RESP 200 quiz 2 study guide

Equations to know:

• PAO₂

• A-a gradient

• Predicted A-a gradient by age equation

• P/F Ratio

• PaO₂ / PAO₂

• CaO₂

• PAO₂ = (PB – PH2O) FiO₂ – (PACO₂)(1.25)

• A-a gradient= PAO₂ - PaO₂

• Predicted A-a gradient by age equation: (Age in years + 10 years / 4 )

• P/F Ratio = PaO₂ / FiO₂

• PaO₂ / PAO₂

• CaO₂ = (SaO₂ × 1.34 x Hb) + (PaO₂ × 0.003)

critical values for oxygenation

P/F: <200

PaO2/PAO2: <0.15

A-a gradient: >450

PaO2: <70 mmhg on 60% FiO2

What is the alveolar air equation

PAO₂ = (PB – PH2O) FiO₂ – (PACO₂)(1.25)

PB= barometric pressure

PH2O= water vapor pressure

1.25= respiratory quotient

can arterial PO2 be higher than alveolar PO2?

No. this starts at the alveoli → arteries. O2 will be lost on the way to the arteriesn

After calculating PAO2 equation:

• How are mountain climbers affected by low barometric pressure

  • As you climb to a higher altitude, atmospheric/barometric pressure (PB) decreases.

    • ↑ Altitude → ↓ PB → ↓ PAO₂ → ↓ PaO₂ → hypoxemia

      So, giving the climber supplemental oxygen increases the amount/partial pressure of oxygen reaching alveoli, increasing the gradient for oxygen to diffuse from the alveoli into the pulmonary capillary blood.

      PAO₂ is calculated alveolar oxygen; PaO₂ is measured arterial oxygen on the ABG. Increasing FiO₂ directly raises PAO₂, which generally causes PaO₂ to rise as more oxygen diffuses into the blood.

Even though room air is still approximately 21% oxygen, there is less pressure pushing those oxygen molecules into the lungs.

• understand relevance of barometric pressure in high altitude

• explain how increasing FiO2 will impact the PaO2 of the climber

• Higher Altitude → Less atmospheric pressure creates less PO2 at elevation

  • supplemental O2 may help

• Air composition:

  • Nitrogen approx 78%

  • Oxygen = 20.9% or 21%

what is the A-a gradient

• A-a gradient= PAO₂ - PaO₂

• Normal: ~5–20 mmHg (increases with age)

  • in units of mmHg

  
  

• Measures the difference between alveolar & arterial PO2

• Helps distinguishcauses of hypoxemia

what does Normal A-a + hypoxemia indicate

think hypoventilation or low barometric pressure

what does elevated A-a indicate

impaired pulmonary O2 transfer

How do we predict the A-a gradient by age

(age + 10) / 4

what are some factors that affect A-a gradient?

• V/Q mismatch

  • imbalance between the air reaching the tiny air sacs in your lungs and the blood flow surrounding them

What is a Low V/Q (Shunt)

• Air flow is low, but blood flow continues. This happens when fluid or mucus blocks the air sacs.

what is High V/Q (Dead Space)

• Air flow is normal, but blood flow is blocked. This happens when a blood clot stops blood from reaching the area.

What is the P/F ratio

• P/F ratio= PaO2 / FiO2

what are the normal & critical values for P/F ratio

• Normal= >400

• Critical = <200

  • Mild: 200-300

  • Moderat: 100-200

  • Severe: <200

what does the P/F ratio identify & access

• severity hypoxemic respiratory failure

  • does NOT identify the cause of hypoxemia

• commonly used at beside

• useful with supplemental O2

Compare arterial to alveolar oxygenation

• PaO2/PAO2

  • Tells us how effectively oxygen moves from the alveoli → arterial blood

what are the normal & critical values for PaO2/PAO2

• Normal= 0.75-0.95

• Critical= <0.15

what strategies can be used to ↑ PaO2/PAO2 value?

• if @ 100%?

  • ↑PEEP

  • ↑FIO2

  • reposition

What is oxygen content

• once O2 is transferred from alveoli → blood; must be transported

• o2 moves through

air → lungs → blood → tissue

aorta → arteries → arterioles → tissue

• heart pumps O2 rich blood through the arteries to the body’s organs & tissue

equation for oxygen content (CaO2)?

 

CaO2 = (1.34*Hb*SaO2) + (0.003*PaO2)

• 16-22% volume is the normal CaO2 

• How much arterial oxygen content is being carried in the blood 

• Normal CaO2 20ml/dl

 

what is the equation for venous content (CvO2)?

CvO2 = (1.34*Hb*SvO2) + (0.003*PvO2)

• The oxygen content left in the veins returning to the heart

what is the equation for oxygen delivery (DO2)?

DO2 = (CaO2) (Qt) (10) - Qt is the other term for cardiac output, so it's also known DO2 = (CaO2) (CO) (10)

what is aerobic respiration?

process whereby cells convert glucose + oxygen → ATP (energy) + carbon dioxide + H2O

5 factors that impact gas exchange at A/C membrane

Gas exchange at the alveolus depends on the following

1. diffusion coefficient

2. interstitial space width

3. perfusion

4. pressure gradient

5. surface area

how does the diffusion coefficient (transferability) affect gas exchange

CO2 diffuses ~20 - 40x faster than O2

how does interstitial space width affect gas exchange

↑ interstitial fluid (CHF/pulmonary edema) →

↑ diffusion distance → impaired O₂ transferLess perfusion at apices; greater perfusion at bases

how does perfusion affect gas exchange

Less perfusion at apices, greater perfusion at bases

how does the pressure gradient affect gas exchange

↑ FiO2 → ↑O2 pressure gradient → better O2 transfer

how does alveolar surface area affect gas exchange

Oxygenation problem → ↑PEEP →

↑ alveolar recruitment/surface area → better O2 transfer

what are 4 things oxygen transport depends on

• Oxygen pressure in the blood (PaO2)

• Hemoglobin (Hb) concentration

• Saturation (left or right shift)

• Cardiac Output (Qt)

How do we improve oxygen pressure in the blood (PaO2) for oxygen transport

• PaO2 = O2 dissolved in plasma

  • expressed as a pressure (mmHg)

• <1% of total blood O2 is carried this way

How do we improve hemoglobin (Hb) concentration for oxygen transport

• Iron-rich protein found in RBCs

• Each Hb has 4 heme groups -> can carry 4 O2 molecules

• Normal Hb:

  • Male: ~14-18 g/dL

  • Female: ~12-16 g/dL

How do we improve Saturation (left or right shift) for oxygen transport

How do we improve QT for oxygen transport

Analyze a case study with vent settings, barometric pressure, FiO2, blood gas, and utilize the data to determine patient's oxygenation status

Understand different ways to improve oxygenation and when to use them: FiO2, PEEP, patient positioning, sedation, paralytics, etc.

If you attempt to improve the patient's oxygenation with FiO2, PEEP, patient positioning, sedation, etc., how would this impact the PaO2 in the PaO2 / PAO2 equation?

What causes left shift of ODC

• -Left: Hb has ↑ affinity for O₂→ holds O₂ more tightly → LESS unloading to tissues

• Memory: LEFT = LOVES O₂

• Causes: ↑ pH — alkalosis 

• ↓ PCO₂ 

• ↓ temperature 

• ↓ 2,3-DPG 

• COHb, MetHb, abnormal Hbs

What causes right shift of ODC

•  Hb has ↓ affinity for O₂→ releases O₂ more easily 

• → MORE unloading to tissues

• Memory: RIGHT = RELEASES O₂

• Give it away, give it away, give it away now

• Causes:

• ↓ pH — acidosis 

• ↑ PCO₂ 

• ↑ temperature 

• ↑ 2,3-DPG

Analyze a patient case study with specific data that could shift the ODC then explain how this will impact oxygen transport to the tissues

Understand PaO2 vs Hb in terms of how they impact oxygenation

Hemoglobin is the oxygen carrier for your body

• tells you how much oxygen the blood can carry to the tissues. Most oxygen is carried attached to hemoglobin—not dissolved in plasma.

• PaO₂ tells you about the oxygen dissolved in the plasma. It reflects how well oxygen is getting from the alveoli into the blood

Normal Hb levels 

• Females: ~12–16 g/dL or Vol%

• Males: ~14–18 g/dL or Vol%

Utilize Hb levels (know what is normal) in a patient to assess patient's oxygenation and suggest solutions as appropriate, such as packed RBC or blood products

what is oxygen delivery (DO2)?

• Total amount of O2 delivered to tissues per minute

• Formula: DO2 = CaO2 Qt 10

• Normal DO2 ~1000 mL/min

Identify factors that decrease oxygen delivery

• Decreased inspired O2 concentration (FiO2)

• Respiratory disease / impaired oxygenation (Decreased SaO2 / PaO2)

• Decreased Hemoglobin (Decreased Hb)

• Decreased Cardiac Output (Decreased Qt)

Identify factors that decrease oxygen delivery - chart given in class will help

• Decreased Cardiac Output (Qt)

• Increased oxygen consumption

• Exercise

• Seizures

• Shivering

• Hyperthermia

• Increased metabolic rate