ABG Interpretation in Complex Medical Conditions: 25 Multiple Choice Questions
Questions 1-5: Basic ABG Interpretation
Question 1: A 68-year-old patient with COPD presents with acute dyspnea. ABG shows: pH 7.32, PaCO2 58 mmHg, HCO3 28 mEq/L, PaO2 62 mmHg on room air. What is the primary acid-base disturbance?
A) Metabolic acidosis
B) Respiratory acidosis
C) Mixed respiratory and metabolic acidosis
D) Respiratory alkalosis
Correct Answer: B) Respiratory acidosis
Explanation: The pH < 7.35 indicates acidemia, and PaCO2 > 45 mmHg confirms the respiratory component as the primary disturbance 1. The elevated bicarbonate (28 mEq/L, normal 22-26) suggests partial metabolic compensation, which is expected in chronic respiratory acidosis seen in COPD patients 1. This patient requires controlled oxygen therapy targeting SpO2 88-92% and urgent consideration for non-invasive ventilation if pH remains < 7.35 despite optimal medical therapy 1.
Question 2: In a critically ill patient with shock and systolic BP 75 mmHg, what is the preferred method for obtaining blood gas samples?
A) Venous blood gas from any peripheral vein
B) Arterial blood gas from radial artery
C) Capillary blood gas from earlobe
D) Central venous blood gas
Correct Answer: B) Arterial blood gas from radial artery
Explanation: For critically ill patients or those with shock or hypotension (systolic BP <90 mmHg), the initial blood gas measurement must be obtained from an arterial sample 2. While earlobe blood gases may be acceptable for stable patients to measure pH and PCO2, they underestimate PO2 by 0.5-1 kPa and are unreliable in shock states 2. An Allen's test should be performed before radial artery puncture to confirm dual blood supply to the hand 3, 4.
Question 3: A 45-year-old diabetic patient presents with Kussmaul breathing. ABG shows: pH 7.18, PaCO2 22 mmHg, HCO3 8 mEq/L, PaO2 98 mmHg. What is the primary disorder?
A) Respiratory acidosis
B) Metabolic acidosis with respiratory compensation
C) Respiratory alkalosis
D) Mixed metabolic and respiratory acidosis
Correct Answer: B) Metabolic acidosis with respiratory compensation
Explanation: The pH < 7.35 indicates acidemia, and HCO3 < 22 mEq/L confirms metabolic acidosis as the primary disturbance 1. The low PaCO2 (22 mmHg, normal 35-45) represents appropriate respiratory compensation through hyperventilation (Kussmaul breathing) 1. This patient requires urgent evaluation for diabetic ketoacidosis, and ABG analysis is specifically recommended for patients with breathlessness at risk of metabolic conditions such as DKA 2, 3, 4. The normal PaO2 confirms adequate oxygenation, so oxygen therapy is not indicated unless the patient becomes hypoxemic 2.
Question 4: After initiating oxygen therapy in a COPD patient, when should repeat ABG be obtained?
A) Within 15 minutes
B) Within 30 minutes
C) Within 60 minutes
D) Within 4 hours
Correct Answer: C) Within 60 minutes
Explanation: ABG analysis should be performed within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration in patients with COPD or other risk factors for hypercapnic respiratory failure 3, 4. This timing allows assessment of whether adequate oxygenation has been achieved without precipitating respiratory acidosis or worsening hypercapnia 3, 4. After each titration of oxygen flow rate in patients with baseline hypercapnia, ABG must be repeated to monitor for CO2 retention 3, 1.
Question 5: A patient post-cardiac arrest on VA-ECMO has an ABG from the femoral arterial line showing: pH 7.42, PaCO2 38 mmHg, PaO2 450 mmHg. Where should the ABG sample ideally be obtained?
A) Femoral arterial line
B) Right radial arterial line
C) Left radial arterial line
D) Central venous catheter
Correct Answer: B) Right radial arterial line
Explanation: In peripherally cannulated ECMO patients, arterial blood gases should come from a right radial arterial line as this site best represents ascending aortic/innominate and thus cerebral perfusion 2. The femoral arterial line reflects well-oxygenated ECMO blood but does not represent cerebral oxygenation 2. In peripheral VA-ECMO, left ventricular ejection of inadequately oxygenated blood can create differential upper and lower torso oxygenation (Harlequin syndrome), occurring in approximately 10% of peripherally cannulated patients 2. A narrow pulse pressure from the right radial line suggests mixing proximal to the innominate artery, while wide pulse pressure indicates more distal mixing 2.
Questions 6-10: Oxygen Therapy and Target Saturations
Question 6: A 72-year-old with known COPD and previous episodes of hypercapnic respiratory failure presents with acute dyspnea. What is the initial target oxygen saturation before ABG results are available?
A) 94-98%
B) 88-92%
C) 85-88%
D) >95%
Correct Answer: B) 88-92%
Explanation: For patients with known COPD or other risk factors for hypercapnic respiratory failure, a target saturation range of 88-92% is recommended pending blood gas results 2, 3. This is a Grade A recommendation for COPD patients 2. Initial oxygen therapy should use a 24% Venturi mask at 2-3 L/min or 28% Venturi mask at 4 L/min or nasal cannulae at 1-2 L/min 2. The target range should be adjusted to 94-98% if the PCO2 is normal (unless there is history of previous NIV or invasive mechanical ventilation), with blood gases rechecked after 30-60 minutes 2.
Question 7: A 55-year-old with acute pulmonary embolism and SpO2 82% on room air has no history of lung disease. What is the appropriate initial oxygen therapy?
A) Nasal cannulae at 2 L/min
B) Simple face mask at 5 L/min
C) Reservoir mask at 15 L/min
D) 28% Venturi mask at 4 L/min
Correct Answer: C) Reservoir mask at 15 L/min
Explanation: For acutely breathless patients not at risk of hypercapnic respiratory failure who have saturations below 85%, treatment should be started with a reservoir mask at 15 L/min 2. The target saturation for patients without risk factors for hypercapnia is 94-98% 2. Once the patient stabilizes, oxygen concentration can be adjusted downwards using nasal cannulae at 1-6 L/min or simple face mask at 5-10 L/min to maintain target saturation 2. This patient requires urgent ABG analysis due to the unexpected fall in SpO2 below 94% 2, 3.
Question 8: A COPD patient on 28% Venturi mask has ABG showing: pH 7.38, PaCO2 48 mmHg, PaO2 58 mmHg, SpO2 89%. What is the next management step?
A) Increase to 35% Venturi mask
B) Continue current oxygen therapy
C) Decrease to 24% Venturi mask
D) Start non-invasive ventilation
Correct Answer: B) Continue current oxygen therapy
Explanation: This patient's ABG shows compensated respiratory acidosis with pH in normal range (7.38) and PaCO2 slightly elevated but stable 1. The SpO2 of 89% is within the target range of 88-92% for COPD patients, so current oxygen therapy should be continued 2, 3. Increasing oxygen concentration risks worsening hypercapnia, while decreasing it may cause dangerous hypoxemia 2. The target range should only be adjusted to 94-98% if PCO2 is normal, which is not the case here 2. Non-invasive ventilation is indicated when pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) persist despite optimal medical therapy 1.
Question 9: During cardiopulmonary resuscitation, what oxygen concentration should be delivered?
A) 28% via Venturi mask
B) Room air only
C) Highest possible inspired oxygen concentration
D) Titrated to SpO2 94-98%
Correct Answer: C) Highest possible inspired oxygen concentration
Explanation: The highest possible inspired oxygen concentration should be given during CPR until spontaneous circulation has been restored 2, 4. This is consistent with resuscitation guidelines 2. Once spontaneous circulation returns and reliable oximetry readings are available, oxygen can be quickly reduced while maintaining target saturation of 94-98% 2. Even patients with COPD and other risk factors for hypercapnia should receive the same initial high oxygen concentration during critical illness, with controlled oxygen therapy (target 88-92%) or supported ventilation initiated after return of spontaneous circulation based on blood gas results 2.
Question 10: A patient with paraquat poisoning has SpO2 91% on room air. What is the appropriate oxygen therapy?
A) Reservoir mask at 15 L/min targeting SpO2 >94%
B) Nasal cannulae targeting SpO2 94-98%
C) Avoid oxygen unless SpO2 falls below 85%, target 85-88%
D) 28% Venturi mask targeting SpO2 88-92%
Correct Answer: C) Avoid oxygen unless SpO2 falls below 85%, target 85-88%
Explanation: Patients with paraquat poisoning or bleomycin lung injury may be harmed by supplemental oxygen and should avoid oxygen unless hypoxemic, with a target saturation of 85-88% 2. This is one of the rare situations where oxygen therapy can be directly harmful 2. The mechanism involves oxygen-mediated free radical generation that worsens lung injury in these specific toxicities 2. Similarly, patients with pure hyperventilation due to anxiety are unlikely to require oxygen therapy, and rebreathing from a paper bag may cause hypoxemia and is not recommended 2.
Questions 11-15: Hypercapnic Respiratory Failure
Question 11: A COPD patient presents with pH 7.28, PaCO2 68 mmHg, HCO3 30 mEq/L, SpO2 86% on 28% Venturi mask. What is the most appropriate next step?
A) Increase oxygen to reservoir mask
B) Initiate non-invasive ventilation
C) Intubate immediately
D) Continue current therapy and recheck ABG in 4 hours
Correct Answer: B) Initiate non-invasive ventilation
Explanation: Non-invasive ventilation should be initiated when pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg, approximately 68 mmHg in this case) persist despite optimal medical therapy in acute hypercapnic respiratory failure 1. This patient's pH of 7.28 indicates severe respiratory acidosis requiring urgent intervention 1. NIV should be started with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 1. Target SpO2 should remain 88-92% in hypercapnic respiratory failure 1. ABG should be obtained prior to and following NIV initiation, with monitoring for worsening pH and respiratory rate indicating need to escalate to intubation 1.
Question 12: A patient on NIV for COPD exacerbation has repeat ABG after 2 hours showing: pH 7.26 (previously 7.28), PaCO2 72 mmHg (previously 68 mmHg), respiratory rate 38/min. What should be done?
A) Continue NIV and recheck in 2 hours
B) Increase NIV pressure support
C) Prepare for intubation
D) Add supplemental oxygen
Correct Answer: C) Prepare for intubation
Explanation: Criteria for intubation include worsening ABG/pH after 1-2 hours on NIV, lack of improvement after 4 hours of NIV, and respiratory rate >35 breaths/min 1. This patient demonstrates worsening acidosis (pH decreased from 7.28 to 7.26), rising PaCO2 (increased from 68 to 72 mmHg), and tachypnea >35/min, meeting multiple criteria for intubation 1. NIV use should not delay escalation to invasive mechanical ventilation when more appropriate 1. Severe acidosis alone does not preclude a trial of NIV, but it must be administered in an appropriate area with ready access to intubation 1.
Question 13: A morbidly obese patient with acute respiratory distress has ABG: pH 7.31, PaCO2 62 mmHg, HCO3 29 mEq/L, PaO2 55 mmHg on room air. What is the initial target SpO2?
A) 94-98%
B) 88-92%
C) 85-88%
D) >95%
Correct Answer: B) 88-92%
Explanation: Morbid obesity is a recognized risk factor for hypercapnic respiratory failure, and the target saturation range of 88-92% is recommended pending blood gas results 2. This is a Grade D recommendation for conditions other than COPD 2. The ABG confirms acute hypercapnic respiratory failure with pH 7.31 and PaCO2 62 mmHg 1. Initial oxygen therapy should use controlled delivery (24% or 28% Venturi mask or nasal cannulae at 1-2 L/min) rather than high-flow oxygen 2. This patient requires urgent consideration for NIV given pH < 7.35 and PaCO2 > 49 mmHg 1.
Question 14: After starting a COPD patient on oxygen therapy, ABG shows PaCO2 has risen by 9 mmHg (>1 kPa) compared to baseline. What does this indicate?
A) Normal expected response
B) Clinically unstable disease requiring further optimization
C) Need to increase oxygen flow rate
D) Indication for immediate intubation
Correct Answer: B) Clinically unstable disease requiring further optimization
Explanation: Patients who develop respiratory acidosis with a rise in PaCO2 >1 kPa (7.5 mmHg) during oxygen therapy may have clinically unstable disease and should undergo further medical optimization 3. A rise of 9 mmHg exceeds this threshold and indicates the patient is sensitive to oxygen therapy 3. Management should include optimizing bronchodilator therapy, treating infection if present, and considering NIV if pH falls below 7.35 1. For patients with persistent respiratory acidosis despite optimization, nocturnal ventilatory support should be considered 3. The oxygen flow rate should not be increased without addressing the underlying instability 2.
Question 15: A neuromuscular disease patient with progressive dyspnea has SpO2 92% on room air. What additional monitoring is most important?
A) Continuous pulse oximetry only
B) Spirometry and urgent ABG if saturation falls
C) Chest X-ray
D) Echocardiography
Correct Answer: B) Spirometry and urgent ABG if saturation falls
Explanation: Patients with acute and subacute neurological and muscular conditions producing muscle weakness require careful monitoring including spirometry, and if oxygen level falls below target saturation, they need urgent blood gas measurements and are likely to need ventilatory support 2. These patients are at risk for hypercapnic respiratory failure due to respiratory muscle weakness rather than intrinsic lung disease 2. A normal SpO2 does not negate the need for blood gas measurements, as pulse oximetry will be normal in patients with normal PO2 but abnormal pH or PCO2 2, 3. Spirometry helps assess respiratory muscle strength and vital capacity trends 2.
Questions 16-20: Critical Illness and Special Situations
Question 16: A patient with suspected carbon monoxide poisoning has SpO2 reading of 98% on pulse oximetry. What should be done?
A) No oxygen needed, SpO2 is normal
B) Give maximum oxygen via reservoir mask regardless of oximetry
C) Give controlled oxygen targeting SpO2 94-98%
D) Obtain ABG first before starting oxygen
Correct Answer: B) Give maximum oxygen via reservoir mask regardless of oximetry
Explanation: In carbon monoxide poisoning, give as much oxygen as possible using a bag-valve mask or reservoir mask, and a normal or high oximetry reading should be disregarded 2, 4. Pulse oximetry cannot differentiate between carboxyhaemoglobin and oxyhaemoglobin due to their similar absorbances, so readings may be falsely normal 2, 4. The blood gas PO2 will also be normal in these cases despite the presence of tissue hypoxia 2. Carboxyhaemoglobin levels must be checked to confirm diagnosis 2. Oxygen therapy should not be delayed to obtain ABG in this emergency situation 2.
Question 17: A septic shock patient has ABG: pH 7.22, PaCO2 28 mmHg, HCO3 11 mEq/L, lactate 6.2 mmol/L, PaO2 88 mmHg on 40% FiO2. What is the acid-base disturbance?
A) Respiratory acidosis
B) Metabolic acidosis with respiratory compensation
C) Mixed metabolic and respiratory acidosis
D) Respiratory alkalosis
Correct Answer: B) Metabolic acidosis with respiratory compensation
Explanation: The pH < 7.35 indicates acidemia, and HCO3 < 22 mEq/L confirms metabolic acidosis as the primary disturbance 1. The low PaCO2 (28 mmHg) represents appropriate respiratory compensation through hyperventilation 1. The elevated lactate confirms lactic acidosis from septic shock, and ABG analysis is specifically recommended in critical illness including sepsis and shock 4. This patient requires aggressive fluid resuscitation, vasopressor support, source control, and broad-spectrum antibiotics 4. The requirement for increased oxygen concentration (40% FiO2) is an indication for urgent clinical reassessment 2. The PaO2 of 88 mmHg on 40% FiO2 suggests impaired oxygenation requiring close monitoring 2.
Question 18: A post-cardiac arrest patient has return of spontaneous circulation. When should ABG be obtained?
A) Immediately after ROSC
B) Only if SpO2 falls below 90%
C) After 30 minutes of observation
D) Only if patient remains intubated
Correct Answer: A) Immediately after ROSC
Explanation: After return of spontaneous circulation following cardiopulmonary resuscitation, an ABG measurement should be taken to guide ongoing oxygen therapy 4. This is critical for post-cardiac arrest care 4. During active resuscitation, the highest possible inspired oxygen should be given, but once spontaneous circulation returns, oxygen should be titrated based on ABG results to avoid hyperoxemia 2, 4. Patients in peri-arrest situations or requiring immediate medical intervention should have ABG measurement 4. The ABG helps assess adequacy of ventilation, oxygenation, and metabolic derangements that require correction in the post-arrest period 2, 4.
Question 19: A patient with acute severe asthma has SpO2 88% on reservoir mask at 15 L/min. ABG shows: pH 7.38, PaCO2 42 mmHg, PaO2 58 mmHg. What is the most concerning finding?
A) Low PaO2
B) Normal pH
C) Normal PaCO2
D) Low SpO2
Correct Answer: C) Normal PaCO2
Explanation: In acute severe asthma, a normal or rising PaCO2 (42 mmHg in this case) despite tachypnea indicates impending respiratory failure and is an ominous sign 4. Patients with severe asthma typically hyperventilate, resulting in low PaCO2 1. A "normal" PaCO2 of 42 mmHg suggests the patient is tiring and can no longer maintain adequate ventilation 1. This patient requires urgent senior review, consideration for ICU admission, and preparation for possible intubation 2. ABG is specifically recommended in acute asthma requiring oxygen therapy 4. The combination of severe hypoxemia (PaO2 58 mmHg) despite maximum oxygen and normalizing PaCO2 indicates life-threatening asthma 2.
Question 20: A pregnant patient at 32 weeks gestation has SpO2 93% on room air with no respiratory symptoms. What is the appropriate management?
A) Start oxygen immediately targeting SpO2 >94%
B) Obtain ABG first
C) No oxygen therapy unless patient is truly hypoxemic
D) Start oxygen targeting SpO2 88-92%
Correct Answer: C) No oxygen therapy unless patient is truly hypoxemic
Explanation: Oxygen therapy may be harmful to the fetus if the mother is not hypoxemic 2. An SpO2 of 93% may be within normal range for some individuals, and transient dips to 90% or less can occur during sleep in normal participants 2. The presence of a normal SpO2 does not negate the need for blood gas measurements if there are clinical concerns 2. However, in an asymptomatic pregnant patient with SpO2 93%, clinical assessment should determine if true hypoxemia exists before initiating oxygen therapy 2. If the patient develops respiratory symptoms or SpO2 falls below 94% unexpectedly, ABG should be obtained 2, 4.
Questions 21-25: Advanced Interpretation and Mixed Disorders
Question 21: A patient has ABG: pH 7.40, PaCO2 60 mmHg, HCO3 36 mEq/L. What is the interpretation?
A) Normal ABG
B) Compensated respiratory acidosis
C) Compensated metabolic alkalosis
D) Mixed disorder
Correct Answer: B) Compensated respiratory acidosis
Explanation: Although pH is 7.40 (normal range), both PaCO2 (60 mmHg, elevated) and HCO3 (36 mEq/L, elevated) are abnormal 1. The systematic approach to ABG interpretation requires examining all components, not just pH 1. The elevated PaCO2 indicates chronic respiratory acidosis, and the elevated HCO3 represents complete metabolic compensation bringing pH back to normal 1. This pattern is typical of chronic COPD with long-standing CO2 retention 2. These patients require controlled oxygen therapy with target SpO2 88-92% 2, 3. A sudden deterioration in such patients (fall in SpO2 ≥3% or increasing breathlessness) requires urgent ABG analysis 2, 4.
Question 22: A diabetic patient has ABG: pH 7.28, PaCO2 38 mmHg, HCO3 17 mEq/L, glucose 420 mg/dL, anion gap 24. What additional calculation is most helpful?
A) Alveolar-arterial gradient
B) Delta gap
C) Base excess
D) Osmolar gap
Correct Answer: B) Delta gap
Explanation: The delta gap calculation helps address multiple concurrent metabolic disturbances and clarify interpretation of mixed acid-base conditions 5. In this case of diabetic ketoacidosis with high anion gap metabolic acidosis, the delta gap (change in anion gap divided by change in HCO3) helps identify if there is a concurrent non-anion gap metabolic acidosis or metabolic alkalosis 5. The formula is: Delta gap = (Measured anion gap - 12) / (24 - Measured HCO3) 5. A ratio <1 suggests concurrent non-anion gap acidosis, >2 suggests concurrent metabolic alkalosis, and 1-2 suggests pure anion gap acidosis 5. This patient requires urgent treatment for DKA, and ABG analysis is specifically recommended for patients at risk of diabetic ketoacidosis 2, 3, 4.
Question 23: A cirrhotic patient being evaluated for liver transplant has ABG: pH 7.44, PaCO2 38 mmHg, PaO2 68 mmHg on room air. The patient is 70 years old. What P(A-a)O2 gradient cutoff should be used for hepatopulmonary syndrome diagnosis?
A) ≥10 mmHg
B) ≥15 mmHg
C) ≥20 mmHg
D) ≥25 mmHg
Correct Answer: C) ≥20 mmHg
Explanation: For hepatopulmonary syndrome diagnosis in patients aged ≥65 years, a P(A-a)O2 ≥20 mmHg cutoff should be used instead of ≥15 mmHg 1. This age-adjusted criterion accounts for normal age-related increases in alveolar-arterial gradient 1. The diagnostic criteria include PaO2 <80 mmHg or P(A-a)O2 ≥15 mmHg (≥20 mmHg if age ≥65) 1. Patients with hepatopulmonary syndrome and PaO2 <60 mmHg should be evaluated for liver transplantation 1. Severe hypoxemia (PaO2 <45-50 mmHg) is associated with increased post-transplant mortality, and ABG should be performed every 6 months in these patients 1.
Question 24: A patient on VA-ECMO has right radial ABG: pH 7.38, PaCO2 42 mmHg, PaO2 75 mmHg with narrow pulse pressure. What does the narrow pulse pressure indicate?
A) Adequate ECMO flow
B) Mixing point proximal to innominate artery
C) Mixing point distal to innominate artery
D) LV distension
Correct Answer: B) Mixing point proximal to innominate artery
Explanation: A narrow pulse pressure from the right radial arterial line suggests a mixing point proximal to the innominate artery, whereas a wide pulse pressure due to LV ejection indicates the mixing point is more distal 2. In peripheral VA-ECMO, the mixing point between antegrade under-oxygenated blood from LV ejection and retrograde well-oxygenated ECMO flow determines upper body oxygenation 2. Management of differential oxygenation (Harlequin syndrome) includes increasing ECMO flow to move the mixing point proximally, manipulating ventilator settings to improve oxygenation, or inserting an oxygenated return cannula in the jugular vein (V-AV ECMO) 2. The PaO2 of 75 mmHg is relatively low for a patient on ECMO and may indicate inadequate cerebral oxygenation 2.
Question 25: A COPD patient with known baseline PaCO2 of 55 mmHg presents with acute exacerbation. Current ABG shows: pH 7.32, PaCO2 68 mmHg, HCO3 32 mEq/L. What is the most appropriate interpretation?
A) Chronic compensated respiratory acidosis
B) Acute-on-chronic respiratory acidosis
C) Pure acute respiratory acidosis
D) Mixed respiratory and metabolic acidosis
Correct Answer: B) Acute-on-chronic respiratory acidosis
Explanation: The elevated baseline HCO3 (32 mEq/L) indicates chronic metabolic compensation for the patient's baseline respiratory acidosis 1. However, the pH of 7.32 (acidemic) with PaCO2 of 68 mmHg (13 mmHg above baseline) indicates an acute worsening superimposed on chronic disease 1. This is acute-on-chronic respiratory acidosis, requiring urgent intervention with controlled oxygen therapy (target SpO2 88-92%) and consideration for NIV given pH <7.35 1. The distinction between acute and chronic is critical because acute respiratory acidosis requires more aggressive intervention 1. Patients with COPD and acute exacerbation should have ABG checked when starting oxygen therapy, especially if they have known CO2 retention 3, 4. This patient's rise in PaCO2 of 13 mmHg from baseline indicates clinically unstable disease requiring optimization 3.
Key Clinical Pearls
Critical Pitfalls to Avoid:
Never rely solely on pulse oximetry in critically ill patients—normal SpO2 does not exclude significant acid-base disturbances, hypercapnia, or anemia 2, 3, 4
Always repeat ABG within 60 minutes after initiating or changing oxygen therapy in patients at risk for hypercapnia 3, 4
In carbon monoxide poisoning, pulse oximetry readings are falsely normal—give maximum oxygen regardless of SpO2 2, 4
A "normal" PaCO2 in severe asthma indicates impending respiratory failure, not stability 4
In peripheral VA-ECMO, obtain ABG from right radial artery to assess cerebral oxygenation, not from femoral line 2