Flashcards in Anaesthetics - Pulse oximetry and non invasive blood pressure Deck (19)
How do automated blood pressure machines use oscillometry to measure blood pressure?
Pressure in the cuff inflates. As it does so there are pressure changes in the cuff as the oscillations of the device record. The cuff pressure increases to minimise background oscillations in the machine. The cuff then deflates whilst looking at the changes in cuff pressure. It reaches a point at which these oscillations are at a maximum and deems this to be MAP. Cuff pressure then decreases and the oscillations die down. Systolic BP is essentially the mean of these oscillatory readings, but diastole is more difficult. The cuff looks at the shape of the envelope of pressure changes and makes an estimate as to where diastole is.
What errors can occur in non invasive blood pressure recording?
Cuff size and position:
- too small - over reads
- bladder must be at the front so correct application is important
- varies between devices
- care in diagnosis of hypertension and obstetrics
Unreliable in AF
What is the solubility of oxygen in blood?
The solubility of oxygen in blood is low, such that a normal arterial PO2 of 13kPa has only 3mL of dissolved oxygen present. If you think that oxygen consumption during exercise can rise to 4000mL/min, then relying entirely on dissolved oxygen would not sustain oxygen demand. The main function of the red blood cell pigment, haemoglobin, is to carry large quantities of oxygen needed by tissues. However, an important point to remember is that it is only the dissolved oxygen that can contribute to partial pressure (Henry's law) not that bound to haemoglobin.
What is the structure of haemoglobin?
Haemoglobin is composed of 4 subunits, each containing a protein chain (globin) and a haem group. Normal adult haemoglobin (HbA) contains two identical alpha chains and two identical beta chains. The haem group is attached to each chain at a histidine residue and each has an iron atom in the ferrous form, which binds to an oxygen molecule. Each haemoglobin molecule can bind up to four molecules of oxygen. When haemoglobin binds with oxygen it is called oxyhaemoglobin. The haemoglobin that does not bind with oxygen is called deoxyhaemoglobin.
How much oxygen can each gram of haemoglobin bind with?
Each gram of haemoglobin can bind with 1.34 mL of oxygen. Oxygen binds rapidly and reversibly to haemoglobin: O2 + Hb HbO2
The amount of oxyhaemoglobin is a function of the partial pressure of oxygen in the blood. In the pulmonary capillaries, in which PO2 is high, the reaction is shifted to the right to form oxyhaemoglobin. This keeps the free oxygen concentration low and maintains the diffusion gradient for oxygen. In tissue capillaries in which PO2 is low, the reaction is shifted to the left and oxygen is unloaded.
What is the oxygen carrying capacity?
The maximum amount of oxygen that can be carried by haemoglobin is called the oxygen carrying capacity - about 20mL O2/dL of blood. This value is calculated by assuming a normal haemoglobin concentration of 15g Hb/dL of blood (1.34 x 15 = 20.1) The oxygen carrying capacity varies with Hb concentration.
What is oxygen content? How does this relate to saturation?
The amount of oxygen actually bound to haemoglobin (whereas capacity is the amount that can potentially bind). The percentage saturation of haemoglobin is calculated by the ratio of oxyhaemoglobin content over capacity:
SO2 = HbO2 content/ (HbO2 + Hb) x 100
Oxygen content is the amount of bound oxyhaemoglobin plus the amount dissolved.
Why is the shape of the oxygen dissociation curve "S" shaped?
The curve plots oxygen saturation, partial pressure and oxygen content. The curve is "S" shaped because the haemoglobin affinity for oxygen increases as more binding sites are occupied - called co-operative binding due to allosteric modulation of the quarternary Hb structure wit successive binding of oxygen molecules.
Explain the plateau region of the oxygen dissociation curve.
This is the loading phase, in which oxygen is loaded onto haemoglobin to form oxyhaemoglobin in pulmonary capillaries. The plateau region illustrates how oxygen saturation and content remain stable over a wide range of alveolar PO2 partial pressures. For this reason, oxygen content cannot be appreciably raised by hyperventilation or decreased in mild hypoventilation. More severe reductions in PO2 to levels in the steep region of the curve (<8kPa) are associated with significant reductions in oxygen saturation and content. Therefore, breathing oxygen enriched air may significantly raised arterial oxygen content and exercise capacity at high altitude and in patients with chronic hypoxic respiratory disease.
Explain the steep slope of the oxygen dissociation curve.
The steep unloading phase of the curve allows large quantities of oxygen to be unloaded from haemoglobin in the tissue capillaries, in which a lower capillary PO2 occurs. This is because at mixed venous partial pressures, Hb affinity for oxygen is much lower. The partial pressure gradient for diffusion into the tissue is maintained in two ways: First, the tissues consume oxygen, keeping their PO2 low. Second, the lower affinity for O2 ensures O2 will be unloaded more readily from Hb; unbound O2 is free in the blood, creates a partial pressure and the PO2 of the blood is kept relatively high. Because the PO2 of the tissue is kept relatively low, the partial pressure gradient that drives O2 diffusion from blood to tissues is maintained.
What is the P50 value?
The P50 is the PO2 at which 50% of the haemoglobin is saturated. It provides a measure of the binding affinity of Hb for oxygen. The normal P50 in arterial blood is about 4kPA. A high P50 signifies a decrease in haemoglobin's affinity for oxygen and results in a rightward shift in the oxyhaemoglobin equilibrium curve, whereas a low P50 signifies the opposite and shifts the curve to the left. A shift in the P50 in either direction has the greatest effect on the steep phase and only a small effect on the loading of oxygen in the normal lung, because loading occurs during the plateau phase.
What is the Bohr effect?
Several factors can affect the binding affinity of Hb for O2, including temperature, arterial carbon dioxide tension, and arterial pH. A rise in CO2, a fall in pH and a rise in temperature all shift the curve to the right. The effect of carbon dioxide and hydrogen ions on haemoglobin oxygen affinity is known as the Bohr effect. A right shift in the curve means that for a given partial pressure of oxygen the affinity is lower so more is unloaded. A leftward shift (think "L" for lower - e.g. low CO2, low temp, high pH) increases the affinity of haemoglobin for oxygen, which lowers its ability to release oxygen to the tissues.
What is 2,3-DPG?
Red blood cells contain 2,3-DPG, an organic phosphate compound that can also affect the affinity of Hb for oxygen. In red cells, 2,3-DPG are much higher than in other cells because erythrocytes lack mitochondria. An increase in 2,3-DPG facilitates unloading of oyxgen from the red cell at the tissue level (shifts the curve to the right). An increase in RBC 2,3-DPG occurs in hypoxia and exercise.
How does anaemia affect the shape of the oxygen dissociation curve?
It is oxygen content rather than PO2 and SaO2 that keeps us alive. A person can have a normal arterial PO2 and SaO2 but reduced oxygen content. This situation is seen in patients who have anaemia. A patient with anaemia who has half the normal Hb concentration will have a normal PO2 and SaO2 but oxygen content will be reduced to half of normal. A patient with anaemia has a normal SaO2 because the content and capacity are proportionately reduced. The shape of the oxygen dissociation curve is the same, but if content rather than saturation is plotted then the curve is seen to move downwards reflecting reduced content.
What explains the colour of reduced and oxygenated red blood cells?
In physics, objects are a certain colour because the absorb light of all other wavelengths but reflect back the light in the spectrum they represent. Oxygenated RBCs are bright red, meaning that they absorb light heavily in the high frequency blue spectrum of light and reflect low frequency red light. The opposite occurs in reduced Hb.
What is the Beer-Lambert relation? How is it relevant to pulse oximetry?
When a solution of certain concentration (c) absorbs light of a specific intensity/ frequency (I0) some intensity is lost as light passes through (I1) the solution. The Beer-Lambert law enables calculation of the absorbance of a solution that light passes through which also takes into account the path length.
Oxyhaemoglobin is seen to absorb light at the blue end of the visible spectrum as predicted. Unexpectedly, reduced haemoglobin absorbs light in a similar pattern to oxyhaemoglobin up to about 640nm where the absorption spectra become different. They cross over at a point called the isosbestic point. At this point it is impossible to differentiate reduced and oxygenated haemoglobin. So, saturation is calculated by the proportion of Hb molecules absorbing light in the blue end of the spectrum compared to the isosbestic point, which reflects total absorption.
What factors can cause false low readings?
Venous congestions/ pulsatations - tricuspid regurgitation, high airway pressures, Valsalva manouvre
What can cause false high readings?
Classically this is carbon monoxide poisoning. Carboxyhaemoglobin has a much higher affinity for Hb than oxygen does at much lower partial pressures. Oximeters cannot differentiate between oxyhaemoglobin and carboxyhaemoglobin.