Impaired Gas Exchange Flashcards
Gas Transport Blood
OXYGEN 98% oxyhaemoglobin rest is dissolved in blood plasma PaO2 CO2 20-30% carbaminohaemoglobin rest dissolved in plasma as bicarbonate ions
CO2 Equation
CO2 + H2O - H2CO3 - HCO3- + H+
Causes
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)
Clinical Significance
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
Clinical Features - moderate hypoxaemia
abnormal ABGs/pulse oximetry (GOLD STANDARD) 40-60mmHg PaO2 increased minute ventilation restlessness confusion tachycardia hypertension
Clinical Features - severe hypoxaemia
abnormal ABGs/pulse oximetry (GOLD STANDARD) <40mmHg PaO2 bradycardia arrhythmias hypotension coordination loss impaired judgement coma brain damage
Clinical Features - Hypercapnia
increased PaCO2 warm peripheries flushed skin bounding pulse impaired concentration drowsiness
Clinical Features - Hypocapnia
decreased PaCO2 paraesthesia in hands, face and trunk dizziness headache tremor hypotension tachycardia
Physio Management
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
Respiratory Failure - definition and types
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
pH
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
PaO2
partial pressure of O2 dissolved in arterial blood
normal = 85mmHg
80-100
<60mmHg indicates hypoxaemia
PaCO2
partial pressure of CO2 dissolved in the arterial blood
normal = 35-45mmHg
HCO3-
buffer - a solution that can resist pH change
normal = 22-26mmol/L
SaO2
percentage of oxygen bound to haemoglobin
Respiratory Buffer System
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
Renal Buffer System
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-
Different levels of compensation
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
ABGs Types of Disorders
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
ABGs Interpretation
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?
Limitations of ABGs
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
