Intermediate: ICU Principles

Question 1

What is the oxygen content of arterial blood (CaO2):

Answer 1

Hb X Sata X 1.34

Sata: arterial hgb oxygen saturation

Question 2

Why is PaO2 so important?

Answer 2

The PaO2 evaluates the efficacy of gas exchange. As mentioned above (question 1), the amount of dissolved oxygen in blood is very little compared to that bound to Hb. Therefore Hb is the principle determinant of the ‘level’ or content of oxygen. Hb saturation curve is plotted against PaO2, because the PaO2 at various levels determines the % saturation of Hb.

Question 3

Normal VO2 and VCO2:

Answer 3

Note that when VO2 is 250 and VCO2 of 200 the RQ is 0.8.

Question 4

What is dysoxia?

Normal Satmv?

Answer 4

The point where VO2 decreases due to the DO2 decreasing (no more oxygen can be extracted) is called dysoxia.
Basically, when VO2 starts outpacing DO2, the mixed venous sat will start dipping below 70% (70-75% is around normal)

Question 5

So what do you do when your patient has a Satmv?

Answer 5

Consider if DO2 is ok: arterial saturation low (increase FiO2, PEEP), Hb low (transfusion), cardiac output low (inotrope, fluid bolus, etc). If you have no further optimization options for DO2, start thinking about decreasing VO2 (decrease temperature, decrease nutrition (generally bad idea), paralyze (generally bad idea), etc).

Question 6

patient in the ICU with severe hypoxia and inadequate delivery of oxygen to tissues is found to have an increasing level of lactic acid and a decreasing pH. Which of the following are true regarding this patient:

A. Glycolysis has stopped occurring
B. Anaerobic metabolism is associated with lactic acid production, but does not explain the low pH
C. The pH is low because there is insufficient oxygen in the final step in the electron transport chain
D. This is a result of overfeeding with TPN

Answer 6

C: The pH is low because there is insufficient oxygen in the final step in the electron transport chain

When oxygen is not present, the ETC comes to a halt (O2 accepts the electrons donated by the citric acid cycle). In this situation cellular respiration is anaerobic and far fewer ATP molecules are produced. Additionally, lactic acid accumulates causing an anion gap metabolic acidosis.

Question 7

True or False? NTG primarily decreases preload, not afterload.

Answer 7

TRUE

Question 8

A patient has an elevated Satmv, which of the following is most likely:

A. Cirrhosis, CI 5.2, arterial Sat 92%
B. Septic, Hb 6.5, PaO2 62
C. Severe COPD with erythrocytosis, EF 35%, sat 90%, Hb 18
D. Healthy 25 year old in hyperbaric chamber at 2 atmospheres, FiO2 100%

Go through these

Answer 8

A: Cirrhosis, CI 5.2, arterial Sat 92%

This is a tough one, lets go through each in turn. A cirrhotic patient tends to have high cardiac index (CI) as well as a propensity for peripheral AV shunting both of which cause an increased mixed venous sat (SVO2), since a lot of oxygen is delivered and with AV fistulas, less O2 will be consumed. Low arterial saturations are common in ESLD patients due to an increased shunt fraction due to atalectasis and pulmonary AV fistulas which . This does limit DO2 (oxygen delivery), but the very high CO compensats for the low arterial saturation.

The septic patient tends to have an elevated Satmv (on exams!) because of high CO and decreased utilization of oxygen (sick mitochondria), but given the very low Hb and a PaO2 which correlates to a saturation in the low 90s, the oxygen delivery is still likely lower than the patient in choice A.

The patient in choice C has a lot of Hb, but a low CO in all likelihood. Even though each cc of blood can carry more oxygen than the other patients, he is unable to substantially increase the DO2 due to the poor heart function.

A hyperbaric chamber can increase PaO2, but cannot make a Hb molecule more than 100% saturated which accounts for the great majority of oxygen content. This patient would likely have a high-normal Satmv, not an elevated one.

Question 9

Why is the SatMV higher in sepsis?

Answer 9

It should be noted the reason for this is not entirely clear. Mitochondrial oxygen utilization is impaired, which would in that sense decrease the consumption of oxygen. However, there is a wide spread activation of the immune system, referred to as the ‘respiratory burst,’ where overall oxygen consumption is in fact increased. The increase in Satmv is most likely due to increased oxygen delivery primarily through increased CO.

Question 10

A leftward shift of the oxygen Hb saturation curve secondary to hypocarbia is called:

A. Fick principle
B. Bohr effect
C. Haldane effect
D. Law of LaPlace

Explain them all

Answer 10

This is a backwards stated question because Bohr effect essentially means that increasing CO2 binding to Hb (by increasing PaCO2) will cause an increased release of oxygen (in other words right shift). Therefore, the opposite must be true, that hypocarbia causes a left shift. Haldane effect is similar. This states that when deoxygenated, Hb can bind more CO2 (and therefore work as a buffer). Fick principle is what we’ve been talking about all along, relating oxygen delivery to consumption. LaPlace’s law relates pressure to wall tension [Pressure = (2 X Tension X Wall Thickness)/ Radius].

Question 11

patient’s respiratory quotient increases from 0.8 to 0.85, what is the most likely reason:

Answer 11

D: Increase in carbohydrate proportion in TPN

The RQ is the balance between the production of CO2 and consumption of O2, or VCO2/ VO2. Nutrition can effect CO2 production in that carbohydrates, protein, and fat produce 1.0, 0.8, and 0.7 units of CO2 for every unit of O2 consumed. In patient’s nearing respiratory failure, decreasing carbohydrates and increasing lipids can decrease the overall RQ meaning the patient needs to expel LESS CO2 per O2 consumed. The other answers will not change the RQ.

Question 12

Does Respiratory quotient change in preggo?

Answer 12

No

Question 13

The majority of blood CO2 is held as _____ in ______

Answer 13

bicarbonate in the plasma and RBCs

Question 14

A patient’s PaCO2 is 40 mm Hg, what is the likely mixed venous PCO2 (PmvCO2) (in mm Hg):

Answer 14

C: 46

The PmvCO2 is normally about 46 as it enters the lungs and is 40 as it exits (PaCO2).

Question 15

Wedge pressures in ARDS:

Answer 15

Classically, the definition has included a wedge pressure < 18 to essentially rule out a cardiogenic cause of the pulmonary oedema.

Question 16

ARDS treatment strategies; including plateau pressures:

Answer 16

Plateau pressures less than 30 are associated with improved outcomes. To make things easy for us simpletons, an algorithm was formed where tidal volumes are initially set at 6 cc/ kg of ideal weight ** and decreased if plateau pressures are elevated (> 30). PEEP is added for two reasons: First, and foremost, to improve oxygenation. By opening previously closed alveoli, gas exchange is improved. The second benefit is to reduce the trauma of continual opening and closing which is another story in and of itself. The concept of permissive hypercapnea means that so long as the patient is oxygenating, ventilation need not be increased to normalize the pH (answer A)

Question 17

ARDS TVs:

Answer 17

according to ARDSnet protocol, an 8cc/kg Tv is initially set and titrated down to 6 cc/kg over 4 hours. Although in the big picture, this is irrelevant, the ABA has reportedly asked about this subject!

Question 18

TV caveat in ARDS:

Answer 18

When plateau pressures rise above 30 cm H20, this is reason to decrease tidal volumes, BUT the converse is not true: when plateau pressures are below 30 cm H2O, it is not a reason to increase tidal volumes.

Question 19

When dealing with ARDS questions, keep an eye on-

Answer 19

BP and PEEP!

Question 20

Two most common triggers for ARDS:

Answer 20

Pneumonia and sepsis

Question 21

A patient with ARDS is on assist control (AC) for 10 days, now has a tidal volume of 6 cc/kg, plateau pressure of 24 cm H2O, FiO2 of 60%, PEEP of 10, rate of 12, and an ABG (pH, PaO2, PaCO2) of 7.25/ 55 / 58. Which of the following is the next best step:

A. Increase PEEP
B. Increase FiO2
C. Increase tidal volume
D. Decrease tidal volume
E. Change to SIMV with pressure support

Answer 21

A: Increase PEEP

The patient is hypoxic which can likely be overcome by either increasing PEEP or FiO2. Both have potential disadvantages. Increased FiO2, especially over 60% for > 48 hours may result in increased oxygen free radicals and oxygen toxicity, therefore worsening the underlying disease (ARDS). Increased PEEP increases mean airway pressures, decreases preload, and high levels can itself cause diffuse alveolar damage. Additionally, PEEP can lead to pneumothorax, pneumopericardium, and subQ air. Between the two, PEEP is probably safer in the long term. There is no indication to decrease tidal volume as plateau pressures are less than 30 already. As discussed above, there probably is never a time to increase tidal volume. Changing from AC to SIMV makes no significant change in ventilation strategy as at high pressure supports, AC and SIMV are nearly indistinguishable.

As an aside, as an astute M5 member pointed out, strictly adhering to the ARDSnet protocol, the next move would be to increase FiO2, not PEEP. That being said, the ABA has steadfastly chosen increasing PEEP as the correct answer in this and similar situations, and I suggest you choose increasing PEEP provided you have room to go up on the plateau pressure

Question 22

A patient on SIMV with a rate of 6, tidal volume of 500 and a pressure support (PS) of 5 and PEEP of 5 has a respiratory rate (RR) of 26 and a minute volume of 7 liters. His ABG (pH, PaO2, PaCO2) is 7.38 / 100 / 40. What would the most likely response to increasing PS be:

A. A decrease in RR, minimal change in pH
B. A decrease in RR, increase in pH
C. Minimal change in RR, minimal change in pH
D. An increase in RR, minimal change in pH
E. An increase in RR, increase in pH

Answer 22

The correct answer is: A: A decrease in RR, minimal change in pH

By increasing PS, spontaneous breaths now only supported with a PS of 5 cm H2O will have an increased tidal volume.

Question 23

Tell me about airway pressure release ventilation.

Answer 23

APRV is an increasingly utilized ventilatory strategy which employs two levels of continuously applied positive pressure (essentially two levels of CPAP), one very high, and one very low. During the high pressure time, alveoli are opened and oxygenation is improved. Just like increasing PEEP with traditional modes, increasing the high CPAP level can also improve oxygenation. A brief period (0.4-0.8 seconds) of low pressure (0-5 cm H2O) interrupts relatively long periods of high CPAP (4-8 seconds), allowing for CO2 removal (ventilation). The patient can spontaneously breathe at both levels of CPAP comfortably, unlike inverse ratio ventilation which is very distressing to patients. Pressure support can also be added to spontaneous breathing, but mean airway pressures (think plateau pressures) should be checked and ensure they stay below 30.