B2 W2 - Pathophysiology of Respiratory Failure Flashcards
1
Q
What does the oxygen cascade describe?
A
The oxygen cascade outlines the stages in which the partial pressure of oxygen (PO2) decreases from the air we breathe to the mitochondria in our cells.
2
Q
What is the partial pressure of oxygen (PO2) in dry air at sea level?
A
21.2 kilopascals (kPa).
3
Q
What is the partial pressure of oxygen at the level of the mitochondria?
A
Between 0.5 to 3 kPa, varying with the tissue, individual cell and even region of the cell.
4
Q
What are the key stages of the oxygen cascade?
A
Humidification of inspired air in the trachea and conducting airways.Alveolar gas exchange.Diffusion across the alveolar-capillary membrane.
5
Q
How does humidification affect the partial pressure of inspired oxygen (PiO2)?
A
Inspired air is warmed to body temperature (37°C) and fully saturated with water vapour in the trachea, diluting the oxygen and reducing its partial pressure.
6
Q
What is the equation for calculating PiO2 after humidification?
A
PiO2 = FiO2 (PB - PSVP water).FiO2 = fraction of inspired oxygen (0.21 for room air).PB = atmospheric pressure (kPa), which is dependent on altitude.PSVP water = saturated vapour pressure of water (kPa).
7
Q
What is the approximate PiO2 after humidification?
A
19.95 kPa.
8
Q
What two main factors determine the partial pressure of oxygen in the alveolus?
A
The amount of oxygen supplied to the alveolus by ventilation.The amount of oxygen diffusing into the bloodstream and removed by the pulmonary capillaries.
9
Q
Why is the alveolar gas equation important in understanding the oxygen cascade?
A
It allows us to calculate the alveolar PO2, which cannot be measured directly, and helps us understand how changes in ventilation and inspired oxygen concentration affect alveolar PO2.
10
Q
What is the alveolar gas equation?
A
PAO2 = PiO2 - (PaCO2 / R).PAO2 = alveolar partial pressure of oxygen.PiO2 = partial pressure of inspired oxygen (after humidification).PaCO2 = partial pressure of carbon dioxide in arterial blood.R = respiratory exchange ratio or respiratory quotient (usually estimated at 0.8).
11
Q
How does the alveolar gas equation relate to ventilation?
A
As ventilation increases, PaCO2 decreases, leading to an increase in PAO2. Conversely, decreased ventilation leads to increased PaCO2 and decreased PAO2.
12
Q
What is the impact of increasing the fraction of inspired oxygen (FiO2)?
A
It increases the alveolar partial pressure of oxygen more effectively than hyperventilation.
13
Q
How does diffusion affect the oxygen cascade?
A
Oxygen diffuses across the alveolar-capillary membrane into the bloodstream, driven by the partial pressure difference between the alveolus and the capillary blood.
14
Q
How does the diffusion of carbon dioxide compare to that of oxygen?
A
Carbon dioxide diffuses about 20 times faster than oxygen due to its higher solubility coefficient.
15
Q
What are the implications of the difference in diffusion rates between oxygen and carbon dioxide?
A
Diseases affecting the diffusion barrier, like pulmonary fibrosis, impact oxygen diffusion more significantly than carbon dioxide diffusion.
16
Q
What does it mean for gas exchange to be perfusion-limited?
A
Under normal conditions, oxygen diffusion is perfusion-limited because the PO2 in the alveolus and capillary blood reach equilibrium about a third of the way along the capillary. Therefore, increasing blood flow is the primary way to enhance oxygen transfer.
17
Q
What conditions can make gas exchange diffusion-limited?
A
Thickening of the alveolar-capillary membrane.Strenuous exercise, which reduces capillary transit time.Increasing altitude, which lowers the alveolar PO2 due to lower atmospheric pressure.
18
Q
What happens to inhaled gas in the trachea and conducting airways?
A
The gas is humidified: heated to body temperature (around 37°C) and fully saturated with water vapour.
19
Q
How does humidification affect the partial pressure of oxygen?
A
The addition of water vapour dilutes the oxygen, thereby reducing its partial pressure.
20
Q
What is the symbol for the partial pressure of inspired oxygen after humidification?
A
PiO2.
21
Q
What is the equation for calculating PiO2?
A
PiO2 = FiO2 (PB - PSVP water)
22
Q
What does FiO2 stand for, and what is its value in room air?
A
FiO2 stands for the fraction of inspired oxygen, which is the concentration of oxygen in the gas mixture. In room air, it is 0.21 (representing 21% oxygen).
23
Q
Does FiO2 change with altitude?
A
No, FiO2 remains consistent irrespective of altitude.
24
Q
What does PB stand for, and how does it relate to altitude?
A
PB represents atmospheric pressure, measured in kilopascals (kPa). Atmospheric pressure decreases as elevation above sea level increases.
25
How does a change in PB affect PiO2?
As PB decreases with increasing altitude, PiO2 also decreases.
26
What does PSVP water stand for?
PSVP water represents the saturated vapour pressure of water, measured in kPa.
27
What is the significance of PSVP water in the PiO2 equation?
It accounts for the dilution of oxygen caused by humidification.
28
What is the approximate value of PiO2 after humidification in the trachea?
Around 19.95 kPa.
29
Is the reduction in PiO2 due to humidification significant?
While there is a reduction, it is not considered a significant reduction.
30
What is the purpose of the alveolar gas equation?
It allows us to calculate the alveolar partial pressure of oxygen (PAO2) because it can't be directly measured.
31
What is the alveolar gas equation?
PAO2 = PiO2 - (PaCO2 / R)
32
What is PiO2?
The partial pressure of inspired oxygen after it has been humidified.
33
What happens to the PiO2 when a person breathes a gas mixture of 40% oxygen, assuming all other factors remain constant?
It increases.
34
What happens to the PiO2 when a person stands at the top of Mount Everest, assuming all other factors remain constant?
It decreases because atmospheric pressure decreases with increasing altitude.
35
What is PaCO2?
The partial pressure of carbon dioxide in arterial blood.
36
Why is PaCO2, and not the partial pressure of carbon dioxide in the alveolus, used in the equation?
While the equation requires the partial pressure of carbon dioxide in the alveolus, PaCO2 is used instead because it's more easily measured and the values can be used interchangeably. This is because the partial pressure of carbon dioxide reaches equilibrium as blood passes the alveoli.
37
What is the respiratory exchange ratio (R)?
It represents the relationship between the number of carbon dioxide molecules produced and the number of oxygen molecules consumed by metabolism, calculated as CO2 production/O2 consumption.
38
What does the respiratory exchange ratio depend on?
The type of fuel being metabolised: fat, carbohydrates, or protein.
39
What is the typical estimated value for R?
0.8, which indicates that more oxygen is consumed than carbon dioxide is produced.
40
How does the alveolar gas equation relate to ventilation?
As ventilation increases, more carbon dioxide is eliminated, leading to a decrease in PaCO2. Because PaCO2 is inversely proportional to ventilation, the equation predicts that PAO2 will increase. Conversely, a decrease in ventilation will lead to an increase in PaCO2 and a decrease in PAO2.
41
What is the effect of increasing FiO2 on PAO2?
Increasing FiO2 by giving someone a gas mixture to breathe with a higher concentration of oxygen than normal air increases PAO2 more than hyperventilation.
42
Does increasing FiO2 mean that the amount of oxygen carried in the blood also increases proportionally?
No. Oxygen carriage is limited by the carrying capacity of haemoglobin and the oxygen saturation curve.
43
How does the alveolar gas equation help us predict changes in PAO2?
By taking into account the balance between the amount of oxygen supplied to the alveoli (by ventilation) and the amount removed from the alveoli (by diffusion into the bloodstream), the equation can be used to predict how changes in ventilation and FiO2 will affect PAO2.
44
In clinical practice, what is the only factor we can readily change to influence the rate of diffusion of a gas across the alveolar-capillary membrane?
The partial pressure difference; for example, by increasing the fraction of inspired oxygen.
45
How much faster is the rate of diffusion of carbon dioxide compared to oxygen?
About 20 times faster.
46
Why is the rate of diffusion of carbon dioxide higher than that of oxygen?
Carbon dioxide has a much higher solubility coefficient.
47
How do diseases affecting the diffusion barrier, like pulmonary fibrosis, impact the diffusion of oxygen and carbon dioxide?
They affect oxygen diffusion to a much greater extent than carbon dioxide diffusion.
48
What are the two ways that gas exchange across the alveolar-capillary membrane can be described?
Perfusion-limited or diffusion-limited.
49
Under normal conditions, is the diffusion of oxygen perfusion-limited or diffusion-limited?
Perfusion-limited.
50
Why is oxygen diffusion perfusion-limited under normal conditions?
Because the partial pressures of oxygen in the alveolus and the blood reach equilibrium about a third of the distance along the pulmonary capillary.
51
How can the net transfer of oxygen be increased, assuming all other factors remain constant?
By increasing blood flow, which effectively brings in new blood faster, maintaining the partial pressure gradient with the alveolar gas.
52
In what situations can oxygen transfer become diffusion-limited?
Thickening of the alveolar-capillary membrane, strenuous exercise, and increasing altitude.
53
How does thickening of the alveolar-capillary membrane affect oxygen diffusion?
It decreases the rate of diffusion, potentially preventing equilibrium between alveolar and arterial partial pressure of oxygen from being achieved by the time blood has passed through the pulmonary capillary.
54
How does strenuous exercise potentially lead to diffusion-limited oxygen transfer?
Increased cardiac output reduces transit time for blood in the capillary, possibly leading to hypoxaemia, especially if there is an impaired diffusion barrier.
55
How does increasing altitude affect oxygen transfer?
Lower atmospheric pressure at higher altitude results in a lower alveolar partial pressure of oxygen, reducing the partial pressure gradient and the rate of oxygen diffusion.
56
What can happen to patients with mild lung disease at high altitude?
They may develop impaired diffusion at altitude, even if they can achieve equilibrium at rest and sea level.
57
Can oxygen transfer become diffusion-limited in healthy individuals at high altitude?
Yes, during exercise.
58
Under normal conditions, is oxygen exchange perfusion-limited or diffusion-limited?
Perfusion-limited.
59
What does it mean for oxygen exchange to be perfusion-limited?
The partial pressure of oxygen in the alveolus and the blood reach an equilibrium before the blood reaches the end of the pulmonary capillary. This means that increasing blood flow is the only way to increase oxygen transfer.
60
How far along the pulmonary capillary does the partial pressure of oxygen in the alveolus and blood reach equilibrium under normal conditions?
About a third of the way.
61
Why does the partial pressure of oxygen in the capillary blood initially remain low?
Oxygen that diffuses into the capillary binds to haemoglobin, which maintains a low partial pressure of dissolved oxygen in the blood.
62
When does the partial pressure of oxygen in the capillary blood start to rise?
When haemoglobin is fully saturated with oxygen.
63
What conditions can cause oxygen transfer to become diffusion-limited?
Thickening of the alveolar-capillary membrane, strenuous exercise, and increasing altitude.
64
Why does thickening of the alveolar-capillary membrane cause oxygen transfer to become diffusion-limited?
Thickening of the membrane decreases the rate of diffusion. This means an equilibrium between alveolar and arterial partial pressure of oxygen may not be reached by the time the blood has passed through the pulmonary capillary.
65
How does strenuous exercise cause oxygen transfer to become diffusion-limited?
Increased cardiac output reduces the transit time for blood along the capillary, meaning there is less time for oxygen to diffuse into the blood. This can result in hypoxaemia, especially in individuals with an impaired diffusion barrier.
66
How does increasing altitude cause oxygen transfer to become diffusion-limited?
Lower atmospheric pressure at higher altitude results in a lower alveolar partial pressure of oxygen. This reduces the partial pressure gradient for diffusion and can lead to diffusion-limited oxygen transfer, even in healthy individuals during exercise.
67
What can happen to patients with mild lung disease at high altitude?
They can develop impaired diffusion at altitude, even if they are able to achieve equilibrium at rest and at sea level.
68
What is the partial pressure of oxygen in dry air at sea level?
21.2 kilopascals.
69
What is the partial pressure of oxygen at the mitochondria?
Between 0.5 and 3 kilopascals, depending on the tissue, individual cell, and the region of the cell.
70
What happens to inhaled gas in the trachea and conducting airways?
It is humidified by being heated to body temperature (around 37 degrees Celcius) and becoming fully saturated with water vapour.
71
How does humidification affect the partial pressure of oxygen?
Adding water vapour dilutes the oxygen, reducing its partial pressure.
72
What is PiO2?
The partial pressure of inspired oxygen under humidified conditions.
73
What is FiO2?
The fraction of inspired oxygen, which is the concentration of oxygen in the gas mixture.
74
What is the FiO2 of ambient air?
0.21 (21%).
75
Does FiO2 change with altitude?
No, it remains consistent.
76
What is PB?
Atmospheric pressure in kilopascals.
77
How does altitude affect PB?
As elevation above sea level increases, atmospheric pressure decreases.
78
How does altitude affect PiO2?
As altitude increases, atmospheric pressure decreases, resulting in a lower PiO2.
79
What is PSVP water?
The saturated vapour pressure of water in kilopascals, which represents the dilution of oxygen during humidification.
80
What is the partial pressure of oxygen in inspired air after it is humidified?
Around 19.95 kilopascals.
81
What is the partial pressure of oxygen in the alveoli?
Around 13.3 kilopascals.
82
What two main factors influence the partial pressure of oxygen in the alveolus?
The amount of oxygen supplied to the alveolus by ventilation, and the amount of oxygen that has diffused into the bloodstream and been removed by the pulmonary capillaries.
83
Why is it important to understand the factors influencing the partial pressure of oxygen in the alveolus?
Because one of the key factors that determines the rate of diffusion of a gas across the alveolar-capillary membrane is the partial pressure difference, and the alveolar partial pressure of oxygen cannot be measured directly.
84
What does the alveolar gas equation allow us to do?
Calculate the alveolar partial pressure of oxygen based on a given partial pressure of inspired oxygen (PiO2), the alveolar partial pressure of carbon dioxide, and the respiratory exchange ratio.
85
What does the first part of the alveolar gas equation measure?
The amount of oxygen in the air supplied to the alveoli.
86
What happens to the alveolar partial pressure of oxygen when breathing a gas mixture of 40% oxygen?
PiO2 increases.
87
What happens to the alveolar partial pressure of oxygen when standing at the top of Mount Everest?
PiO2 decreases.
88
What does the second part of the alveolar gas equation measure?
The amount of oxygen being removed from the alveolus.
89
Why is the partial pressure of carbon dioxide in arterial blood used in the alveolar gas equation instead of the partial pressure in the alveolus?
Because it is more easily measured, and the values can be used interchangeably as the partial pressure of carbon dioxide reaches an equilibrium as blood passes the alveoli.
90
What is the respiratory exchange ratio?
The number of carbon dioxide molecules produced relative to the number of oxygen molecules consumed by metabolism (carbon dioxide production/oxygen consumption).
91
What does the respiratory exchange ratio depend on?
The fuel being metabolised (fats, carbohydrates, or proteins).
92
What is the usual estimated value of the respiratory exchange ratio?
0.8.
93
What does an RER of 0.8 mean?
More oxygen is consumed than carbon dioxide is produced.
94
How can we calculate how much oxygen is being taken up by the blood passing the alveolus?
By knowing the amount of carbon dioxide and the relationship between carbon dioxide production and oxygen consumption (respiratory exchange ratio).
95
What does the difference between the amount of oxygen supplied to the alveoli and the amount removed by the blood tell us?
The partial pressure of oxygen in the alveoli.
96
How does the alveolar gas equation allow us to predict changes in alveolar partial pressure of oxygen with ventilation?
The amount of carbon dioxide in the alveolus is dependent on the amount produced, but also the amount removed by ventilation. As PaCO2 is inversely proportional to ventilation, increases in ventilation will increase carbon dioxide eliminated and decrease PaCO2, thus increasing the alveolar partial pressure of oxygen.
97
How does increasing the fraction of inspired oxygen (FiO2) affect the alveolar partial pressure of oxygen?
It increases the alveolar partial pressure of oxygen more than hyperventilation.
98
Does the amount of oxygen carried in the blood increase or decrease linearly with changes in alveolar partial pressure of oxygen?
No, oxygen carriage is limited by the carrying capacity of haemoglobin and the oxygen saturation curve.
99
What is the only factor that can be readily changed in clinical practice to influence the rate of diffusion of a gas across the alveolar-capillary membrane?
The partial pressure difference.
100
How can the partial pressure difference across the alveolar-capillary membrane be changed clinically?
By increasing the fraction of inspired oxygen.
101
How much higher is the rate of diffusion of carbon dioxide compared to oxygen?
About 20 times higher.
102
Why is the diffusion rate of carbon dioxide higher than that of oxygen?
Carbon dioxide has a much higher solubility coefficient.
103
How do diseases affecting the diffusion barrier, such as pulmonary fibrosis, affect the diffusion of oxygen and carbon dioxide?
They affect oxygen diffusion to a much greater degree than carbon dioxide diffusion.
104
Under normal conditions, is the diffusion of oxygen perfusion-limited or diffusion-limited?
Perfusion-limited.
105
What does it mean for oxygen diffusion to be perfusion-limited?
The partial pressure of oxygen in the alveolus and blood reach an equilibrium about a third of the distance along the pulmonary capillary, meaning that increasing blood flow is the only way to increase the net transfer of oxygen.
106
What happens to the partial pressure of oxygen in the capillary blood as it moves along the pulmonary capillary?
It initially remains low as oxygen diffuses into the capillary and binds to haemoglobin, maintaining a low partial pressure of oxygen in the blood. It starts to rise once the haemoglobin is fully saturated, reaching an equilibrium with the alveolar partial pressure about a third of the way along the capillary.
107
Under what conditions can the transfer of oxygen become diffusion-limited?
Thickening of the alveolar-capillary membrane, strenuous exercise, and increasing altitude.
108
Why does thickening of the alveolar-capillary membrane result in diffusion-limited oxygen transfer?
It decreases the rate of diffusion, which can prevent equilibrium from being reached between alveolar and arterial partial pressures of oxygen by the time blood has passed through the pulmonary capillary.
109
Why does strenuous exercise potentially lead to diffusion-limited oxygen transfer?
Increased cardiac output reduces the transit time of blood in the capillary, which can result in hypoxaemia, especially in the presence of an impaired diffusion barrier.
110
Why does increasing altitude potentially lead to diffusion-limited oxygen transfer?
Lower atmospheric pressure at higher altitudes results in a lower alveolar partial pressure of oxygen, reducing the partial pressure gradient for diffusion. This can lead to diffusion-limited oxygen transfer, even in healthy individuals during exercise.
111
Can patients with mild lung disease experience diffusion limitations at high altitude?
Yes, even if they can achieve equilibrium at rest and at sea level.
112
Why is the diffusion rate of carbon dioxide higher than that of oxygen in the lungs?
Carbon dioxide has a much higher solubility coefficient, making its diffusion rate approximately 20 times higher than oxygen's.
113
What is the alveolar-arterial gradient?
It is the difference between the calculated alveolar partial pressure of oxygen and the measured arterial partial pressure of oxygen in systemic blood.
114
Why is the arterial partial pressure of oxygen in the systemic circulation typically lower than the alveolar partial pressure of oxygen?
A small amount of blood bypasses the alveoli through a physiological shunt and does not participate in gas exchange. This results in a slightly lower arterial partial pressure of oxygen in the systemic circulation, usually around 13 kPa.
115
What is the normal alveolar-arterial gradient in young, healthy adults?
Less than 1.5 kilopascals.
116
What causes the alveolar-arterial gradient to increase with age?
Worsening ventilation/perfusion (V/Q) matching in the lungs.
117
What is an anatomical shunt, and what are some examples?
It is when deoxygenated blood enters the left side of the heart without going through the pulmonary circulation due to anatomical reasons. Examples include bronchial blood flow draining directly into the left side of the heart, and some venous drainage from the myocardium via thebesian veins also draining into the left side of the heart.
118
What is a functional shunt?
Blood bypasses areas with a low V/Q ratio.
119
What can cause an increased alveolar-arterial gradient?
Diffusion impairment: this can be due to a thickened diffusion barrier, as discussed in Part One of the lecture, or a decreased surface area for diffusion, such as in emphysema where there is damage to the alveoli.Right-to-left shunt: blood is shunted from the right to the left side of the heart without going through the pulmonary circulation.V/Q mismatch.
120
Why does alveolar hyperventilation result in a normal alveolar-arterial gradient?
Because alveolar hyperventilation also reduces the alveolar partial pressure of oxygen, so the difference between alveolar and arterial partial pressure of oxygen (i.e. the A-a gradient) remains unaffected.
121
What is high V/Q mismatch, and what is another term for it?
When alveoli are ventilated but not perfused. This is also known as dead space.
122
What are some examples of conditions that can cause a high V/Q mismatch?
Pulmonary embolism and reduced right ventricular stroke volume.
123
What is low V/Q mismatch, and what is another term for it?
When alveoli are perfused but not ventilated. This is also known as a pulmonary shunt.
124
What are some examples of conditions that can cause a low V/Q mismatch?
Conditions that prevent air from getting into or filling the alveoli, such as pneumonia, atelectasis, and asthma.
125
How does a low V/Q mismatch affect the total oxygen content of the blood?
Blood flowing past poorly ventilated alveoli does not pick up any extra oxygen. When this poorly oxygenated blood mixes with oxygenated blood from areas of the lung that are better ventilated, it lowers the total oxygen content of the blood.
126
How can oxygen administration improve arterial partial pressure of oxygen in cases of mild V/Q mismatch?
By increasing the FiO2 (fraction of inspired oxygen). This increases the concentration of oxygen available for diffusion and the partial pressure gradient across the alveolar-capillary membrane, therefore increasing the arterial partial pressure of oxygen to a certain degree.
127
Why is oxygen administration less effective in cases of large V/Q mismatch (where V/Q is approaching zero)?
Because the non-ventilated alveoli will not be able to benefit from the additional oxygen, and blood passing alveoli in the well-ventilated areas is already fully saturated. This situation is essentially the same as a right-to-left shunt where blood bypasses the lungs altogether; this blood cannot pick up any extra oxygen no matter how much is given.
128
What is the impact of low V/Q areas on arterial partial pressure of carbon dioxide compared to the effect on partial pressure of oxygen?
The impact on arterial partial pressure of carbon dioxide is less than the effect on partial pressure of oxygen. This is because the elimination of carbon dioxide can be increased in areas of high V/Q.
129
How can increased alveolar ventilation in high V/Q areas help maintain arterial partial pressure of carbon dioxide within a normal range?
Increased alveolar ventilation reduces the partial pressure of carbon dioxide in the alveoli, which increases the partial pressure difference and thus increases the rate of diffusion. This allows arterial partial pressure of carbon dioxide to be kept in a normal range to a certain degree.
130
What happens to the partial pressure of oxygen as blood flows through the systemic circulation?
Oxygen is removed by metabolising tissues, so the partial pressure of oxygen progressively falls from the arterial end to the venous end.
131
Do all the arterial blood flow through capillary beds in the systemic circulation?
No. Pre-capillary sphincters control how much blood flows through each capillary bed, and some blood will bypass the capillaries and flow directly into the venules via arteriovenous anastomoses.
132
What is the partial pressure of oxygen in the mitochondria, and how is this linked to the rate of metabolism?
The mitochondrial partial pressure of oxygen is very low, to allow oxygen to diffuse from the capillaries to the mitochondria. In tissues with a high rate of metabolism, oxygen utilisation is higher and the partial pressure of oxygen in the mitochondria will be even lower, which creates a greater partial pressure gradient between the cell and the blood, increasing the rate of diffusion.
133
What are the two ways in which oxygen is transported from the lungs to the tissues?
98% of oxygen is bound to haemoglobin, and 2% is dissolved in the plasma.
134
What is global oxygen delivery, and what is it a product of?
It is the amount of oxygen delivered to the whole body from the lungs. It is the product of cardiac output and the oxygen content of arterial blood.
135
What are the requirements for cells to be able to utilise oxygen?
Adequate partial pressure of oxygen in the arterial blood, adequate oxygen carrying capacity (adequate haemoglobin concentration), adequate cardiac output and arterial flow, and adequate mitochondrial function.
136
What is the difference between hypoxia and hypoxaemia?
Hypoxia is inadequate levels of tissue oxygenation for aerobic metabolism, whereas hypoxaemia is specifically low arterial partial pressure of oxygen. Hypoxia caused by hypoxaemia is called hypoxaemic hypoxia.
137
What is hypoxia?
Inadequate levels of tissue oxygenation for aerobic metabolism.
138
What is the difference between hypoxia and hypoxaemia?
Hypoxia is a broader term that refers to inadequate oxygen levels in tissues. Hypoxaemia specifically refers to low partial pressure of oxygen in arterial blood.
139
What is hypoxaemic hypoxia?
Hypoxia that is caused by hypoxaemia (low arterial partial pressure of oxygen).
140
What are some causes of hypoxaemic hypoxia?
Low partial pressure of inspired oxygen (e.g. at altitude), hypoventilation, V/Q mismatch, right-to-left shunt, or diffusion abnormality.
141
What is the impact of reduced PaO2 on haemoglobin saturation and oxygen carrying capacity?
When PaO2 is reduced, haemoglobin saturation and the total volume of oxygen that can be carried by the blood are also reduced.
142
What is anaemic hypoxia, and what are some possible causes?
Anaemic hypoxia occurs when PaO2 is normal, but the oxygen carrying capacity of the blood is reduced. This can be caused by severe anaemia or carbon monoxide poisoning (where carbon monoxide binds to haemoglobin, preventing oxygen from binding).
143
What is stagnant or circulatory hypoxia?
This occurs when the PaO2 and haemoglobin concentration are normal, but the delivery of oxygen to the tissues is reduced.
144
What are some examples of conditions that can cause stagnant hypoxia?
A systemic problem such as heart failure, where the heart is not able to pump sufficient blood around the body; or a localised problem such as ischaemia due to an embolus blocking an artery.
145
What is cytotoxic hypoxia?
Impaired ability of the tissues to metabolise oxygen, despite normal oxygen levels and normal delivery.
146
What are some examples of conditions that can cause cytotoxic hypoxia?
Cyanide poisoning and severe sepsis.
147
What is the clinical definition of respiratory failure?
Respiratory failure occurs when the respiratory system can no longer meet the body's metabolic demands due to inadequate oxygenation, with or without inadequate carbon dioxide removal.
148
What is the key clinical marker of respiratory failure?
Hypoxaemia, specifically a partial pressure of oxygen in arterial blood (PaO2) of less than 8 kPa while breathing room air (FiO2 of 0.21), indicates respiratory failure.
149
How can the respiratory system be conceptualised to understand the different types of respiratory failure?
The respiratory system can be thought of as a bellows responsible for ventilation and a gas exchange system responsible for oxygenation.
150
What components of the respiratory system are represented by the 'bellows'?
The bellows represents the conducting airways, chest wall, pleura, respiratory muscles, the nerves supplying them, and the higher centres in the brain.
151
What components of the respiratory system are represented by the 'gas exchange system'?
The gas exchange system represents the alveoli, the pulmonary capillaries, and the pulmonary circulation.
152
What are the two main types of respiratory failure?
The two main types are Type 1 and Type 2.
153
Broadly, what is the difference between Type 1 and Type 2 respiratory failure?
In Type 1, only oxygen exchange is impaired, while in Type 2, both oxygen and carbon dioxide exchange are impaired.
154
What is the physiological cause of Type 1 respiratory failure?
Type 1 respiratory failure is primarily caused by a deficiency in the gas exchange system, leading to a failure of oxygenation.
155
What characterises Type 1 respiratory failure in terms of blood gas measurements?
Type 1 respiratory failure presents with a PaO2 of less than 8 kPa and a normal or low PaCO2.
156
What is the physiological cause of Type 2 respiratory failure?
Type 2 respiratory failure is caused by a deficiency in the bellows, leading to alveolar hypoventilation and a failure of both oxygenation and carbon dioxide removal.
157
What characterises Type 2 respiratory failure in terms of blood gas measurements?
Type 2 respiratory failure presents with hypoxaemia (PaO2 less than 8 kPa) and hypercapnia (raised PaCO2).
158
What is the primary physiological cause of Type 1 respiratory failure?
Type 1 respiratory failure is primarily caused by a ventilation/perfusion (V/Q) mismatch.
159
What is the defining characteristic of Type 2 respiratory failure?
Type 2 respiratory failure is characterized by both hypoxaemia and hypercapnia, caused by a failure of ventilation.
160
How does impaired lung movement contribute to Type 2 respiratory failure?
Impaired lung movement, as seen in conditions like chest wall deformity, obesity, or neurological impairment, prevents the lungs from effectively pumping air in and out, leading to Type 2 respiratory failure.
161
How can excessive work of breathing lead to Type 2 respiratory failure?
When the work of breathing becomes excessive, such as in COPD or a near-fatal asthma attack, the body may be unable to maintain adequate ventilation, resulting in Type 2 respiratory failure.
162
What is the most common clinical feature of respiratory failure?
Breathlessness, or dyspnoea, is the most common feature of respiratory failure.
163
What are some other clinical signs of respiratory failure?
Other signs include tachypnoea (high respiratory rate), tachycardia (high heart rate), and cyanosis (blue discolouration of the skin) due to haemoglobin desaturation.
164
How does hypoxaemia affect the brain?
Hypoxaemia can lead to agitation, confusion, drowsiness, and even progress to coma and death due to interference with aerobic metabolism and cellular function in the brain.
165
What are the clinical features of hypercapnia?
Hypercapnia can cause headaches, confusion, drowsiness, tachycardia, a bounding pulse, vasodilation, and CO2 retention flap (asterixis).
166
What is a serious consequence of hypercapnia?
One of the most important consequences of hypercapnia is respiratory acidosis.
167
What is CO2 retention flap (asterixis)?
CO2 retention flap, also known as asterixis, is a clinical sign of hypercapnia characterized by a flapping tremor of the hands when the wrists are extended.
168
Explain the relationship between alveolar ventilation and PaCO2 in the context of respiratory failure.
Alveolar ventilation and PaCO2 have an inverse relationship. When alveolar ventilation decreases, as in Type 2 respiratory failure, PaCO2 increases, leading to hypercapnia.
169
Why is the partial pressure of carbon dioxide normal or low in Type 1 respiratory failure?
In Type 1 respiratory failure, although oxygenation is impaired, the body can still maintain adequate ventilation to remove CO2, resulting in a normal or even low PaCO2 due to compensatory hyperventilation.
170
Why does respiratory acidosis occur in Type 2 respiratory failure?
In Type 2 respiratory failure, inadequate ventilation leads to the accumulation of CO2 in the blood. CO2 reacts with water to form carbonic acid, increasing hydrogen ion concentration and lowering blood pH, resulting in respiratory acidosis.
171
What is the body's initial response to hypoxaemia?
The body responds to hypoxaemia by increasing the rate and depth of breathing (hyperventilation).
172
What role do peripheral chemoreceptors play in the compensatory response to hypoxaemia?
Peripheral chemoreceptors are activated by a PaO2 below 8 kPa and trigger hyperventilation to increase oxygen intake.
173
How do accessory muscles contribute to compensating for hypoxaemia?
Accessory muscles, such as those in the neck and chest, are recruited to aid in ventilation when breathing becomes laboured.
174
Explain the role of hypoxic pulmonary vasoconstriction in hypoxaemia.
Hypoxic pulmonary vasoconstriction redirects blood flow from poorly ventilated lung areas to better-ventilated areas, optimising gas exchange.
175
How does the sympathetic nervous system respond to hypoxaemia?
The sympathetic nervous system increases cardiac output in response to hypoxaemia to enhance oxygen delivery to the tissues.
176
How does the body compensate for reduced oxygen-carrying capacity in hypoxaemia?
The body increases haemoglobin concentration, acutely via splenic contraction and in the long term by increasing erythropoietin production.
177
What happens to the oxyhaemoglobin dissociation curve in response to hypoxaemia, and why is this beneficial?
The oxyhaemoglobin dissociation curve shifts to the right due to increased 2,3-diphosphoglycerate and acidosis, making it easier for haemoglobin to release oxygen to the tissues.
178
What is the main compensatory mechanism for hypercapnia?
Hypercapnia stimulates ventilation to expel excess carbon dioxide.
179
Under what circumstances might the compensatory mechanism for hypercapnia be absent?
If the cause of Type 2 respiratory failure is hypoventilation and the patient cannot increase their respiratory rate, the compensatory drive to increase ventilation might be absent.
180
How does the body compensate for respiratory acidosis caused by hypercapnia?
The kidneys compensate for respiratory acidosis by retaining bicarbonate, which helps buffer the excess acid and normalise pH.
181
Is metabolic compensation for respiratory acidosis a rapid process?
No, metabolic compensation is slow as it relies on the kidneys adjusting bicarbonate reabsorption, taking several days to develop.
182
How can you differentiate acute from chronic respiratory acidosis using ABG results?
Evidence of metabolic compensation (increased bicarbonate) indicates chronic respiratory acidosis.
183
Can respiratory or metabolic compensation fully overcompensate for an acid-base imbalance?
Overcompensation does not occur. If it seems to be the case, consider a mixed respiratory-metabolic disorder.
184
What does an elevated lactate level in a septic patient's ABG results signify?
An elevated lactate in a septic patient indicates anaerobic metabolism due to insufficient tissue oxygenation, reflecting the severity of sepsis.
185
What is the primary treatment for hypoxaemia in respiratory failure?
Supplemental oxygen is the primary treatment for hypoxaemia in respiratory failure.
186
When might mechanical ventilation be necessary in the management of respiratory failure?
Mechanical ventilation is often required in severe cases of respiratory failure when the patient's respiratory effort is inadequate to maintain sufficient oxygenation and ventilation. [This information is implied in the sources as a logical next step in management when oxygen therapy alone is insufficient. It is important to verify this with clinical guidelines and expert advice.]
187
Besides addressing hypoxaemia and hypercapnia, what is another crucial aspect of managing respiratory failure?
Identifying and treating the underlying cause of respiratory failure is essential for successful management.
188
What is the purpose of an arterial blood gas (ABG)?
An ABG helps assess a patient's respiratory status (oxygenation and ventilation) and acid-base balance. It also provides other valuable measurements like lactate, glucose, and electrolytes.
189
Describe the key steps involved in interpreting ABG results.
Interpreting ABG results involves a systematic approach:Verify patient details and oxygen supplementation status.Evaluate PaO2 for hypoxaemia and respiratory failure.Analyse pH for acidaemia or alkalaemia.Assess PaCO2 to determine the respiratory component and type of respiratory failure.Examine bicarbonate levels for evidence of metabolic compensation.Consider other components like lactate, glucose, and electrolytes in the context of the patient's presentation.
190
What is the expected PaO2 in a patient receiving supplemental oxygen?
The expected PaO2 in a patient on oxygen should be roughly 10 kPa less than the percentage of inspired oxygen (FiO2).
191
How does ABG analysis help differentiate between Type 1 and Type 2 respiratory failure?
The PaCO2 level distinguishes the types of respiratory failure: Type 1 has normal or low PaCO2, while Type 2 presents with elevated PaCO2 (hypercapnia).
192
What ABG findings indicate partially compensated respiratory acidosis?
Partially compensated respiratory acidosis presents with:Low pH (acidaemia)High PaCO2 (hypercapnia)High bicarbonate, indicating metabolic compensation, but not fully correcting the pH.
