Analysis and Monitoring of Gas Exchange

RESD 60Cardiopulmonary Assessment

Egan Chapter 11

Respiration

Process of moving oxygen to tissues for aerobic metabolism & removal of carbon dioxide.

Involves gas exchange at lungs & tissues• O2 from atmosphere to tissues for aerobic metabolism

• Removal of CO2 from tissues to atmosphere

Diffusion

– Gas moves across system by simple diffusion

– Oxygen moves from PO2 of 159 mm Hg in atmosphere to intracellular PO2 of ~5 mm Hg

– CO2 gradient is reverse from intracellular CO2 ~60 mm Hg to atmosphere where it = 1 mm Hg

Alveolar oxygen tensions (PAO2)

– PIO2 is primary determinant

– In lungs, it is diluted by water vapor & CO2

– Alveolar air equation accounts for all these factors

PAO2 = FIO2 (PB – 47) – (PaCO2/0.8)

Alveolar Oxygen Tension (PA02)

• PAO2 = FIO2 (PB – 47) – (PACO2/0.8)

– Dalton’s law of partial pressures accounts for first part of formula; second part relates to rate at which CO2 enters lung compared to oxygen exiting• Ratio is normally 0.8.

– If FIO2 > 0.60, (PACO2/0.8) can be dropped from equation

Diffuison

• Diffusion occurs along pressure gradients

• Barriers to diffusion

– A/C membrane has 3 main barriers:• Alveolar epithelium• Interstitial space & its structures• Capillary endothelium - RBC membrane

Fick’s Law

• Fick’s law: The greater the surface area, diffusion constant, & pressure gradient, the more diffusion will occur.

• What factors can decrease each of these?

Time limits to diffusion:

• Pulmonary blood is normally exposed to alveolar gas for 0.75 second, during exercise may fall 0.25 second

• Normally equilibration occurs in 0.25 second

• With diffusion limitation or blood exposure time of less then 0.25 seconds, there may be inadequate time for equilibration

Shunts

• PaO2 normally 5–10 mm Hg less than PAO2 due to presence of anatomic shunts, Portion of cardiac output that returns to left heart without being oxygenated by exposure to ventilated alveoli

– Two right-to-left anatomic shunts exist:• Bronchial venous drainage• Thebesian venous drainage• These drain poorly oxygenated blood into

arterial circulation lowering CaO2

V/Q ratio & regional differences

– Ideal ratio is 1, where V/Q is in perfect balance

– In reality lungs don’t function at ideal level • High V/Q ratio at apices >1 V/Q (~3.3)

– ↑PAO2 (132 mm Hg), ↓PACO2 (32 mm Hg)

• Low V/Q ratio at bases <1.0 (~0.66)– Blood flow is ~20 times higher at bases– Ventilation is greater at bases but not 20– ↓PAO2 (89 mm Hg), ↑PACO2 (42 mm Hg)

• See Table 11-1

Oxygen Transport

• Oxygen transport is the mechanism by which oxygen is carried from the lungs to the capillary bed.

• How oxygen is used by the tissues is oxygen consumption.

Factors that Determine 02 Transport

• Oxygen Content

• Cardiac Output – CO or Qt

• Distribution of cardiac output

• Oxyhemoglobin dissociation curve

Oxygen Content – Ca02

• Transported in 2 forms

• Physically dissolved in plasma– Gaseous oxygen enters blood & dissolves. – Henry’s law allows calculation of amount dissolved

• Dissolved O2 (ml/dl) = PO2 0.003

• Chemically bound to hemoglobin (Hb)– Each gram of Hb can bind 1.34 ml of oxygen.– [Hb g] 1.34 ml O2 provides capacity.– 70 times more O2 transported bound than dissolved

Hemoglobin Saturation

–Saturation is % of Hb that is carrying oxygen compared to total Hb

•SaO2 = [HbO2/total Hb] 100

•Normal SaO2 is 95% to 100%

Total oxygen content of blood

– Combination of dissolved & bound to Hb

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

– Normal is 1620 mL/dL

• Normal arteriovenous difference (~5 mL/dL)

• C(a v)O2

Abnormalities of Gas Exchange & Transport

• Impaired oxygen delivery (DO2)

– DO2 = CaO2 Qt

– When DO2 is inadequate, tissue hypoxia ensues

– What can cause impaired oxygen delivery?

Impaired Oxygen Delivery

• Decreased Qt – (CO)

• What can decrease CO?

• How can you assess this? What are the signs and symptoms?

Impaired Oxygen Delivery

• Decrease in Ca02

• What can decrease Ca02?

• How do you assess?

Hypoxemia

– Defined as abnormally low PaO2

– Most common cause is V/Q mismatch

– Other causes: hypoventilation, diffusion defect, shunting, & low PIO2 (altitude)

Physiologic shunt

– Where perfusion exceeds ventilation, includes:• Capillary or absolute anatomic shunts• Relative shunts seen in disease states diminish

pulmonary ventilation

– Relative shunts can be caused by:• COPD• Restrictive disorders• Any condition resulting in hypoventilation

Shunt equation

– Quantifies portion of blood included in V/Q mismatch

– Usually expressed as % of total cardiac output

– What is the shunt equation?

Deadspace Ventilation

• Ventilation that doesn’t participate in gas exchange

• Waste of energy to move gas into lungs

– Two types: alveolar & anatomic

Alveolar Deadspace:

– Ventilation that enters into alveoli without any, or without adequate perfusion

• Disorders leading to alveolar deadspace :– Pulmonary emboli– Partial obstruction of the pulmonary vasculature– Destroyed pulmonary vasculature (as can occur in

COPD)– Reduced cardiac output

Anatomic Deadspace

– Ventilation that never reaches alveoli for gas exchange

– Normal individuals have fixed anatomic deadspace

– Becomes problematic in conditions where tidal volumes drop significantly• Significant % of inspired gas remains in

anatomic deadspace

Increased Deadspace

• In face of increased deadspace, normal ventilation must increase to achieve homeostasis

• Additional ventilation comes at cost:– Increase in WOB– Consumes additional oxygen – Further adding to burden of external ventilation

Quick Quiz

• Oxygen delivery to the tissues (DO2) may be substantially impaired due to all of the following, except:

A. Low Hb levels (anemia)

B. V/Q matching

C. abnormal cardiac output

D. presence of Carboxyhemoglobin (COHb)