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Arterial Blood Gases
Transcript of Arterial Blood Gases
Pre class material
By the end of this session you should be able to:
identify normal values on a blood gas
understand the compensation of metabolic and respiratory function
identify basic conditions that lead to acid base dysfunction
identify and classify issues on arterial blood gases
identify the physiotherapy issues
identify oxygen sensitive patients and suggest management strategies in relation to oxygen therapy / ventilation
What are ABGs?
pH: 7.35 – 7.45
PaCO2: 4.7 – 6 kPa
PaO2: 10.7 – 13.3 kPa
HCO3 22 – 26 mmol/L
BE: -2 - +2 mEq/litre
But what does this mean?!
What do ABGs tell us?
Acid Base Balance
Working it out
Your body strives to maintain normality.
The pH tells us how acid or alkali the blood is.
It is inversely proportional to the number of hydrogen ions in the blood, therefore the more H+ ions there are the lower the pH will be.
pH is measured from 1 -14.
The normal pH of blood is 7.35 – 7.45.
When the pH is less than 7.35 the blood is said to be acidic, when it is greater than 7.45 it is said to be alkaoltic
When there is a shift in the status of the blood the body’s functions will alter.
If the blood is acidic there is a decrease in the force of cardiac contractions and certain medication may not work as effectively.
If the blood is alkalotic this
can affect tissue
and muscular functioning.
The body strives to maintain the pH within normal ranges,
It achieves this by a using a buffer system between the respiratory and renal systems.
If one system is overwhelmed and there is change in blood pH the other system will compensate.
The respiratory system is able to start compensating within minutes to hours, however the metabolic system takes days.
A buffer is a substance that can either bind or release hydrogen ions, this maintains pH. There are 4 buffers in the blood: Haemoglobin, plasma proteins, phosphate and bicarbonate.
(Hennessey & Japp 2007)
A little bit of chemistry…
This equilibrium equation shows us that
if either CO2 or HCO3- are altered then the pH will be affected.
the pH of the blood is dependent on a ratio of CO2 to HCO3 rather than absolute amounts. This equation demonstrates that’s when CO2 is dissolved in the blood it becomes an acid. The more CO2 there is in the blood the more carbonic acid (H2CO3) is generated, this dissociates and excess H+ ions are released.
What does this mean physiologically?
CO2 is the by-product of cellular metabolism. In the lungs excess CO2 combines with H2O and carbonic acid is produced, the level of carbonic acid will alter the pH of the blood. The lungs are able to alter the rate and depth of respiration, until there is the correct level of CO2. This process starts to occur within 1-3 minutes.
Respiratory compensation - the lungs as a buffer:
The kidneys excrete or retain Bicarbonate HCO3-. If the pH decreases the kidneys will retain HCO3-, if the pH increases the kidneys excrete HCO3- via urine. This process takes days to occur.
Metabolic compensation - the kidneys as a buffer:
A respiratory problem will be compensated by the metabolic system and that a metabolic problem will be compensated by the respiratory system
It can be seen that:
Acid-Base Disorders: Respiratory Acidosis
Results from an increase in PaCO2 (which then combines with H2O in the body to form carbonic acid, which lowers pH) caused by hypoventilation.
CNS suppression – head injury / drugs
Weak respiratory muscles – MND
Fatigue of respiratory muscles – exhaustion
Chest wall injury / deformity/ abdomen distension
Chest conditions such as pneumonia, pneumothorax, pulmonary oedema
Potential causes of respiratory acidosis :
Physiotherapy may help with these problems
Acid - Base Disorders Respiratory Alkalosis:
Results from a decrease in PCO2 caused
by alveolar hyperventilation.
Potential causes of respiratory alkalosis :
Excess mechanical ventilation
increased metabolic demands - fever, pregnancy or sepsis
Physiotherapy may help with these problems
Results from excess hydrogen ions in the body, which reduces bicarbonate
Acid - Base Disorders Metabolic Acidosis:
Potential causes of metabolic acidosis :
Lactic acid build up in surgery
Deficit of base:
NOTE: the respiratory system will compensate by hyperventilation - the patient will look breathless but this is NOT a physiotherapy problem
Acid - Base Disorders Metabolic Alkalosis :
Results from an increase in bicarbonate in the body, or a fall in the hydrogen ions.
Potential causes of metabolic alkalosis :
Too much base:
Too many antacids
Excess use of bicarbonates
Use of lactate in dialysis
Loss of acid:
Physiotherapy is unlikely to help these patients
:7.35 – 7.45
Partial pressure of CO2 dissolved in arterial blood (4.7 – 6 kPa or 35-45mmHg) .
Partial Pressure of O2 dissolved in arterial blood (10.7 – 13.3 kPa or 80-100mmHg)
Saturation of O2 we will cover this in more depth in week 4 as there is variation.
The calculated level of bicarbonate in the bloodstream (22 – 26 mmol/L)
Base excess indicates the quaity of excess or insuffcient level of bicarbonate in the blood (-2 - +2 mEq/litre)
You may find the following on an ABG sample report:
Arterial blood Gases (ABGs) are an important diagnostic tool. They tell us a great deal about how the respiratory and metabolic systems are working.
ABGs are normally taken from an artery such as radial, brachial or femoral.
They can be painful for the patient.
Sample taken by Doctor, Nurse or Physiotherapist with extended training
Our bodies use oxygen to generate energy, this produces CO2 as a waste product. ABGs help us to assess the effectiveness of gas exchange in the blood by analysing the partial pressure of O2 (PaO2) and CO2 (PaCO2).
The partial pressure of a gas is a measure of the concentration of a gas in a mixture of gases
Gas exchange the basics
Where to Start?
Look at oxygen first. Although it is not involved in the process of working out ABGs.
Oxygen level allows you to see if supplementary oxygen is required or if they are reviving too much oxygen therapy.
If the patient has a PO2 of less than 8kpa, supplementary oxygen maybe required.
Patients who have chronic hypercapnia maybe oxygen sensitive.
These patients do not regulate there breathing based on levels of CO2 in the blood. Instead these patients have adapted to rely on PaO2 level to drive ventilation, this is known as hypoxic drive.
If these patients are given too much O2 this can depress ventilation and cause an increase an PaCO2.
These patients will have a SaO2 of 88-92% and a longstanding respiratory condition such as COPD and a longstanding raised CO2.
If you have an oxygen sensitive patient, and they become drowsy or confused consider that they may be given too much O2.
The flowchart below provides the basis for working out ABGs.
Working it out Summary
Final Step....Respiratory Failure
Type 1 Resp Failure:
hypoxemia without hypercapnia. It is typically caused by a ventilation/perfusion (V/Q) mismatch. Can be acute (infection) or chronic (COPD).
Type 2 Resp Failure:
Hypoxemia (PaO2 <8kPa) with hypercapnia (PaCO2 >6.0kPa). Cause –impaired resp drive. Can be acute (severe actute asthma) or Chronic – late stage COPD
and enter code 4910F1BE
Answers can be:
Fully or Partially Compensated
There can be type 1 or type 2 Respiratory failure.