Hypercapnia & Hypoxia Flashcards
Define hypoxia
reduced O2 in tissues
Define hypoxaemia
reduced O2 in the blood
List the mechanisms behind hypoxia and hypoxaemia
reduced oxygen inspiration
cannot get 02 to lungs
cannot get 02 inot blood
reduced systemic O2
cannot bind to Hb
Provide a list of causes relating to reduced oxygen inspiration
Equipment related
Anatomy
Altitude
Hypoventilation
URT Obstruction
Pneumothorax
Provide a list of causes relating to inability to get oxygen into the lungs
Equipment
URT Obstruction
Pneumothorax
Provide a list of causes relating to reduced systemic O2
Anaemia (see pulse oximetry slide later)
Heart disease (e.g. resulting in output failure)
Hypovolaemia
-Periphery shuts down as preferential shunting to vital organs occur
Extreme vasoconstriction
-In cold environment the periphery is shut down for preferential shunts to vital organs
Sepsis (discussed more in later lectures)
Provide a list of causes relating to oxygen being unable to bind to Hb and/or cannot get off of Hb
Haemoglobinopathy
Methemoglobinemia
Provide a list of causes relating not being able to get oxygen into the blood
VQ mismatch
Intrapulmonary Shunts
Diffuse alveolar/interstitial disease
Explain how equipment reduces oxygen inspiration and reduces the amount of O2 getting into the lungs
Kinked/obstructed ET (endotracheal) tube.
If you have the luxury of a Capnograph, this will be spotted quickly (the wave form will look a bit like a sharks fin).
Obstructed due to a build up of mucous or it could be something physiological such as bronchoconstriction.
Rarely, it could be a kinked or damaged tube on the breathing system. These tubes are normally pretty robust so it wouldn’t be a common problem.
Breathing system may not be working properly due to leak (leak tests down prior to anaethesia)
Issue with oxygen supply
Resolution:
Unkink tube or re-intubate with a new tube.
If it is physiological, treat the root cause.
Explain how anatomy reduces oxygen inspiration
BOAS (brachycephalic obstructive airway syndrome)
-narrowed airways mean inspiration of reduced amounts of O2
-doesnt always lead to hypoxia but means higher risk
-airways can become easily obstructed
Resolution:
Short term intubate
Some may require surgery in the long term
Explain how altitude reduces oxygen inspiration
Lower atmospheric pressure which means decreased partial pressure of oxygen
Hyperventilation due to low atmospheric pressure
Leads to hypocapnia
An example of the link between CO2 and O2 is how the body compensates for lower atmospheric pressures by hyperventilating.
Explain how hypoventilation reduces oxygen inspiration
Results in hypercapnia
Very common cause
Partial pressure of oxygen in the alveoli (PAO2) decreases
Partial pressure of carbon dioxide in the alveoli (PACO2) rises
Resulting in hypercapnia
Explain how URT obstruction reduces oxygen inspiration and reduced the amount of oxygen getting into the lungs
Loss of pharyngeal muscle tone
Regurgitation
Vomiting
Laryngospasm
Stick injury
Laryngeal oedema
Tracheal collapse
Foreign body
Depending on the severity of the obstruction there may either be reduced O2 inspired or complete inability to get O2 to the lungs
Explain how pneumothorax reduces oxygen inspiration and reduced the amount of oxygen getting into the lungs
Air has reached the lungs but it escapes into the space between lung and chest wall and cannot take part in efficient gas exchange
Explain how Haemoglobinopathy lead to the inability of oxygen binding to Hb
Carbon monoxide binds to haemoglobin with much greater affinity and prevents the carriage of oxygen.
Shifts the haemoglobin-oxygen dissociation curve to the left
Exposure to even small concentrations of CO (carbon monoxide) hinder the ability of Hb to deliver oxygen to the body, because carboxyhaemoglobin forms more readily than oxyhaemoglobin (HbO2) does.
Carbon monoxide is produced in normal metabolism and is a common chemical, but excessive exposure can lead to big problems.
Explain how Methemoglobinemia leads to the inability of Hb to remove oxygen
Elevated methaemoglobin in the blood, haemoglobin iron is in the oxidized or ferric state and cannot reversibly bind oxygen – paracetamol toxicity
Methaemoglobin is a type of haemoglobin which carries oxygen through the blood but doesn’t release it to the cells
When there is too much methaemoglobin produced, it starts to replace normal haemoglobin, and therefore oxygen is not released and doesn’t reach tissues
Briefly outline some causes of VQ mismatch
All lung dysfunction lowers PaO2 via V-Q imbalance, e.g., asthma, pneumonia, atelectasis, pulmonary oedema, horse on its back (common problem in Equine anaesthesia!)
Explain the effect of gravity on VQ ratio
Since the pressure of blood being pumped into the lungs is so low, where the blood ends up is usually determined by gravity.
Similarly ventilation is affected by gravity however not as significantly .
Positioning makes a difference; in a standing animal with normal lungs, there will be a small degree of V/Q mismatching with minimal perfusion in the dorsal lung fields and higher in the ventral areas.
Explain the effect of diffusion abnormalities on VQ ratio
Oxygen enters alveolus, then diffuses across the one-cell layer thick epithelium of the lung, then across the one-cell layer thick endothelium, into the pulmonary artery then into the pulmonary capillary bed where gas exchange takes place and then into the pulmonary vein
When there is a diffusion abnormality, it is often caused by a blockage/interstitial deposition between these two areas within the unit. This is often seen in patients with infiltrative disease of the lungs such as pulmonary fibrosis and it hinders the oxygen from diffusing through the unit sufficiently, which can ultimately lead to hypoxaemia.
Explain mismatch as a result of a pulmonary embolism
Pulmonary artery splits off into various areas of the lungs
If there is a clot formed, it will be pumped in to a specific area and can then get lodged.
As a result, perfusion to a certain segment of the lung is blocked.
That area is then classified as an area of high V/Q because there is very low perfusion.
The blood that would normally go to this area has to go somewhere else, therefore you end up with a low V/Q in multiple areas.
Describe how pulmonary shunts affect VQ
In the normal lung, oxygen enters the alveoli and raises the saturation from the venous level of 70% to 100% by the time it reaches the arterial side.
In shunt, no oxygen can get into the alveoli, so the venous saturation is never increased.
In low SvO2 situations, the alveoli are not able to raise the low venous saturation to the normal arterial level. When these two problems are both present, the arterial desaturation becomes even worse.
Blood enters the systemic arterial system without going through ventilated areas of lung.
Most caused by cardiac disorders with right to left shunting such as reverse PDA (Patent Ductus Arteriosus)
May also occur when large areas of the lung are not ventilated e.g., atelectasis
List some of the consequences of hypoxia/hypoxaemia
- Decreased brain function (including ventilation)
- Decreased cardiac function (including output)
- Made worse where there is concurrent hypercapnia
- Cardiac dysrhythmia
- Cardiac arrest
Outline the importance of tissue perfusion
Perfusion of O2 to tissues is one of the basic needs to sustain life.
Brain tissues will die within minutes if their supply of O2 is interrupted.
Tissues of major organs last only 15-20 minutes without O2.
We cannot afford to wait to see if the animal gets better with cage rest, we need to be proactive and aggressive with our treatments to re-perfuse tissue.
Discuss some of the presenting signs of hypoxia/hypoxaemia
Tachypnoea, increased respiratory effort, tachycardia (common in patients with adequate compensatory ability).
Patients unable to compensate for hypoxaemia (exhaustion, neurologic or neuromuscular disease) may not show outward evidence of respiratory distress due to respiratory muscle weakness
Cyanosis may be detected by the human eye if the hypoxia is particularly bad
Obtundation or coma may occur in patients with prolonged uncompensated hypoxemia
Death
