ABG Flashcards
ABG measurement give indication of
- ventiliation
- gas exchange
- acid-base status
Fi02
‘fraction of inspired o2’ which is the amount of oxygen that a patient is receiving
respiratory drive
- brain
- centeral (in the brainstem) and peripheral (in the heart) chemoreceptors
- mechanoreceptors is the lungs/chest wall (smooth muscle)
- other factors- pain/fear affect rate and depth
partial pressure
air moves in when
- Air moves in when air pressure inside the lungs is less than that in the atmosphere.
- Air moves through a concentration gradient
partial pressure
air moves out
- Air moves out when the pressure inside the lungs is greater than the pressure outside.
- Air moves through a concentration gradient
inhalation
- PP
- how is it achieved
- pressure in alveoli must be lower than atmospheric pressure for air to flow into the lungs
this is achieved by increasing the pressure in the lungs
increase in volume of the lungs mechanics
- Achieved by contraction of the diaphragm and external intercostals. As the diaphragm contracts it flattens thus increasing the vertical diameter of the thorax.
- As the external intercostals contract, the ribs elevate (bucket handle movement) thus increasing the anterior posterior and lateral diameters of the chest wall.
- As the size of the thoracic cavity increase, the volume increases and the pressure decreases. Air continues to flow as long as there is a pressure difference
- This inverse relationship is known as Boyle’s Law.
If someone doesn’t have the ability to change their lung volumes i.e. contract their muscles
- they cannot achieve adequate ventilation.
- (weak – weaning patient; fatigued – pt working hard for a long time; lack of innervation – SCI or progressive NMD)
- OR they could have abnormal MSK eg kyphoscoliosis
exhalation
- occurs due to the elastic recoil of the chest wall and lungs - doesn’t normally involve any muscle contraction.
- as muscles relax the size of thoracic cavity decreases –> volume decreases–> alveolar pressure increases
exhalation pressure
when the pressure inside the lungs is greater than the pressure in the atmosphere
factors affecting exhalation ihalation
- airway patency
- air flow
- respiratory drive
- skeletal structure
- muscle function
- BMI
obstructed airway can be affected by eg
sputum
bronchospasm
resp drive can be affected by
- GCS
- medication - opioids
- muscular dystrophy conditions
- Severe kyphosis or kyphoscoliosis affects chest wall biomechanics
skeletal structure affects breathing
overuse of muscles to fatigue them- need to build the muscles back up so theres no exhaustion of respiratory muscles to the point they cant ventilate themselves
obesity affects breathing by
increased subcutaneous fat lungs stay the same size but there’s an increased load on their lungs which makes it a lot harder to take in breaths
partial pressure definition
pressure exerted by each individual gas in a mixture of gases (air)
- gas diffuses down a pressure gradient (from high to low)
ficks law of diffusion
The rate of diffusion is proportional to the surface area and concentration difference and is inversly proportional to the thickness of the membrane
efficient diffusion needs
- a concentration gradient, a large surface area and a thin membrane.
- Large surface area created by the millions of alveoli.
Any disease state that can reduce the number of functioning alveoli will affect the surface area e.g. sputum, collapse, conditions such as emphysema
other factors affecting diffusion
- Other factors affecting diffusion – thickened membrane between alveoli e.g. fibrosis
- Bullae- big areas in lungs with lost lung tissue reduce surface area- affecting oxygenation
oxygen transport in blood
- 2% in plasma
- 98% RBC
oxygen heamoglobin dissociation curve
- Partial pressure is high in the capillaries in the lungs and so almost all of the Hb is saturated with O2
- Areas where partial pressure is low e.g. capillaries supplying the tissues of the body, Hb will release it’s O2 and therefore supply the cells with O2 for respiration.
o2 Hb dissociation shift to left and right
- Shift to the left – increased affinity for O2 and won’t release it as readily
- Shift to the right – reduced affinity for O2 and releases it more easily e.g. during exercise so more available to the tissues
Giving supplementary oxygen can increase
The oxygen available and therefore increase the oxy-Hb saturation
CO2 and control of respiration
7 point
- Stimulus disrupts homeostasis
- Increases arterial blood Pco2 or decreases pH or Po2
- Chemo receptors in medulla or peripheral chemoreceptors in the aortic or carotid arteries
- In put nerve impulses to the control centre - inspiratory area in medulla oblongata
- Output nerve impulses to the effectors muscles of inhalation and exhalation contract more forcefully and more frequently (hyperventilation)
- Decrease in arterial blood Pc02 increase in pH and increase in P02
- Return to homeostasis when response brings arterial blood Pc02, pH and P02, back to normal
If blood CO2 increases and the pH drops = becomes more acid (due to increase in hydrogen ions)
- chemoreceptors respond by sending nerve impulses to the respiratory control centre in the brain
- which innervates the respiratory muscles and RR increases
- patient hyperventilates to try and reduce the level of CO2 and therefore H+
hypoxic drive
3
- considered in patients with a history of hypercapnia
- some patients have chronically high PaCO2 levels resulting in desensitisation of the CO2 receptors
- they depend on low PaO2 levels for their respiratory drive
acid base balance
- disturbed if there are any abnormalities with CO2 removal from the lungs or production and removal of acid from the tissues
- Normal 7.35-7.45
- Alkalotic - high pH ( 7.45-7.8)
- Acidotic - low pH- too much co2 (7.35-6.8)
buffer systems
- prevents rapid, drastic pH changes by neutilising acids and bases by reactions that give up or absorb hydrogen ions
buffer system
5
- CO2 dissolves in water (blood) to produce carbonic acid.
- ·The more CO2 there is the more H2CO3 carbonic acid, which dissociates to form more H+.
- Free hydrogen ions are what affect the pH. The more there are the more acidic the pH.
- HCO3 is a base and therefore accepts hydrogen ions, so reduced the concentration of H+ in the blood.
- All happening within the RBCs and alongside other mechanisms.
repiratory mechanisms (if buffering isnt adequate)
- Increased ventilation, more CO2 exhaled and the equation is driven to the left, H+ falls and the pH increases.
- Decreased ventilation, Co2 is retained and the concentration increases, thus driving the reaction to the right and H+ increases thus lowering the pH.
renal (metabolic) mechanisms
- HCO3- is a base so will accept H+ thus reducing the H+ ions in the blood.
- Main site of HCO3 reabsorption is the proximal tubule of the kidney.
PH normal value
7.35-7.45
PaO2 normal value
10.7-13.3 kPa (80-100 mmHg)
PaCO2 normal value
- 4.7-6 kPa (35-45 mmHg)
HCO3 (bicarbonate) normal value
- 22-26 mmol/L
BE= (base excess) normal value
-2 to +2
SaO2 (o2 saturation) normal value
95-100%
abnormalities in PaO2
hypoxia
hypoxaemia
hypoxia
reduced oxygen at the tissue level
* can result from hypoxaemia
hypoxaemia
reduced oxygen in arterial blood
V/Q
V= ventilation - air entering the alveoli
Q= perfusion - amount of blood flowing through the capillaries surrounding the alveoli
causes of hypoxia
- hypoxaemia
- reduced cardiac output
- reduced o2 carrying capacity of the blood
- reduced/restricted blood flow
causes of hypoxaemia
- V/Q mismatch due to wasted perfusion or ventilation
- hypoventilation
- diffusion abnormality
- avalible O2
reduced V/Q causes
- Collapsed alveoli (wasted perfusion)
- sputum in alveoli,
- blood clot in the capillaries (wasted ventilation)
Type 1 respiratory failure
Pa02- lower than 8KPa
type 1 respiratory impairment
Pa02- 8-10.7
Type 2 resp failure
- pH- acidotic
- PaC02 higher than 6 KPa
- Pa02- low
**Acute **2- pH acidotic
chronic 2- pH may be normal due to compensation
type 2 failiure cause
- occurs due to inadequate alveolar ventilation
- COPD
- fatigue
- kyphoscoliosis
- drigs- opiates
- nueromuscular disorders
type 1 respiratory failure causes
- pneumonia
- pulmonary embolus
- pneumothorax
- pulmonary odema
- acute asthma
- acute respiratory distress syndrome
- fibrosis
- COPD
- COVID
pH low/high
lower than 7.35= acidotic
higher than 7.45= alkalotic
PaCO2 low/high
lower than 4.7 KPa = respiratory alkalosis
higher than 6.0 KPa = respiratory acidosis
causes of hypocapnia
low CO2- hyperventilation
causes of hypercapnia
high CO2- hypoventilation
HCO3 & BE high low
HCO3 lower than 22mmol/L & BE lower than -2 = metabolic acidosis
HCO3 higher than 26mmol/L & BE higher than +2 = metabolic alkalosis
Causes or metabolic acid/alka
acid
- diabetic ketoacidosis, lactic acidosis, diarrhoea/high GI output, renal failure
alka
- vomitting, over diuresis
HCO3 & BE indicate exetend of renal compensation and thus metabolic contribution to an acid-base disturbance
