Blood Gases Flashcards
(43 cards)
Why blood gases are important for physiologists
Oxygen service – primarily to monitor hypoxia, however also monitor CO2 retention is patients who utilise the ‘hypoxic drive’
Sleep service
Hypo/normocapnic patients includes some patients with CSA and those with a Cheyne-Stokes breathing (CSB) pattern
Hypercapnic patients have high normal or elevated wake PaCO2which may rise further in sleep. This includes patients with CSA due to drug or substance and primary CSA of infancy. This group also includes patients with obesity hypoventilation syndrome, thoracic cage disorders, neuromuscular disorders and other hypoventilation syndromes
NIV – to monitor CO2 retention and hypoxia and titrate pressures accordingly
Hypoxaemia terminology
- ↓ Blood O2
- Decreased PO2
Hypoxia terminology
- Decreased SaO2
- ↓ Tissue O2
Signs and symptoms of hypoxia
Restlessness, confusion
Tachypnoea, dyspnoea
Tachycardia, dysrhythmia, hypertensive
Clubbing – Area of Scientific Debate
Right heart failure
General physical appearance
Hypocapnia / Hypercapnia
Decreased PCO2 / Increased PCO2
Signs and Symptoms of Hypercapnia
Visual : dimmed sight
Respiratory : SOB
Muscular : tremor
Heart : increased heart rate and blood pressure
Skin : sweating
Central : drowsiness, mild narcosis, dizziness. confusion, headache, unconsciousness
pH
- Measure of acidity or alkalinity
(hydrogen ion activity)
Alkalaemia/ Acidaemia
Increased pH / decreased pH
Measured parameters
pH
reference range 7.35 - 7.45
Partial pressure of carbon dioxide (pCO2)
reference range 4.7 - 6.0 kPa
Partial pressure of oxygen (pO2)
reference range 11 - 14 kPa
Serum Bicarbonate (HCO3-) [AKA: Standard Bicarb]
reference range 22 - 26 mmol/L
Base Excess (BE)
reference range -3 to 3 mmol/L
The base excess is defined as the amount of H+ ions that would be required to return the pH of the blood to 7.35 if the pCO2 were adjusted to normal.
What is pH?
pH is defined as the negative logarithm to base 10 of the hydrogen ion concentration.
pH = -log10 [H+]
Normal reference range 7.35 - 7.45.
Measure of acidemia or alkalemia and is vital to the diagnosis and management of a wide range of conditions.
Outside the acceptable range of pH, proteins are denatured and digested, enzymes lose their ability to function, and death may occur.
H+ reference range is 35–45 nmol/L (nM)
An 0.1 unit fall in pH from 7.4 to 7.3 represents a 25% increase in [H+]
Acid/Base Balance
pH is primarily regulated by factors in the Henderson-Hasselbalch equation and “buffering”
pH = pK + log [HCO3-]
α x pCO2
pK = acid dissociation constant
a= the CO2solubility coefficient (derived from Henry’s law – 0.03).
HCO3- from lungs
pCO2 from lungs
Numerator change = metabolic acidosis (reduced HCO3-) or metabolic alkalosis (elevated HCO3-);
Denominator change = either respiratory alkalosis (reduced PaCO2) or respiratory acidosis (elevated PaCO2).
The pK’a value is dependent on the temperature, [H+] and the ionic concentration of the solution. It has a value of 6.099 at a temperature of 37C and a plasma pH of 7.4. At a temperature of 30C and pH of 7.0, it has a value of 6.148. For practical purposes, a value of 6.1 is generally assumed and corrections for temperature, pH of plasma and ionic strength are not used except in precise experimental work.
Which Systems are Involved in Acid/Base Balance?
The acid/base status is controlled by a balance between 3 primary functions:
Chemical Buffering (i.e. Carbonic Acid - Bicarbonate System)
Respiratory buffering control of CO2.
Metabolic, Renal control of HCO3-.
These components are used to distinguish between Respiratory and Metabolic disturbances of the acid/base status.
Buffers in blood
The most important pH buffer is the Carbonic Acid - Bicarbonate System
CO2 + H20 <-> H2CO3 <-> H+ + HCO3-
Excess carbonic acid can be converted into carbon dioxide gas and exhaled through the lungs; this prevents too many free hydrogen ions from building up in the blood and dangerously reducing its pH
Mechanism of carbonic acid - bicarbonate system
The major buffer system in the ECF is the CO2-bicarbonate buffer system. This is responsible for about 80% of extracellular buffering. It is the most important ECF buffer for metabolic acids but it cannot buffer respiratory acid-base disorders.
Carbonic acid is an intermediate step on the transport of CO2out of the body viarespiratory gas exchange. The hydration reaction of CO2is generally very slow in the absence of a catalyst, butred blood cellscontaincarbonic anhydrase, which both increases the reaction rate and dissociates a hydrogen ion (H+) from the resulting carbonic acid, leavingbicarbonate(HCO3−) dissolved in theblood plasma. This catalysed reaction is reversed in the lungs, where it converts the bicarbonate back into CO2and allows it to be expelled.
When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, excess carbonic acid can be converted into carbon dioxide gas and exhaled through the lungs; this prevents too many free hydrogen ions from building up in the blood and dangerously reducing its pH; likewise, if too much OH–is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH
Because carbon dioxide is non-polar, it cannot dissolve in the bloodstream to travel to the lungs. As such,carbonic anhydraseconverts carbon dioxide and water into carbonic acid, which is polar and thus can travel through the body. In the lungs, carbonic anhydrase converts carbonic acid back into water and carbon dioxide to expel carbon dioxide through the lungs without losing water.
Bicarbonate Control
The concentration of HCO3- is controlled by the Kidneys.
The regulation of HCO3- takes hours to days to affect the acid/base balance.
Bicarbonate can be the key to assessing whether a blood gas result is acute, chronic or acute-on-chronic.
If this form of pH modulation is prolonged it may/will lead to renal failure – Kidneys are working considerably harder than usual.
Acute = I.e. someone having an asthma attack. Chronic = I.e. someone with long standing COPD. Acute on Chronic – i.e. someone with COPD having an exacerbation. Acute on chronic typically presents as respiratory failure with partial compensation and particularly deranged PO2/PCO2
Metabolic and respiratory components of carbonic exchange
Resp component:
Intracellular metabolism produces metabolites that form acids in water (Carbonic Acid)
Approximately 98% of this acid load is in the form of CO2
CO2 + H2O H2CO3
Excess CO2 is exhaled by the lungs
Metabolic component:
Function of the kidneys
Base bicarbonate: Na HCO3
Process of kidneys excreting H+ into the urine and reabsorbing HCO3- into the blood from the renal tubules 1) active exchange Na+ for H+ between the tubular cells and glomerular filtrate 2) carbonic anhydrase is an enzyme that accelerates hydration/dehydration CO2 in renal epithelial cells
How does the kidney participate in homeostasis
The kidney participates in whole-body homeostasis, regulating acid base balance, electrolyte concentrations, extracellular fluid volume, and regulation of blood pressure. The kidneys have two very important roles in maintaining the acid-base balance: to reabsorb bicarbonate from urine, and to excrete hydrogen ions into urine. The kidney can correct any imbalances by:
Removing excess acid (hydrogen ion) or bases (bicarbonate) in the urine and restoring the bicarbonate concentration in the blood to normal
The Partial Pressure of a Gas
The pressure (Tension) of an individual gas in a mixture is referred to as a partial pressure.
The pressure of a mixture of non-reacting gases is the sum of the partial pressures of its constituents. (Dalton’s law of partial pressures).
e.g. the atmosphere contains 20.9% oxygen and has a pressure of 101 kPa therefore:
pO2 = 20.9 x (101-6.3) kPa = 19.8kPa
100
Arterial Blood Gas sampling
Radial artery the most common site.
Purpose designed ABG Syringe.
~2mL blood.
Lithium Heparin anticoagulant
Eliminate air bubbles.
Rapid analysis (<5min) or on ice water slurry.
ABG Alternative
ABGs often used in acute settings in which blood gas status requires urgent assessment – Painful for patient!
Capillary Blood Gas (CBG) samples are an acceptable alternative in non-urgent situations (i.e. O2 assessment, NIV outpatient visit)
Earlobe must be adequately vascularised i.e. warmed with a heat source or use of transvasin cream
Equally accurate for PCO2 but PO2 can be underestimated by 0.5-1.0kPa on CBG
Arterial Blood Gas Levels
Blood gas levels depend upon gases passing from an area of high partial pressure to an area of low partial pressure.
e.g. A high pressure of O2 in alveolar gas causes O2 to diffuse into the pulmonary capillary blood.
In this “oxygen cascade” diagram below a PaO2 <55mmHg will result in tissue hypoxia that produces organ damage.
Chemo-receptors and control of ventilation
Central chemo-receptors in blood-brain barrier are [H+] sensitive -> drives respiratory accelerator centre.
Hypoxic receptors in carotid bodies drive apneustic centre. [H+] receptors drive “pneumotaxic” centre.
With voluntary, emotional, limbic and humeral control over-riding these.
Hypercapnia causes a significant swing in ventilation when compared with hypoxia.
Oxygen Homeostasis
Normal range 11 - 14kPa.
Oxygen homeostasis is dependent upon a number of factors:
Inspired oxygen concentration (FiO2).
Inspired oxygen partial pressure.
Tidal volume (VT) Alveolar ventilation (VA).
Homeostasis The property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH.
In thenormallung, the V and the Q arenot equal, the normal ratio is about 0.8.This is due to two main reasons:gravityandair. The diagram to the right can be simplified as follows. There is more air in the top of the lung, and there is more blood in the bottom of the lung (because of gravity). This means that some of the blood in the bottom of the lung is not oxygenatedandsome of the air in the top ofthe lung does not have its oxygen extracted.This concept is critical to understand. Disruptions of V and Q are how pulmonary embolisms, pneumonia, and other lung pathologies kill patients.
What’s the atmosphere in the alveolus?
The amount of CO2 is a function of;
-CO2 production (metabolism)
- CO2 Unloading
- And minute alveolar ventilation
CO2 production is constrained by metabolism
Alveolar ventilation is a function Vd/Vt
If alveolar CO2 increases -> causing CO2 retention then in the tissues, PAO2 must also decrease. Displacement of oxygen molecules by CO2 molecules.
