Preoxygenation Techniques — MCQs
According to neonatal resuscitation protocol, how much oxygen to give in a term neonate with apnea and bradycardia initially?
Which of the following is required for the Direct Fick method of measuring cardiac output?
Highest concentration of oxygen is delivered through?
Patient with BMI 40 presents for emergency surgery. All are correct about airway management EXCEPT:
Functional residual capacity (FRC) is defined as the volume of air remaining in the lungs at which specific moment in the respiratory cycle?
Time required for pre-oxygenation before tracheal intubation:
What is the most reliable indicator to prevent esophageal intubation?
Which of the following parameters is most critical for maintaining optimal oxygenation?
During rapid sequence intubation in a child after taking brief history and clinical examination next step is:
A 25 year old male is undergoing incision and drainage of abscess under general anaesthesia with spontaneous respiration. Which of the following is the most efficient anaesthetic circuit which can be used in this patient?
Preoxygenation Techniques Indian Medical PG Practice Questions and MCQs
Question 1: According to neonatal resuscitation protocol, how much oxygen to give in a term neonate with apnea and bradycardia initially?
- A. 100%
- B. 21% (Correct Answer)
- C. 50%
- D. 30%
Explanation: ***21%*** - According to **NRP (Neonatal Resuscitation Program) 2020 guidelines**, for **term neonates (≥35 weeks gestation)** requiring resuscitation, the initial recommendation is to use **room air (21% oxygen)** to minimize the risk of hyperoxia and oxidative injury. - Multiple randomized controlled trials have demonstrated that room air is as effective as 100% oxygen for initial resuscitation. - Supplemental oxygen is only added if **oxygen saturation targets** are not met despite adequate ventilation, and should be titrated using **pulse oximetry**. *30%* - This concentration is **higher than room air** and is not the initial recommendation for term neonates needing resuscitation. - Starting with a higher oxygen concentration can lead to **oxidative stress** without immediate benefit. - Higher initial concentrations (21-30%) are reserved for **preterm neonates (<35 weeks)**. *100%* - Administering **100% oxygen** can be harmful to a neonate, potentially causing **oxidative injury** to developing organs, including the lungs, brain, and retina. - This was the old practice but has been **discontinued** based on evidence showing increased mortality and morbidity. - High concentrations are no longer recommended even in severe cases; oxygen should be titrated to saturation targets. *50%* - While lower than 100%, 50% oxygen is still **not the initial recommended concentration** for term neonates in resuscitation protocols. - The goal is to start with **21% oxygen** and gradually increase based on **pulse oximetry monitoring** and target saturation ranges if 21% is insufficient.
Question 2: Which of the following is required for the Direct Fick method of measuring cardiac output?
- A. O2 content of arterial blood
- B. O2 consumption per unit time
- C. O2 content of venous blood
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - The **Direct Fick method** calculates **cardiac output (CO)** using the formula: **CO = VO₂ / (CaO₂ - CvO₂)**, where VO₂ is oxygen consumption, CaO₂ is arterial oxygen content, and CvO₂ is mixed venous oxygen content. - Therefore, all three measurements—**O₂ content of arterial blood**, **O₂ consumption per unit time**, and **O₂ content of venous blood**—are essential components required for this calculation. - Each component plays a critical role in determining cardiac output: **O₂ content of arterial blood (CaO₂)** - Represents the oxygen delivered by the **arterial circulation** to the tissues - Essential for calculating the **arteriovenous oxygen difference (A-V O₂ difference)**, which reflects oxygen extraction by tissues - Typically measured from a systemic arterial sample **O₂ consumption per unit time (VO₂)** - Measures the body's **total oxygen utilization** per minute - Typically obtained through **spirometry** or metabolic cart measurements - Forms the **numerator** of the Fick equation, representing total oxygen uptake by tissues **O₂ content of venous blood (CvO₂)** - Indicates the **oxygen remaining in the blood** after tissue extraction - Must be measured from **mixed venous blood** (typically from pulmonary artery via right heart catheterization) - Combined with arterial O₂ content to determine the **A-V O₂ difference** (denominator of the equation) *Why other individual options are incomplete* - Selecting only one or two components would provide insufficient data to calculate cardiac output using the Direct Fick principle - The method fundamentally requires measuring both oxygen delivery (arterial content) and return (venous content), plus total consumption, to determine flow rate
Question 3: Highest concentration of oxygen is delivered through?
- A. Bag and mask
- B. Venturi mask
- C. Nasal cannula
- D. Mask with reservoir (Correct Answer)
Explanation: ***Mask with reservoir*** - A mask with a reservoir bag, particularly a **non-rebreather mask**, delivers the highest concentration of oxygen (up to 95-100%). - The **reservoir bag** and **one-way valves** prevent entrainment of room air and re-breathing of exhaled CO2, maximizing oxygen delivery. *Bag and mask* - While capable of delivering high oxygen concentrations, its effectiveness is highly dependent on a **proper seal** and the technique of the rescuer. - Its primary role is for **manual ventilation** rather than sustained high-concentration oxygen delivery alone. *Venturi mask* - The Venturi mask is known for delivering **precise and controlled oxygen concentrations**, not necessarily the highest. - It uses a jet of oxygen to entrain fixed amounts of room air, maintaining a **consistent FiO2** (fraction of inspired oxygen). *Nasal cannula* - A nasal cannula delivers relatively **low concentrations of oxygen** (24-44%), as it mixes with a large volume of room air. - It is suitable for **mild to moderate hypoxemia** but cannot provide the high FiO2 needed in critical situations.
Question 4: Patient with BMI 40 presents for emergency surgery. All are correct about airway management EXCEPT:
- A. Extended ramping
- B. Avoid cricoid pressure (Correct Answer)
- C. Rapid sequence induction
- D. Avoid preoxygenation
Explanation: ***Avoid cricoid pressure*** - While **cricoid pressure** (Sellick's maneuver) is used to prevent **aspiration** by compressing the esophagus, its effectiveness in **obese patients** is highly debated and often hindered by excess neck tissue. - In obese patients, cricoid pressure can actually worsen the view during laryngoscopy, making intubation more difficult and potentially causing airway trauma. *Extended ramping* - **Ramping** the patient, where the head and shoulders are elevated, is crucial in **obese patients** to align the **oral, pharyngeal, and laryngeal axes**. - This position improves the view during laryngoscopy and facilitates successful intubation by effectively displacing excess tissue. *Rapid sequence induction* - **Rapid sequence induction (RSI)** is often indicated in **obese patients** undergoing emergency surgery due to their increased risk of **gastric reflux** and **pulmonary aspiration**. - RSI involves administering a sedative and a paralytic agent in rapid succession, followed immediately by intubation, to minimize the time the airway is unprotected. *Avoid preoxygenation* - **Preoxygenation** is essential in **obese patients** to maximize their **oxygen reserves** before intubation. - Obese patients have reduced **functional residual capacity (FRC)** and increased **oxygen consumption**, making them desaturate rapidly during apnea, so preoxygenation significantly prolongs safe apnea time.
Question 5: Functional residual capacity (FRC) is defined as the volume of air remaining in the lungs at which specific moment in the respiratory cycle?
- A. During active expiration
- B. After normal expiration (Correct Answer)
- C. At peak inspiration
- D. During active inspiration
Explanation: ***After normal expiration*** - **Functional residual capacity (FRC)** is the volume of air remaining in the lungs at the end of a **normal, passive expiration**. - It represents the sum of the **expiratory reserve volume (ERV)** and the **residual volume (RV)**. *During active expiration* - **Active expiration** involves the use of accessory muscles to force more air out of the lungs than during normal expiration. - This process would result in a lung volume less than FRC, closer to the **residual volume**. *At peak inspiration* - **Peak inspiration** represents the total lung capacity (TLC), which is the maximum volume of air the lungs can hold after a maximal inspiratory effort. - This is the largest lung volume, significantly greater than FRC. *During active inspiration* - **Active inspiration** is the process of inhaling air, which increases lung volume. - FRC is a static volume measured at the end of expiration, not during the dynamic process of inhaling.
Question 6: Time required for pre-oxygenation before tracheal intubation:
- A. 1 min
- B. 5 min
- C. 3 min (Correct Answer)
- D. 2 min
Explanation: ***3 min*** - Pre-oxygenation typically involves administering 100% oxygen for **3 minutes** via a tight-fitting face mask. - This duration allows for **denitrogenation** of the functional residual capacity (FRC), replacing nitrogen with oxygen to create an oxygen reserve. *1 min* - **One minute** of pre-oxygenation is generally insufficient to adequately denitrogenate the FRC, especially in patients with normal respiratory function. - This duration would lead to a shorter safe apnea time and increased risk of **hypoxemia** during intubation. *5 min* - While 5 minutes of pre-oxygenation provides a slightly larger oxygen reserve, it is usually not necessary and offers little additional benefit over **3 minutes** in a healthy adult. - Prolonged pre-oxygenation can be less practical in emergency settings and could potentially delay intubation without significant clinical advantage. *2 min* - **Two minutes** of pre-oxygenation may provide some benefit but is generally considered suboptimal compared to 3 minutes for maximizing the **oxygen reserve**. - Healthy individuals can typically be safely intubated after 2 minutes of pre-oxygenation, but 3 minutes allows for a more robust safety margin.
Question 7: What is the most reliable indicator to prevent esophageal intubation?
- A. Oxygen saturation on pulse oximeter
- B. Direct visualization of passing tube beneath vocal cords
- C. Auscultation over chest
- D. Measurement of CO2 in exhaled air (EtCO2). (Correct Answer)
Explanation: ***Measurement of CO2 in exhaled air (EtCO2)*** - The presence of **carbon dioxide** in exhaled air confirms tracheal intubation as the esophagus does not contain CO2. - This method provides a **real-time**, objective assessment of tube placement that is highly reliable because even small amounts of CO2 are detected. *Oxygen saturation on pulse oximeter* - This indicator measures **oxygenation**, which can remain adequate for several minutes after esophageal intubation due to pre-oxygenation. - A **delayed drop in saturation** might indicate esophageal intubation, but it's not immediate and therefore not the most reliable early indicator. *Direct visualization of passing tube beneath vocal cords* - While helpful, **direct visualization** can sometimes be misleading due to difficult airways or poor visibility, where the tube might appear to pass correctly but enter the esophagus. - This method is **operator-dependent** and its reliability can vary based on the intubator's experience and the patient's anatomy. *Auscultation over chest* - **Auscultation** can detect breath sounds; however, sounds can be transmitted from the stomach or surrounding tissues, leading to false positives. - It is also very difficult to reliably distinguish between **esophageal and tracheal sounds**, especially in noisy environments or with inexperienced personnel, making it less reliable than EtCO2.
Question 8: Which of the following parameters is most critical for maintaining optimal oxygenation?
- A. FiO2
- B. Respiratory rate
- C. PEEP (Correct Answer)
- D. Tidal volume
Explanation: ***PEEP*** - **Positive End-Expiratory Pressure (PEEP)** is crucial for maintaining optimal oxygenation because it prevents **alveolar collapse** at the end of expiration, thereby increasing the **functional residual capacity** and improving gas exchange. - By keeping alveoli open, PEEP increases the number of available alveoli for ventilation, preventing **atelectasis** and optimizing the **venous admixture** from non-ventilated lung units. *FiO2* - While **Fraction of Inspired Oxygen (FiO2)** is essential for providing sufficient oxygen, simply increasing FiO2 without proper alveolar recruitment and patency (often achieved with PEEP) can be less effective and potentially harmful due to **oxygen toxicity**. - High FiO2 can improve oxygenation in cases of **hypoxemia**, but it doesn't address underlying problems like **alveolar collapse** or **ventilation-perfusion mismatch** as directly as PEEP does. *Respiratory rate* - **Respiratory rate** primarily affects **carbon dioxide elimination** (PaCO2) and, to some extent, alveolar ventilation. - While an adequate respiratory rate is necessary for overall gas exchange, it is not the most direct or critical parameter for optimizing **oxygenation** compared to PEEP's role in maintaining alveolar patency. *Tidal volume* - **Tidal volume** also primarily affects **carbon dioxide elimination** and plays a role in overall minute ventilation. - Excessive tidal volume can lead to **ventilator-induced lung injury (VILI)**, while insufficient tidal volume can reduce minute ventilation, but it does not directly optimize oxygenation by preventing **alveolar collapse** in the same way PEEP does.
Question 9: During rapid sequence intubation in a child after taking brief history and clinical examination next step is:
- A. Administer oxygen (Correct Answer)
- B. Analgesic injection with Fentanyl
- C. Preanaesthetic medication with atropine and lignocaine
- D. IV anesthetic Diazepam/Ketamine
Explanation: ***Administer oxygen*** - Pre-oxygenation with 100% oxygen is critical before **rapid sequence intubation (RSI)** to maximize **oxygen reserves** and extend the safe apnea time. - This step helps prevent **hypoxemia** during the intubation procedure, especially in children who have lower functional residual capacity. *Analgesic injection with Fentanyl* - While fentanyl is often used in RSI for its **analgesic** and **sedative properties**, it typically follows pre-oxygenation and is administered as part of the **induction phase**, often concurrently with a paralytic. - Administering fentanyl alone without prior oxygenation or other induction agents would not be the immediate next step in a structured RSI protocol. *Preanaesthetic medication with atropine and lignocaine* - **Atropine** may be used in children to prevent **bradycardia** during intubation, particularly in infants, but it's not the immediate next step after initial assessment; pre-oxygenation is more critical. - **Lidocaine** can be used to blunt the sympathetic response to intubation or to suppress cough, but it's not universally required and comes after pre-oxygenation and other induction medications. *IV anesthetic Diazepam/Ketamine* - **Diazepam** and **ketamine** are **induction agents** that cause sedation and loss of consciousness, but they are administered after pre-oxygenation and often just before the paralytic agent. - Administering an induction agent without adequate pre-oxygenation would increase the risk of **hypoxemia** during the subsequent apnea.
Question 10: A 25 year old male is undergoing incision and drainage of abscess under general anaesthesia with spontaneous respiration. Which of the following is the most efficient anaesthetic circuit which can be used in this patient?
- A. Mapleson A (Correct Answer)
- B. Mapleson D
- C. Mapleson B
- D. Mapleson C
Explanation: ***Mapleson A*** - This circuit is highly efficient for **spontaneous respiration** due to its design, which effectively sweeps away exhaled CO2 with a low fresh gas flow. - The **reservoir bag** is close to the patient, and the APL valve is at the machine end, preventing rebreathing of carbon dioxide during spontaneous breathing. *Mapleson D* - While versatile, the Mapleson D circuit is considered far more efficient for **controlled ventilation** rather than spontaneous respiration, requiring higher fresh gas flows in the latter. - It features the **APL valve near the patient** and a longer expiratory limb, leading to potential rebreathing of CO2 if fresh gas flows are not sufficiently high during spontaneous breathing. *Mapleson B* - This circuit is generally considered **inefficient for both spontaneous and controlled ventilation** compared to other Mapleson systems. - The fresh gas inlet and APL valve are both near the patient, leading to significant rebreathing unless very high fresh gas flows are used. *Mapleson C* - Similar to Mapleson B, the Mapleson C circuit is also considered **inefficient for spontaneous respiration**, requiring high fresh gas flows to prevent CO2 rebreathing. - It has a very short expiratory limb and the APL valve near the patient, making it less effective for maintaining normocapnia during spontaneous breathing compared to Mapleson A.
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