Hemodynamics Exam 2

Tissue oxygenation cannot be directed measured so use indirect assessment techniques (4)

• VD, shunt calculations (distribution of ventilation)

• AaDO2, a/A (ability to cross AC membrane)

• PaO2, CaO2, QT (transport)

• C(a-v)O2 (O2 extraction by tissues)

Abnormal V/Q Relationships (4)

• Dead Space ventilation (VD)

• Dead Space effect

• Intrapulmonary shunt (Qs/QT)

• Shunt effect (venous admixture)

Anatomical Dead Space (increased) (6)

• Rapid, shallow breathing

• Male

• Some pulmonary diseases

  • Bronchiectasis

• Mechanical deadspace

  • Add tubing on ventilator to give deadspace

• PEEP

• Increased VT is more effective to increase alveolar ventilation than RR

Anatomical Dead space (decreased) (5)

• Diseases with bronchial obstruction

• Lung resection

• Pneumonectomy

• Tracheotomy

• Intubation

Deadspace Effect (Increased) (3)

• Mechanical ventilation

• Decreased cardiac output

• I.e., anything that increases ventilation and/or decreases lung perfusion

(PB-H2O)FiO2 stands for?

Partial pressure going in (PIO2)

Equations for Dead Space:

• VD/VT = (PaCO2- PECO2) / PaCO2

• VDphys = ((PaCO2 - PECO2) x VT) / PaCO2

Normal VD/VT

• Spontaneous breathing: 0.2-0.4 (20%-40%)

• On vent: <0.60 (60%)

Intrapulmonary Shunt

Defined as the portion of the cardiac output that returns to the left heart without being oxygenated (Perfusion w/o Ventilation)

Equation for Intrapulmonary Shunt and the normal

• Total(physiologic) = anatomic + capillary

  • Normal = <0.10 (10%)

  • > 0.30 (30%) = need for mechanical ventilation

Anatomic Shunt (3)

• Is the major portion of total shunt

• 2-5% of cardiac output

• Normal blood flow through:

  • Pleural veins

  • Thebesian veins (deposits blood back into the left side of the heart (left atrium)

  • Bronchial veins (deposits blood into the pulmonary vascular system/pulmonary veins)

    • Helps to dump into the pulmonary venous system (all converge into pulmonary vein)

Anatomic shunt is increased by (3):

• Congenital heart disease

• Intrapulmonary fistula

• Vascular tumors

  All 3 require surgical intervention

Capillary Shunt (5)

• True shunt

• Small amount is normal

• Perfusion of non-ventilated alveoli

• Capillary shunt is refractory to oxygen therapy

  • Alveoli are unable to ventilate

  • Blood passing by good alveoli cannot carry more O2 once it is fully saturated

• Treatment is PEEP/CPAP

Causes of increased capillary shunt (5)

• Atelectasis or other types of alveolar collapse

• Pneumonia or alveolar consolidation

• Pneumothorax 

• Pulmonary edema or alveolar fluid accumulation

• Complete airway obstruction

Shunt Effect (Venous Admixture) (Relative Shunt) (3)

• V/Q mismatch when perfusion is in excess of ventilation (Perfusion > Ventilation)

• Responsive to O2 therapy (add VT, try putting on non invasive)

• I.e., anything that decreases ventilation and/or increases perfusions

Relative Shunt causes (3)

• Hypoventilation

• Diffusion defects of the alveolar capillary membrane

• Uneven distribution of ventilation (Bronchospasm) (Think CBABE)

  • Ventilation of distal airways

Alveolar-Capillary Diffusion Defects

There is an abnormality in the structure of the AC membrane that slows the movement of O2 between the alveoli and pulmonary capillary bed

Causes of diffusion defects (3)

• Interstitial pulmonary edema

• Interstitial lung disorders

• May also occur following the administration of drugs that cause an increase in cardiac output or dilation of the pulmonary vessels

Shunt equation is useful tool (2):

• To evaluate effectiveness of O2 therapy

• To differentiate shunt or V/Q imbalance as cause of hypoxemia

Normal cardiac output

5-8 liters per minute

How do you calculate shunt percentage from AaDO2?

every 80 mmHg = 1% shunt

1.25 or 1/8 is the?

Respiratory quotient (do not use when FiO2 is greater than or equal to 60)

P(A-a)O2 or Alveolar-arterial oxygen tension gradient in mmHg

• On room air, the P(A-a)O2 should be less than 4 mmHg for every 10 years in age.

• On 100% oxygen, every 50 mmHg difference in P(A-a)O2 approximates 2% shunt

Normal a/A ratio

• >60%

Tissue Oxygenation - Step 1

• Oxygen must be made available to alveoli, depends on:

  • FiO2

  • Distribution of ventilation

Tissue Oxygenation - Step 2

• Oxygen must cross the alveolar-capillary membrane

  • Pressure gradient

  • Alveolar capillary membrane thickness

  • Surface area of alveolar-capillary membrane

    • What kinds of patients have surface area problems with alveolar-capillary membrane?

      • Emphysema (COPD) because their alveoli is floppy/flat

      • Atelectasis

Tissue Oxygenation - Step 3

• Oxygen must load into blood and be transported

  • Hemoglobin content

  • SaO2

  • Cardiac output

  • Cardiovascular status

Tissue Oxygenation - Step 4

• Tissues must uptake oxygen

  • Pressure gradient

Tissue Oxygenation - Step 5

• Tissues utilize oxygen

  • Tissue metabolism