Respiratory Physiology and Critical Care

Question 1

What is the most common cause of cyanide toxicity? What is the treatment?

Answer 1

Cyanide toxicity is often due to smoke inhalation during home or commercial fires.

First-line treatment is with hydroxocobalamin

Question 1

What is the equation for Pulmonary Vascular Resistance?

Answer 1

PVR = (mean PA pressure - wedge pressure) / (cardiac output x 80)

Normal PVR is 50 to 150

Question 2

Describe functional residual capacity

Answer 2

FRC = Expiratory reserve volume + Residual volume

Decreases with pregnancy, ascites, neonates, general anesthesia, obesity, and supine position

Increased with PEEP

Question 3

During apnea, how fast will PaCO2 rise?

Answer 3

6 mm Hg during the first minute and then 3 to 4 mm Hg each minute after

Question 4

What is the equation for BMI?

Answer 4

kg/m^2

Question 5

Define anatomic dead space, alveolar (functional) dead space, and physiologic dead space

Answer 5

Anatomic dead space - volume of gas in the conducting airways of the lungs

Alveolar dead space - volume of gas in ventilated, but not perfumed alveoli

Physiologic dead space - sum of anatomic and alveolar dead space

Question 6

What is the normal O2 requirement in adults?

Answer 6

3 to 4 mL/kg/min

250 mL/min in typical adult

Question 8

What is the equation for O2 Content?

Answer 8

(1.39 x Hgb x Sat) + (0.003 x PaO2)

Question 9

What is the Bohr dead space equation?

Answer 9

Dead space ratio (VD/VT) = (PaCO2 - EtCO2) / PaCO2

Question 10

What is the mechanism for the compensatory shift of the oxyhemoglobin dissociation curve in response to chronic acid base abnormalities?

Answer 10

Altered metabolism of erythrocyte-2,3-DPG

Question 11

What is the P50 of adult hemoglobin?

Answer 11

27 mm Hg

Question 12

What is mixed venous O2 saturation (SvO2)? What is the equation? What would cause a decreased SvO2? An increased SvO2?

Answer 12

Mixed venous oxygen saturation is the percentage of oxygen bound to hemoglobin in the blood that returns to the right side of the heart
- Measured from a pulmonary artery catheter and includes all of the venous blood returning from the SVC, IVC, and coronoary sinus

SvO2 = SaO2 - VO2/(13.9 x CO x Hgb)

SvO2 is decreased by:

• Increased O2 consumption (hyperthermia, pain)
• Decreased CO (MI, hypovolemia)
• Decreased hemoglobin
• Decreased SaO2

SvO2 is increased by:

• Increased hemoglobin (transfusion)
• Increased SaO2
• Decreased O2 consumption (cyanide, sepsis, CO poisoning)
• Increased CO (sepsis, L to R shunt)

Question 12

A change in PCO2 of 10 mm Hg will alter the pH by what amount?

Answer 12

~0.1 pH units

Question 13

What is normal vital capacity in an adult?

Answer 13

60 to 70 mL/kg

About 5L in the average adult

Question 15

How do you calculate the amount of bicarbonate that needs to be given to correct acidosis?

Answer 15

Total body weight (kg) x base deficit x 0.2

When administering sodium bicarbonate, 1/2 calculated dose is given and measurements are repeated

Question 16

What is the magnitude of compensatory reaction to respiratory alkalosis?

Answer 16

Maximum decrease in [HCO3-] of 5 mEq/L for 10 mm Hg decrease in PCO2 under 40 mm Hg

Question 17

What is the P50 of sickle cell hemoglobin?

Answer 17

31 mm Hg (decreased affinity for oxygen)

Question 18

List some conditions that cause a rightward shift of the oxy-hemoglobin dissociation curve

Answer 18

Results in decreased affinity, increased unloading of oxygen

Acidosis
Hyperthermia
Increased 2,3-DPG
Pregnancy
Abnormal hemoglobins (sickle, thalassemia)
Volatile anesthetics

Question 19

List some conditions that cause a leftward shift of the oxy-hemoglobin dissociation curve

Answer 19

Results in increased affinity, decreased unloading of oxygen

Alkalosis
Hypothermia
Different hemoglobin species
Decreased 2,3-DPG

Question 20

List the O2 saturation at specific PaO2 values

Answer 20

PaO2 of 27 mm Hg = 50% sat

PaO2 of 40 mm Hg = 75% sat

PaO2 of 60 mm Hg = 90% sat

PaO2 of 100 mm Hg = 98% sat

Question 21

What is a general estimation of shunt fraction?

Answer 21

A-a gradient / 20

Question 21

In a healthy, resting adult, what % of total body O2 consumption is due to the work of breathing?

Answer 21

1 to 3%

Question 22

Describe Ventilation/Perfusion ratios in the lungs

Answer 22

Because alveoli in the dependent portion of the lungs are more compliant than alveoli in the non-dependent lung, alveolar ventilation increases from top to bottom. Pulmonary blood flow also increases from top to bottom, but more so than alveolar ventilation,

Ultimately, ventilation/perfusion ratios are higher in the non-dependent areas of the lung and low in the dependent areas of the lung.

Thus, PaO2 and pH are higher in the apex and PCO2 is higher at the base

Question 23

How much anatomic dead space does an average adult have?

Answer 23

2 mL/kg

In an average adult, dead space is 150 mL

Question 24

A change of 10 mm Hg of PCO2 results in what magnitude of change in plasma [K+]?

Answer 24

0.5 mEq/L in the same direction

Question 25

A change in [HCO3-] of 10 mEq/L will alter the pH by what amount?

Answer 25

0.15 pH units

Question 27

Describe the negative effects of acidosis

Answer 27

CNS depression Increased ICP Cardiovascular depression Cardiac dysrhytmias Vasodilation Hypovolemia Pulmonary HTN Hyperkalemia

Question 28

Describe Organophosphate poisoning

Answer 28

Organophosphates bind with acetylcholinesterase and produce signs of excess parasympathetic activity: - Diarrhea - Urination - Miosis (pinpoint) - Bronchocontriction - Emesis - Lacrimation - Salivation Treatment is with atropine and pralidoxime

Question 28

How does recombinant human activated protein C (Xigris) work? What is it used for?

Answer 28

APC is indicated for the treatment of septic shock It works via inhibition of factors Va and VIIIa, reducing inflammation and microthrombi. It also blocks production of TNF. Major side effect is hemorrhage.

Question 29

Describe transfusion related lung injury (TRALI)

Answer 29

A serious potential complication of transfusing any product containing plasma, usually diagnosed 1 to 2 hours later - leading cause of transfusion-related mortality ``` Key features: Wide A-a gradient Non-cardiogenic pulmonary edema Leukopenia (2/2 lung sequestration) Fever Hypotension ```

Question 30

Describe the difference between dead space and shunt

Answer 30

Dead space - ventilation without perfusion Leads to elevated PCO2 Shunt - perfusion without ventilation Leads to decreased PO2 and widened A-a gradient

Question 31

What are some causes of dead space?

Answer 31

``` PE COPD ARDS Fibrosis Shock ```

Question 32

Define closing capacity

Answer 32

= closing volume + residual volume The volume at which small airways begin to close Increases with age, bronchitis, LV failure, surgery, and smoking At 44 y/o, CC = FRC when supine At 66 y/o, CC = FRC when standing

Question 33

Describe Flow-Volume Loops

Answer 33

Expiratory above, Inspiratory below TLC to RV (left to right) Fixed lesions: reduction in both inspiratory and expiratory components Extrathoracic lesions: reduce inspiratory component Intrathoracic lesions: reduce expiratory component

Question 34

List the lung volumes and capacities

Answer 34

Tidal volume (TV) - volume of air inhaled/exhaled during normal breathing Inspiratory reserve volume (IRV) - maximum volume of gas that can be inhaled after normal inspiration Expiratory reserve volume (ERV) - maximum volume of gas that can be exhaled after normal expiration Residual volume (RV) - volume of gas remaining in lungs after a forced exhalation Vital capacity (VC) = IRV + TV + ERV - maximum volume of gas that can be exhaled after maximum inhalation Inspiratory capacity (IC) = IRV + TV - maximum amount of gas that can be inhaled after normal expiration Functional residual volume (FRC) = ERV + RV - remaining lung volume at the end of normal expiration Total lung capacity (TLC) = IRV + TV + ERV + RV - lung volume after maximum inspiration

Question 35

How much deoxyhemoglobin is required to appear cyanotic?

Answer 35

At least 5g deoxyhemoglobin

Question 36

How does carbon monoxide exert its toxic effects? What are the symptoms? What will an ABG look like? What is the treatment?

Answer 36

Carbon monoxide effects: - Impaired mitochondrial oxidative phosphorylation - Impaired muscle oxygen utilization from impaired myoglobin function - Functional anemia and leftward shift of oxygen-Hb curve - Vasodilation 2/2 release of NO Initial symptoms include headache, nausea, dizziness, and vomiting - Cyanosis is NOT common An ABG will show a metabolic acidosis with a normal PaO2 and a falsely elevated calculated SaO2 and measured SpO2 - PaO2 is not reliable in setting of carboxyhemoglobinemia since it is only a measure of dissolved oxygen and not bound oxygen Treatment is O2 - Hyperbaric O2 only indicated for coma, cardiac ischemia, or levels over 40%

Question 37

What is Diffusing Capacity of the Lungs (DLCO)?

Answer 37

A measure of the gas exchange capacity of the lungs Influenced by the volume of blood in the pulmonary circulation Decreased by: - Anemia - Hypovolemia - Emphysema - Pulmonary HTN - Pulmonary edema - Cystic fibrosis Increased by: - Supine position - Exercise - Obesity - Left to right shunt - Polycythemia - Asthma

Question 38

How will an air bubble effect an ABG?

Answer 38

It will result in elevated PO2 and decreased PCO2

Question 39

What is the equation for the A-a Gradient?

Answer 39

A-a gradient = PAO2 - PaO2 PAO2 = FiO2(713) - PaCO2/0.8 Normal A-a gradient = age/4 + 4

Question 40

How is ARDS diagnosed?What is the treatment?

Answer 40

Characterized by acute onset, bilateral infiltrates on chest X-ray, and hypoxemia (PaO2/FIO2 ratio less than 300) Treatment is mainly supportive, while underlying cause is identified: - Intubation and ventilation - Low tidal volumes (permissive hypercapnia) - Low PIP - High FiO2 - High PEEP Conservative fluid management is advised Long-term antibiotics are NOT recommended

Question 41

What are some causes of elevated peak pressure? What about plateau pressure?

Answer 41

Elevated peak pressures is indicative of high airway resistance: - Bronchospasm - Kinked ET tube - Mucus plug - Foreign body Elevated plateau pressure is indicative of an issue with system compliance: - Poor positioning - Pulmonary fibrosis - Pneumothorax - Obesity - Insufflation

Question 42

What will the pulse oximetry be in a patient with methemoglobinemia? How is the PaO2 affected? What is the treatment?

Answer 42

Pulse oximetry will read ~85% regardless of true oxygen saturation - need co-oximetry for proper analysis PaO2 will be normal or unchanged Treatment is with methylene blue - Patients with G6PD can be treated with ascorbic acid

Question 43

What is the equation for oxygen delivery (DO2)?

Answer 43

= CO x CaO2 = [HR x SV] x [(SaO2)(Hgb x 1.34) + (0.003 x PaO2)]

Question 44

How does positive pressure ventilation cause oliguria?

Answer 44

Decreased preload 2/2 increased intrathoracic pressure - decreased venous return - increased CVP - increased release of ADH from posterior pituitary Increased sympathetic nervous system stimulation - activation of renin-angiotensin system Release of inflammatory cytokines

Question 46

How can antibiotics used to treat sepsis potentially worsen hypotension?

Answer 46

If sepsis is due to gram-negative bacteria, antibiotics may lead to cell wall lysis and release of lipopolysacharide (LPS) LPS induces immune response, release of cytokines, and production of NO, leading to vasodilation

Question 47

What is refeeding syndrome? What is the mechanism behind it? What are the manifestations?

Answer 47

Hypophosphatemia seen during refeeding in alcoholic, malnourished, or debilitated patients Caused by glucose loading - increased secretion of insulin leads to cellular uptake of glucose and phosphate Can result in AMS, hemolytic anemia, and arrythmias - can be a cause of failed weaning from ventilator

Question 48

What drug can be given to intubated patients to reduce the incidence of ventilator associated pneumonia

Answer 48

Sucralfate H2-blockers or PPIs can actually increase the risk as they reduce gastric acidity, thus increasing bacterial growth

Question 49

What is the RSBI? How is it used?

Answer 49

The Rapid Shallow Breathing Index is a test used in the weaning of mechanical ventilation in ICUs RSBI = (respiratory frequency)/(tidal volume) An RSBI less than 65 is considered an indication of weaning readiness

Question 50

What is respiratory quotient? How does diet affect it?

Answer 50

RQ is the amount of CO2 produced per unit of oxygen consumed from specific energy substrates Carbohydrates have a Q of 1 Proteins have a Q of 0.8 Lipids have a Q of 0.7

Question 51

Percutaneous tracheostomy has many advantages over the open approach. However, what are the absolute contraindications of the percutaneous approach?

Answer 51

- Active infection at the site of trach - Uncontrolled bleeding disorder - Unstable cardiopulmonary status - Patient unable to stay still - Abnormal tracheolaryngeal anatomy