Impaired Gas Exchange

Question 1

Gas Transport Blood

Answer 1

OXYGEN 
98% oxyhaemoglobin 
rest is dissolved in blood plasma PaO2
CO2
20-30% carbaminohaemoglobin 
rest dissolved in plasma as bicarbonate ions

Question 2

CO2 Equation

Answer 2

CO2 + H2O - H2CO3 - HCO3- + H+

Question 3

Causes

Answer 3

typically in conjunction with other impairments
shunt - blood moves through lung tissue which isn’t ventilating well, results in low VQ ratio
reduced blood flow through lungs = high VQ ratio
generalised hypoventilation
decreased FiO2
diffusion impairment
imbalance between oxygen consumption and oxygen delivery (malfunctioning equipment)

Question 4

Clinical Significance

Answer 4

If O2 deliver falls below critical threshold, metabolic demands are no longer met 
results in anaerobic metabolism 
increased minute ventilation in order to remove CO2
RER increases 
serum lactate levels increase 
Results in - 
- reduced arterial oxygenation 
- increased arterial CO2
- increased work of breathing 
- increased work of the heart
- impaired tissue oxygenation 
- multi organ system failure

Question 5

Clinical Features - moderate hypoxaemia

Answer 5

abnormal ABGs/pulse oximetry (GOLD STANDARD)
40-60mmHg PaO2
increased minute ventilation 
restlessness 
confusion tachycardia 
hypertension

Question 6

Clinical Features - severe hypoxaemia

Answer 6

abnormal ABGs/pulse oximetry (GOLD STANDARD)
<40mmHg PaO2
bradycardia 
arrhythmias
hypotension 
coordination loss
impaired judgement 
coma 
brain damage

Question 7

Clinical Features - Hypercapnia

Answer 7

increased PaCO2
warm peripheries 
flushed skin 
bounding pulse 
impaired concentration 
drowsiness

Question 8

Clinical Features - Hypocapnia

Answer 8

decreased PaCO2
paraesthesia in hands, face and trunk 
dizziness 
headache 
tremor 
hypotension 
tachycardia

Question 9

Physio Management

Answer 9

Positioning - to improve VQ matching and lung volumes 
Physical activity - demand ventilation 
address underyling reversible causes
optimise oxygen therapy 
non-invasive ventilation 
invasive mechanical ventilation 
extra corporeal membrane oxygenation

Question 10

Respiratory Failure - definition and types

Answer 10

Definition - inability of the pulmonary system to meet the metabolic demands of the body through adequate gas exchange

Type 1 - hypoxaemic
- PaO2 low
- PaCO2 normal 
Type 2 - hypercapnic 
- PaO2 low 
- PaCO2 high

Question 11

pH

Answer 11

measurement of acidity of alkalinity of the blood
normal range 7.35-7.45
if moves above 7.8 or below 6.8 then interferes with cellular functioning and can lead to cell death

Question 12

PaO2

Answer 12

partial pressure of O2 dissolved in arterial blood
normal = 85mmHg
80-100
<60mmHg indicates hypoxaemia

Question 13

PaCO2

Answer 13

partial pressure of CO2 dissolved in the arterial blood

normal = 35-45mmHg

Question 14

HCO3-

Answer 14

buffer - a solution that can resist pH change

normal = 22-26mmol/L

Question 15

SaO2

Answer 15

percentage of oxygen bound to haemoglobin

Question 16

Respiratory Buffer System

Answer 16

lungs retain or excrete CO2 by altering minute ventilation
process if fast - effective within 1-3 mins
Increased PaCO2 - H+ ions excite the respiratory centre
- causes increased ventilation to remove CO2
Decreased PaCO2 - no excitement of the respiratory centre
- ventilation remains low/decreases in order to retain CO2

Question 17

Renal Buffer System

Answer 17

kidneys can excrete acids of bases to compensate for respiratory acidosis or alkalosis
process is long - takes hours/days
Acidosis - kidneys excrete more H+ ions and resorb more HCO3-
Alkalosis - kidneys excrete more HCO3-

Question 18

Different levels of compensation

Answer 18

Full - action of buffers has restored pH to normal range but abnormality in PaCO2 of HCO3- persists
Partial - buffers have started to act but have not yet returned pH to normal
None - no action of the buffers

Question 19

ABGs Types of Disorders

Answer 19

Respiratory Acidosis - inadequate ventilation results in increased PaCO2
Respiratory Alkalosis - increased ventilation results in increased elimination of CO2
Metabolic Acidosis - accumulation of acids of loss of HCO3-
Metabolic Alkalosis - loss of acids or accumulation of bases

Question 20

ABGs Interpretation

Answer 20

look at the pH - acidosis, alkalosis or normal?
look at the PaCO2 - if this value explains the derangement in pH, then the problem is respiratory in nature
look at HCO3- - if this value explains the derangement in pH, then the problem is metabolic in nature
diagnosis the acid-base status and the extent of comepensation by the non-problematic system
look at PaO2 - any coexistent hypoxaemia?

Question 21

Limitations of ABGs

Answer 21

cannot yield a specific diagnosis
doesn’t reflect the degree to which an abnormality actually effects a patient
low PaO2 doesn’t necessarily indicate tissue hypoxia
normal PaO2 doesn’t necessarily indicate adequate tissue oxygenation
cannot be used as a screening tool for pulmonary disease
difficult to separate co-existing abnormalities