Nitrogen Narcosis — MCQs
Diffusion hypoxia is seen with –
Gas commonly used for rapid induction of anesthesia
Which of the following statements about Nitrous Oxide (N2O) is true?
All of the following are used to maintain proper oxygen flow to the patient except:
The dead body of a 20 year old man found in the sea was brought in for postmortem examination. Which of the following findings would you see in seawater drowning? 1. Hemoconcentration 2. Pulmonary edema 3. Hyponatremia 4. Hypernatremia 5. Hemolysis
All of the following causes decrease in CMRO2, CBF and ICP except:-
Second gas effect is exaggerated by which of the following gases when co-administered with halothane?
A patient undergoing a minor surgical procedure is given lignocaine injection. Assertion: Local anaesthetics act by blocking nerve conduction. Reason: Small fibers and non-myelinated fibers are blocked more easily than large myelinated fibers.
What type of narcosis is primarily associated with increased nitrogen solubility under pressure?
In which context can helium replace nitrogen as a diluent gas in oxygen mixtures?
Nitrogen Narcosis Indian Medical PG Practice Questions and MCQs
Question 1: Diffusion hypoxia is seen with –
- A. Cyclopropane
- B. Halothane
- C. Nitrous oxide (Correct Answer)
- D. Ether
Explanation: ***Nitrous oxide*** - **Diffusion hypoxia** occurs when **nitrous oxide** rapidly diffuses out of the blood and into the alveoli during elimination, diluting the partial pressures of oxygen and carbon dioxide. - This rapid diffusion can lead to a transient decrease in the **partial pressure of oxygen** in the alveoli, potentially causing hypoxia if patients are not given supplemental oxygen during recovery. *Cyclopropane* - **Cyclopropane** is an older, potent inhalational anesthetic not commonly used today due to its **flammability** and high risk of **cardiac arrhythmias**. - It does not cause diffusion hypoxia as its blood solubility and elimination characteristics differ significantly from nitrous oxide. *Halothane* - **Halothane** is a potent volatile anesthetic that can cause **dose-dependent myocardial depression** and **hepatotoxicity**. - Its elimination primarily occurs via the lungs, but its relatively high blood solubility and slower wash-out prevent the rapid alveolar gas dilution seen with nitrous oxide, thus not causing diffusion hypoxia. *Ether* - **Ether** (**diethyl ether**) is another historical anesthetic agent known for its strong analgesic properties and slow induction/recovery. - While it has a high blood-gas solubility, its slower kinetics of elimination do not lead to the rapid outward diffusion phenomenon that causes diffusion hypoxia.
Question 2: Gas commonly used for rapid induction of anesthesia
- A. Nitrous oxide
- B. Sevoflurane (Correct Answer)
- C. Isoflurane
- D. Desflurane
Explanation: ***Sevoflurane*** - Sevoflurane has a **low blood:gas partition coefficient**, leading to rapid equilibration between the lungs and blood, and thus rapid induction and emergence from anesthesia. - Its **non-pungent odor** and lack of airway irritation make it particularly suitable for children and adults requiring mask induction. *Nitrous oxide* - While it has a very low blood:gas partition coefficient, it is a **relatively weak anesthetic** and cannot achieve surgical depths of anesthesia on its own. - It is often used as an **adjunct** to other volatile anesthetics to reduce their required dose and speed up induction. *Isoflurane* - Isoflurane has a **higher blood:gas partition coefficient** compared to sevoflurane, resulting in a slower induction and emergence time. - Its **pungent odor** can cause coughing and airway irritation, making it less ideal for mask induction. *Desflurane* - Desflurane has the **lowest blood:gas partition coefficient** among the volatile anesthetics, leading to very rapid induction and emergence. - However, its **irritating effect on the airway** often causes coughing and laryngospasm, making it unsuitable for mask induction, especially in non-premedicated patients.
Question 3: Which of the following statements about Nitrous Oxide (N2O) is true?
- A. Least potent inhalational anesthetic (Correct Answer)
- B. Lighter than air
- C. Effective muscle relaxant
- D. Does not cause diffusion hypoxia
Explanation: **Least potent inhalational anesthetic** - Nitrous oxide has a **high Minimum Alveolar Concentration (MAC)** of approximately 104%, making it the least potent of the commonly used inhalational anesthetics. - Its high MAC means a very high concentration is required to achieve surgical anesthesia, which is why it is typically used as an adjunct to more potent agents. *Lighter than air* - The molecular weight of nitrous oxide (N2O) is 44, which is **heavier than air** (average molecular weight approximately 29 g/mol). - Its density is greater than air, meaning it would tend to sink rather than rise. *Effective muscle relaxant* - Nitrous oxide provides **minimal to no skeletal muscle relaxation** benefits. - If muscle relaxation is required, a neuromuscular blocking agent must be administered separately. *Does not cause diffusion hypoxia* - Nitrous oxide rapidly diffuses out of the blood into the alveoli during emergence, diluting the oxygen and carbon dioxide there. - This rapid diffusion can lead to **diffusion hypoxia** (also known as the "second gas effect"), necessitating the administration of 100% oxygen during recovery to prevent this complication.
Question 4: All of the following are used to maintain proper oxygen flow to the patient except:
- A. Different pin index for nitrogen and oxygen (Correct Answer)
- B. A proportioner between N₂ and O₂ control valves
- C. Calibrated oxygen concentration analyzers
- D. Placement of oxygen flowmeter downstream of the nitrogen flowmeter
Explanation: ***Different pin index for nitrogen and oxygen*** - Oxygen and nitrogen *do not* use pin index safety systems; the **Pin Index Safety System (PISS)** is used for small gas cylinders to prevent wrong gas connection, but nitrogen is a non-medical gas. - While medical gases have specific pin index patterns, this system is for preventing inadvertent connection of gas cylinders to the wrong yoke, not for *maintaining proper oxygen flow to the patient* from the anesthesia machine's internal system. *A proportioner between N₂ and O₂ control valves* - This device, such as the **Ohio proportioner** or **Link 25 system**, mechanically or pneumatically links the **nitrous oxide (N₂O)** and **oxygen (O₂)** flow controls. - It ensures that the inspired oxygen concentration never falls below a preset safe level, typically 25%, thereby **preventing hypoxic gas mixtures**. *Calibrated oxygen concentration analyzers* - **Oxygen analyzers** continuously monitor the inspired oxygen concentration and provide an audible and visual alarm if the level deviates from the set range. - This serves as a critical safety measure to detect and alert anesthesia providers to **hypoxic gas delivery** or machine malfunctions. *Placement of oxygen flowmeter downstream of the nitrogen flowmeter* - Positioning the **oxygen flowmeter downstream** (closest to the patient) of all other gas flowmeters (e.g., nitrous oxide, air) is a crucial safety feature. - This design ensures that **any leak occurring upstream** of the oxygen flow tube will primarily affect other gases, reducing the risk of an **undetected hypoxic mixture** reaching the patient.
Question 5: The dead body of a 20 year old man found in the sea was brought in for postmortem examination. Which of the following findings would you see in seawater drowning? 1. Hemoconcentration 2. Pulmonary edema 3. Hyponatremia 4. Hypernatremia 5. Hemolysis
- A. 2,4 and 5
- B. 2,3 and 5
- C. 1 and 3
- D. 1 and 4 (Correct Answer)
Explanation: ***1 and 4*** - In **seawater drowning**, the hypertonic seawater (3-4% salt) creates an osmotic gradient that pulls plasma fluid from the blood into the alveoli, leading to **hemoconcentration**. - The absorption of hypertonic seawater into the bloodstream results in **hypernatremia** and hyperchloremia. - These are the characteristic electrolyte and hematologic findings specific to seawater drowning. *2, 4 and 5* - While **hypernatremia** is correct, this option incorrectly includes **pulmonary edema** (which is non-specific to drowning type) and **hemolysis** (which is characteristic of freshwater drowning, not seawater). - **Hemolysis** occurs in freshwater drowning when hypotonic water enters the bloodstream, causing red blood cells to swell and lyse. *2, 3 and 5* - This option is incorrect as it includes findings characteristic of **freshwater drowning**: **hyponatremia** and **hemolysis**. - In freshwater drowning, hypotonic water absorption causes hemodilution (not hemoconcentration), leading to hyponatremia and hemolysis. - **Pulmonary edema** is present in both types but does not differentiate between them. *1 and 3* - While **hemoconcentration** is correct for seawater drowning, **hyponatremia** is incorrect. - Hyponatremia is a feature of freshwater drowning due to dilution of serum sodium by absorbed hypotonic water.
Question 6: All of the following causes decrease in CMRO2, CBF and ICP except:-
- A. Etomidate
- B. Propofol
- C. Thiopentone
- D. Ketamine (Correct Answer)
Explanation: ***Ketamine*** - Ketamine is a dissociative anesthetic that typically causes an **increase in cerebral blood flow (CBF)** and **intracranial pressure (ICP)**, while its effect on cerebral metabolic rate of oxygen (CMRO2) can be variable but often does not decrease significantly. - It works by antagonizing **NMDA receptors**, inducing a state of dissociation rather than global cerebral depression. *Etomidate* - Etomidate is an anesthetic agent that effectively **decreases CMRO2, CBF, and ICP**, making it suitable for neurosurgical procedures. - Its mechanism involves enhancing **GABA-A receptor activity**, leading to global central nervous system depression. *Propofol* - Propofol is a commonly used intravenous anesthetic that significantly **reduces CMRO2, CBF, and ICP**. - It primarily acts on **GABA-A receptors** to induce sedation and anesthesia, making it a good choice for patients with elevated ICP. *Thiopentone* - Thiopentone, a barbiturate, is known to produce a dose-dependent decrease in **CMRO2, CBF, and ICP**. - It also enhances **GABA-A receptor-mediated inhibition**, resulting in cerebral vasoconstriction and metabolic suppression.
Question 7: Second gas effect is exaggerated by which of the following gases when co-administered with halothane?
- A. Nitrous oxide (Correct Answer)
- B. Cyclopropane
- C. Nitrogen (inert gas)
- D. Helium (low solubility)
Explanation: ***Nitrous oxide*** - **Nitrous oxide** has a high partial pressure and low blood solubility, leading to its rapid uptake into the blood and then into the alveoli. - This rapid absorption concentrates the remaining inspired gases, including halothane, thereby exacerbating the **second gas effect** and leading to a more rapid increase in halothane's partial pressure in the alveoli. *Cyclopropane* - While cyclopropane is an anesthetic gas with quick induction, it is no longer widely used and its physiological interaction with other gases in exaggerating the second gas effect is not as pronounced or clinically relevant as that of nitrous oxide. - It has a low MAC and high potency, but its role in the **second gas effect** is not primarily due to its rapid tissue uptake and concentration of other gases in the same manner as nitrous oxide. *Nitrogen (inert gas)* - Nitrogen is an **inert gas** and does not readily participate in physiological processes or anesthetic effects. - Its presence in the inspired gas mixture does not significantly affect the uptake of other anesthetic gases to exaggerate the **second gas effect**. *Helium (low solubility)* - Helium has very **low blood solubility** but is not an anesthetic gas. - Although its low solubility might suggest rapid movement, it would not concentrate other anesthetic gases or directly contribute to the **second gas effect** in the manner of nitrous oxide, which is itself an anesthetic agent with high partial pressure.
Question 8: A patient undergoing a minor surgical procedure is given lignocaine injection. Assertion: Local anaesthetics act by blocking nerve conduction. Reason: Small fibers and non-myelinated fibers are blocked more easily than large myelinated fibers.
- A. Assertion is false, but Reason is true
- B. Both Assertion and Reason are true, and Reason is not the correct explanation for Assertion (Correct Answer)
- C. Both Assertion and Reason are true, and Reason is the correct explanation for Assertion
- D. Assertion is true, but Reason is false
Explanation: ***Both Assertion and Reason are true, and Reason is NOT the correct explanation for Assertion*** - The **Assertion** is true: Local anesthetics (like lignocaine) block nerve conduction by inhibiting **voltage-gated sodium channels**, preventing the depolarization necessary for action potential propagation - The **Reason** is also true: Small diameter and non-myelinated fibers (like C and Aδ pain fibers) are blocked more easily than large myelinated fibers (like Aα motor fibers), which explains the **differential blockade** pattern seen clinically - However, the **Reason does NOT explain WHY** local anesthetics block nerve conduction—it describes **WHICH** nerve fibers are blocked preferentially. The mechanism of blocking conduction is sodium channel inhibition, not fiber size selectivity - The differential sensitivity is a consequence of fiber characteristics (surface area-to-volume ratio, number of nodes of Ranvier), not the explanation for the blocking mechanism itself *Both Assertion and Reason are true, and Reason is the correct explanation for Assertion* - While both statements are individually true, the Reason does not explain the **mechanism** by which local anesthetics block nerve conduction - The Reason addresses fiber **selectivity**, which is a separate pharmacological property from the **mechanism of action** (sodium channel blockade) *Assertion is true, but Reason is false* - The Assertion is demonstrably true—local anesthetics block nerve conduction - The Reason is also true—this is well-established pharmacology: autonomic (small) > sensory (medium) > motor (large) fiber blockade sequence *Assertion is false, but Reason is true* - The Assertion is fundamentally correct and represents the primary pharmacological action of local anesthetics - Blocking nerve conduction is the therapeutic goal of local anesthetic administration
Question 9: What type of narcosis is primarily associated with increased nitrogen solubility under pressure?
- A. CO narcosis
- B. CO2 narcosis
- C. N2 narcosis (Correct Answer)
- D. O2 toxicity
Explanation: ***N2 narcosis*** - **Nitrogen narcosis**, also known as **inert gas narcosis** or **depth intoxication**, is caused by the increased partial pressure and resulting increased solubility of nitrogen in body tissues, particularly the brain, at depth. - This leads to altered mental states, similar to alcohol intoxication, including impaired judgment, confusion, and euphoria, posing significant risks to divers. *CO narcosis* - **Carbon monoxide (CO) narcosis** is a rare condition that would only occur if the air supply being breathed by the diver was contaminated with CO. - CO poisoning results from carbon monoxide binding to **hemoglobin** with high affinity, forming **carboxyhemoglobin** and reducing the oxygen-carrying capacity of the blood, leading to tissue hypoxia. *CO2 narcosis* - **Carbon dioxide (CO2) narcosis** occurs due to an excessive buildup of carbon dioxide in the body, which can happen if a diver hypoventilates or if breathing equipment malfunctions, leading to inadequate removal of CO2. - Symptoms include headache, confusion, drowsiness, and in severe cases, loss of consciousness; however, it is not primarily due to increased gas solubility in an inert gas context but rather an imbalance in respiratory gas exchange. *O2 toxicity* - **Oxygen toxicity** is a condition caused by breathing high partial pressures of oxygen for prolonged periods, which can lead to damage in various organ systems, including the central nervous system (CNS) and lungs. - This is a distinct phenomenon from narcosis, where the physiological effects are primarily due to the toxic effects of oxygen on cellular function rather than the inert gas properties of nitrogen dissolving in tissues.
Question 10: In which context can helium replace nitrogen as a diluent gas in oxygen mixtures?
- A. Argon
- B. Xenon
- C. Helium
- D. None of the options (Correct Answer)
Explanation: **None of the options** - This question implies that helium might replace *another noble gas* as a diluent, but the correct application is for helium to replace **nitrogen** in oxygen mixtures, particularly in **diving applications**. This question likely has a flaw in its premise if expecting one of the noble gases listed to be the 'replacement' for nitrogen, as helium *is* the replacement. - Helium is used instead of nitrogen in diving gases (**trimix, heliox**) for deep dives because it is less narcotic than nitrogen under pressure, reducing the risk of **nitrogen narcosis**. *Argon* - **Argon** is denser than nitrogen and has a higher narcotic potential at depth, making it unsuitable as a replacement for nitrogen in diving gases. - It is sometimes used during **dry suit inflation** for insulation due to its low thermal conductivity, but not as a breathing gas diluent. *Xenon* - **Xenon** is a potent anesthetic agent, even at atmospheric pressure, due to its high lipid solubility. - Its use as a diluent would cause severe **narcosis** and render a diver unconscious, making it entirely inappropriate for diving mixtures. *Helium* - While helium is indeed the gas that replaces nitrogen as a diluent in oxygen mixtures for deep diving, it being listed as an option here suggests a misunderstanding of the question's phrasing. The question is asking for **in which context** helium can replace nitrogen, not asking to identify helium itself as the replacement. - Given the other options are noble gases that *cannot* replace nitrogen in this context, "None of the options" is the most accurate answer if the question implies picking from the provided list for a replacement *for helium* or a suitable *alternative* to helium, which isn't the case here.
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