoring devices typically display **inspired oxygen concentration (FiO2)** as a numerical value rather than a waveform. - The waveform shown is indicative of gas exchange dynamics during **exhalation**, not inhalation. *Airway pressure* - Airway pressure tracings typically show a waveform that correlates with the **inspiratory and expiratory phases** of breathing, indicating the pressure within the airway. - However, the specific shape and plateau of the waveform in yellow are distinct from typical **airway pressure** curves and are characteristic of CO2.
Question 5: Endotracheal tube in the esophagus is best assessed by:
- A. Direct laryngoscopy
- B. Auscultation
- C. CO2 Exhalation (Correct Answer)
- D. Chest wall movement
Explanation: ***CO2 Exhalation*** - Measuring **CO2 exhalation** (capnography) is the most reliable method to confirm endotracheal tube placement, as CO2 is present in the trachea but not in the esophagus. - A persistent **waveform on the capnograph** indicates proper tracheal intubation. *Direct laryngoscopy* - While helpful for initial visualization during intubation, **direct laryngoscopy** cannot confirm continuous tracheal placement after the tube is advanced. - It only confirms the tube passing through the vocal cords, not its final position in the trachea versus esophagus. *Auscultation* - **Auscultation** can be misleading because stomach sounds can be transmitted to the chest, and breath sounds can be heard in the epigastrium even with esophageal intubation. - It relies on subjective interpretation and is less definitive than capnography. *Chest wall movement* - Observing **chest wall movement** is not a definitive sign, as the chest can still rise with esophageal intubation due to air entering the stomach. - This method is unreliable and can be mistaken for proper ventilation, leading to dangerous delays in correcting tube misplacement.
Question 6: Capnography is useful for
- A. Determining Vaporizer malfunction or contamination
- B. Determining circuit hypoxia
- C. Detecting concentration of oxygen in the anesthetic circuit.
- D. Determining the appropriate placement of endotracheal (Correct Answer)
Explanation: ***Determining the appropriate placement of endotracheal*** - Capnography provides a direct and continuous measurement of **exhaled CO2**, which confirms proper **endotracheal tube (ETT) placement** in the trachea. - The presence of a square-wave capnogram with a distinct end-tidal CO2 (ETCO2) value indicates CO2 detection, confirming the ETT is in the airway and not the esophagus. *Determining Vaporizer malfunction or contamination* - **Anesthetic gas analyzers**, not capnographs, are used to detect vaporizer malfunctions or contamination by measuring the concentration of specific anesthetic agents. - While a capnograph might show changes in CO2 if ventilation is affected by an issue with the vaporizer, it does not directly diagnose the vaporizer problem itself. *Determining circuit hypoxia* - **Oxygen analyzers** in the anesthetic circuit are used to determine the concentration of oxygen, which helps detect circuit hypoxia. - Capnography monitors CO2 levels, and while changes in CO2 might indirectly result from hypoxia, it doesn't directly measure oxygen concentration or alert to hypoxia. *Detecting concentration of oxygen in the anesthetic circuit.* - **Oxygen sensors or galvanic cells**, integrated into the anesthesia machine, are specifically designed to measure the inspired oxygen concentration. - Capnography measures carbon dioxide, not oxygen, and therefore cannot directly assess the oxygen levels within the anesthetic circuit.
Question 7: What causes sudden decreased end tidal CO2 in GA?
- A. Cardiac arrest (Correct Answer)
- B. Pulmonary embolism
- C. Pulmonary hypertension
- D. Malignant hyperthermia
Explanation: ***Cardiac arrest*** - In **cardiac arrest**, there is a sudden cessation of effective **cardiac output**, which leads to a dramatic reduction in pulmonary blood flow. - As a result, **CO2 is not transported to the lungs** for exhalation, causing an abrupt and severe drop in **end-tidal CO2**. *Pulmonary embolism* - A **pulmonary embolism** causes an acute obstruction of pulmonary arterial blood flow, leading to an **increase in alveolar dead space**. - While it can decrease **end-tidal CO2** due to reduced perfusion, the drop is often less sudden and complete than in cardiac arrest, and the primary mechanism is **ventilation-perfusion mismatch**. *Pulmonary hypertension* - **Pulmonary hypertension** involves chronically elevated pressures in the pulmonary arteries, which can lead to **right ventricular dysfunction** and reduced cardiac output over time. - It typically causes a more gradual and chronic reduction in **end-tidal CO2** due to impaired gas exchange, rather than a sudden, precipitous drop. *Malignant hyperthermia* - **Malignant hyperthermia** is characterized by a rapid and severe increase in **metabolic rate** and CO2 production. - This condition typically leads to a **sudden increase in end-tidal CO2** as the body produces more CO2 than can be eliminated, rather than a decrease.
Question 8: Modern monitors to measure ETCO2 make use of
- A. Laser technology
- B. Scatter technology
- C. Infrared absorption spectroscopy (Correct Answer)
- D. Ultra violet rays
Explanation: ***Infrared absorption spectroscopy*** - This technique relies on the principle that carbon dioxide molecules **absorb infrared light** at specific wavelengths. - A sensor measures the **amount of infrared light absorbed** by the exhaled gases, which is directly proportional to the CO2 concentration. *Laser technology* - While lasers are used in some advanced medical diagnostics, they are **not the primary technology** employed for routine ETCO2 monitoring. - Laser-based techniques for gas analysis are often more complex and **less commonly integrated** into portable or standard capnographs. *Scatter technology* - **Light scattering** methods are typically used to measure particle size or concentration in solutions or aerosols, not gas concentrations. - This technology is **not suitable for detecting specific gas molecules** like CO2 in a breath sample. *Ultra violet rays* - Carbon dioxide molecules **do not significantly absorb ultraviolet (UV) light** in a way that allows for reliable quantitative measurement in medical settings. - UV spectroscopy is used for detecting different types of molecules, but **infrared absorption is specific to CO2** for capnography.
Question 9: Which of the following tubes is used in surgery for cleft palate repair?
- A. Flexometallic tube
- B. RAE tube (Correct Answer)
- C. Robert shaw double lumen tube
- D. None of the above
Explanation: **Explanation:** The **RAE tube** (Ring-Adair-Elwyn) is the gold standard for cleft palate repair. Its design features a pre-formed bend that allows the tube to be directed away from the surgical field (downward over the chin for oral/palatal surgery). This provides the surgeon with an unobstructed view and prevents the tube from being compressed or kinked by the **Doughty or Dingman mouth gags** used during the procedure. **Analysis of Options:** * **RAE Tube (Correct):** Specifically designed for head and neck surgeries. The "South-facing" RAE tube is used for cleft palate to keep the breathing circuit out of the surgeon's way. * **Flexometallic (Armored) Tube:** While resistant to kinking, it lacks the pre-formed anatomical bend of the RAE tube. It is more commonly used in neurosurgery or surgeries where the head is flexed, but it can still obstruct the surgical field in cleft palate repairs. * **Robertshaw Double Lumen Tube:** This is a specialized tube used for **One-Lung Ventilation (OLV)** in thoracic surgery. It is far too large and complex for pediatric cleft palate repair and serves an entirely different purpose (isolating lungs). **High-Yield Clinical Pearls for NEET-PG:** * **North-facing RAE:** Used for ophthalmic or ENT surgeries (tube goes over the forehead). * **South-facing RAE:** Used for cleft lip/palate or intra-oral surgeries (tube goes over the chin). * **Murphy’s Eye:** The small hole at the tip of an ET tube that allows ventilation if the main lumen is obstructed by secretions or the tracheal wall. * **Magill Circuit:** The breathing system of choice for spontaneous ventilation in children, whereas **Bain’s circuit** (Mapleson D) is preferred for controlled ventilation.
Question 10: Bains' circuit is which type of Mapleson circuit?
- A. Type A
- B. Type B
- C. Type D (Correct Answer)
- D. Type E
Explanation: **Explanation:** The **Bain’s circuit** is a **coaxial modification of the Mapleson D circuit**. In this system, the fresh gas flow (FGF) tube is nested inside the wider corrugated expiratory limb. This design allows the inspired gas to be warmed by the exhaled gases (counter-current exchange) and makes the circuit lightweight and convenient for head and neck surgeries. **Analysis of Options:** * **Option C (Correct):** Mapleson D is characterized by having the FGF inlet near the patient end and the APL (pop-off) valve at the machine end. Bain’s circuit follows this exact configuration but in a coaxial format. It is the most efficient Mapleson circuit for **controlled ventilation**. * **Option A (Type A):** Also known as the **Magill circuit**. It is the most efficient for **spontaneous respiration** but inefficient for controlled ventilation. * **Option B (Type B):** Rarely used clinically today; it features the FGF and APL valve both located near the patient. * **Option D (Type E):** Also known as **Ayre’s T-piece**. It has no reservoir bag or valve and is primarily used for pediatric anesthesia (patients <20kg) to minimize resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Efficiency:** For spontaneous breathing, the order of efficiency is **A > D > B > C**. For controlled ventilation, it is **D > B > C > A**. (Mnemonic: **Dog Bites Can Always** help remember controlled ventilation efficiency). * **The Outer Tube:** In Bain's, the outer tube is for expiration; the inner tube is for inspiration. * **Safety Check:** The **Pethick Test** is used to check the integrity of the inner tube of a Bain's circuit. If the inner tube is disconnected or kinked, the patient will rebreathe exhaled gases, leading to hypercapnia.
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