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Tag: Haemoglobin

Blood transfusions, pt 1: clinical signs
I get asked frequently when is the right time to transfuse an anaemic patient?

The difficulty lies in the fact not all anaemic patients require blood transfusions. Just because a patient has pale mucous membranes does not mean the patient needs a transfusion.

The term commonly brought up during the discussion is “transfusion triggers present”.

What constitutes a “transfusion trigger”?

A couple of different definitions exist: classically, it is the PCV or haemoglobin level at which a transfusion is indicated in an individual animal – essentially, if it gets below a certain number, transfusion is required – but it is not always that simple. Just because the PCV is 15%, it doesn’t always mean a transfusion is required.

When the PCV drops low enough, clinical signs of reduced oxygen delivery to the tissues start to develop, these include:
- decreased exercise tolerance
- weakness
- dull mentation
- tachycardia
- tachypnoea
- elevated lactate levels when shock has been addressed
Rapid or slow?

These clinical signs are influenced by the speed at which the anaemia has developed.

If the anaemia has occurred rapidly due to internal bleeding from trauma or a ruptured organ, these clinical signs can present in a matter of minutes, depending on how big the bleed is. This means a transfusion might be indicated when the PCV is still 25%, especially if further rapid blood loss is likely.

If the anaemia developed over days to weeks (slow red cell destruction or anaemia of chronic disease, for example) transfusion triggers might not be present until the PCV drops below 15%, as the body has had time to compensate.

Summary

So, in summary, the decision-making process involves asking the questions:
- What is the PCV?
- How fast has the anaemia developed?
- Are there clinical signs of reduced oxygen delivery?
- Is further loss likely?
When you combine the core aspects of each of the questions above, “transfusions triggers” change from absolute numbers to this:
- PCV under 15% with clinical signs of reduced oxygen delivery.
- Rapid PCV drop to under 20% in dogs and 15% in cats.
- PCV under 25% and surgery or anaesthesia is required, and/or rapid ongoing blood loss is occurring.
Blood products you should use and why will be covered in a future post.

Note: Haemoglobin levels should also be assessed in conjunction with the PCV.
January 3, 2024
Oxyhaemoglobin dissociation curve, pt 4: left and right shift
In various disease and physiological states, the oxyhaemoglobin dissociation curve (OHDC) can shift either left or right. This indicates the increase, or decrease, of the haemoglobin’s (Hgb’s) affinity to oxygen, respectively.

It is important to recognise the situations in which this happens, to manage patients effectively.

Right shift

A shift of the OHDC to the right indicates the Hgb has a reduced affinity to oxygen. This is normally seen in environments where oxygen needs to be released by the Hgb molecules – for example, muscles and placenta.

Four major factors influence this:
- low blood pH (lactic acid)
- high temperature – especially working muscles
- high partial pressure of carbon dioxide (PCO₂)
- increased 2,3-bisphosphoglycerate (2,3-DPG) – an intermediate of glycolysis
2,3-DPG

2,3-DPG is produced in red blood cells during glycolysis. Its production is increased for several conditions in the presence of diminished peripheral tissue oxygen availability, such as hypoxaemia, chronic lung disease, anaemia and congestive heart failure. It promotes oxygen to be released into the tissues and, therefore, makes it harder for oxygen to bind with Hgb in the lungs.

The presence of 2,3-DPG can increase oxygen release to the tissue equivalent to that if the surrounding arterial partial pressure of oxygen (PaO₂) was 10mmHg higher. This is why 2,3-DPG is increased during pregnancy to increase oxygen delivery to the growing fetus.

Another example is during exercise – in the presence of high CO₂ and more hydrogen ions (H+) from lactic acid (lowering the pH), the curve is shifted to the right to help increase oxygen release in the muscles.

Left shift

By the same token, a shift of the curve to the left means the Hgb has an increased affinity for oxygen, so is less likely to release it to the tissue. This normally occurs in the lungs.

The factors this promotes are:
- high blood pH (breathing off CO₂ – an acid)
- low temperature (ambient temperature usually lower than that of the lungs)
- reduced PCO₂ (ventilation)
- decreased 2,3-DPG and the presence of fetal Hgb
So, during exercise, in the presence of lower CO₂ and less H+, the curve shifts to the left to help increase oxygen uptake into the Hgb.

Carbon monoxide toxicity

One pathological situation that causes a left shift of the curve is carbon monoxide (CO) toxicity.

Not only does it cause the OHDC curve to shift to the left, but the Hgb is also 240 times more likely to bind to CO than oxygen. The situation is further complicated by pulse oximeters’ inability to differentiate CO-bound Hgb to O₂-bound Hgb, therefore giving a false reading of normal partial pressure of oxygen in the face of severe hypoxaemia. Similarly, methemoglobinaemia has a similar effect to CO and causes a left shift of the OHDC.

Conclusion

Understanding the relationships between Hgb, oxygen saturation, PaO₂ and the causes of the shifts of the curves will allow accurate assessments of patients.

For example, a patient with unusual hypoxaemia derived from arterial blood gas – despite normal PaO₂ and peripheral oxygen saturation – will immediately increase the suspicion of CO toxicity.

Similarly, a patient presenting with shortness of breath, despite adequate ventilation and oxygen saturation, will suggest Hgb deficiency.
March 13, 2023
Oxyhaemoglobin dissociation curve, pt 3: what the curve means

The oxyhaemoglobin dissociation curve (OHDC) is a graphical description of the relationship between the partial pressure of oxygen (PO₂; x-axis) and the oxygen saturation (y-axis).

To make sense of this graph, we need to first understand the reasons that drive oxygen movement between the haemoglobin (Hgb) and the body tissue.

Oxygen binding affinity

The major determinant of oxygen binding to the Hgb is the PO₂, which the Hgb is exposed to. When the PO₂ is high – typically in the lungs at the alveolar-capillary interface, oxygen readily binds to Hgb because of its increased affinity to oxygen.

The opposite occurs in other body tissues, where the PO₂ is less. This reduces the Hgb’s ability to maintain its binding capacity of oxygen (reduces affinity) and oxygen is released into the tissue instead.

Looking at the OHDC, it can be seen the relationship between the arterial PO₂ (PaO₂) and peripheral oxygen saturation (SpO₂) is not linear, and this has to do with the Hgb’s changing affinity to oxygen.

Initially, when the first oxygen molecule binds to Hgb’s, a change occurs in its conformation that increases its affinity for subsequent molecules of oxygen and, thus, hasten binding. This is depicted by the steep part of the OHDC curve.

However, as more oxygen binds, the available binding sites become saturated and further increase in PaO₂ doesn’t increase further additional binding. This is the flat portion of the curve where the SpO₂ doesn’t increase very much, despite a constant increase in PaO_2.

The only way to further increase SpO₂ is by increasing the haemoglobin content (blood transfusion) and, thus, the total oxygen carrying capacity of Hgb. This is important to factor in when treating anaemic patients, where any further increase in oxygen delivery is unlikely to benefit the overall oxygen capacity of the patient.

Patient application

So, we now understand the oxygen binding affinity to the haemoglobin, but how can that be applied to the patient as a whole?

The curve is actually a graphical representation of the movement of oxygen around different parts of the body. The flat, upper portion of the curve represents the lung/alveolar interface when PO₂ is usually high. In this region, the Hgb’s affinity for the oxygen is high, so very little change occurs in the SpO₂ in a range of PO₂ (80mmHG to 100mmHg).

This is an important defence mechanism of the body that ensures the loading of oxygen on to Hgb is unaffected in the face of varying PO₂. Once PaO₂ decreases beyond 80mmHg, though, a more rapid decrease occurs in SpO₂ for a given decrease in PaO₂. This is the middle, steep, lower portion of the curve. This part of the curve describes the peripheral tissues where the PO₂ is much lower compared to that of the lung/alveolar interface.

Hgb’s affinity for oxygen decreases as PO₂ decreases – this is why a small decrease in PO₂ in this range will result in a significant reduction in SpO₂. This is very important as it means peripheral tissue can withdraw large amount of oxygen for a small drop in PO₂, promoting oxygen diffusion into the tissue.

The norm for peripheral tissue is 40mmHg PaO₂. If you look at the graph closely, it will be equivalent to approximately 75% SpO₂. This means the tissue has been able to extract 25% of oxygen and the Hgb is left with 75% of its oxygen-carrying capacity.

Key to understanding

This is the graph for a typical, healthy individual. Understanding this will help make sense of what actually happens in diseased patients, or patients with physiological changes, that shifts the curve of this graph.

By understanding the changes that occur, it is only then you can implement effective management strategies to help these patients.

March 6, 2023
Oxyhaemoglobin dissociation curve, pt 2: pulse oximetry’s limitations
Pulse oximetry is a useful, non-invasive method of measuring a patient’s oxygen saturation (SO₂) and, under normal physiological circumstances, correlates well to the arterial oxygen saturation (SaO₂).

However, despite its ease of use and accessibility, it is not infallible. Circumstances exist that will undermine the accuracy of these readings – some with dire consequences if not recognised.

Others causes are more technically associated, but also needs recognition.

Unequal to task

Pulse oximetry is incapable of assessing:
- a patient’s haemoglobin levels
- the haemoglobin’s functionality
- the patient’s partial pressure of arterial carbon dioxide (PaCO₂)
The former is particularly apparent in anaemic patients, where peripheral capillary oxygen saturation (SpO₂) readings could be greater than 95%, but animals still severely hypoxic. This is because the total numbers of haemoglobin is reduced; therefore, overall oxygen-carrying capacity is also decreased.

Similarly, haemoglobin can be fully saturated with carboxyhaemoglobin or methaemoglobin strands, giving a misleadingly high SpO₂ reading, yet patients are severely oxygen deprived.

Finally, the ventilation status of the patient is not assessed by pulse oximetry. This is particularly important in animals with respiratory compromise, patients under heavy sedation and those under general anaesthetic or severe respiratory muscle paralysis from envenomation by a tick or snake. These patients can have near normal SpO₂, but a dangerously high PaCO₂.

To overcome these problems, capnography or arterial blood gas analysis with cooximetry, and assessment of haemoglobin concentration is crucial.

Accuracy issues

The accuracy of pulse oximeter readings are also affected by several causes.

Severe hypoxaemia (lower than 70% SpO₂) is not accurately detected by pulse oximetry and requires partial pressure of arterial oxygen (PaO₂) to confirm. Also, any cause of reduced peripheral perfusion can cause erroneously low readings, such as arrhythmias, hypotension, heart failure, hypothermia and severe vasoconstriction.

Physical examination parameters that can indicate perfusion deficits are present include:
- tachycardia
- reduced pulse pressures
- pale mucous membranes
- prolonged capillary refill time
- dull mentation/weakness
- hypothermia
It is not uncommon to stabilise a patient with hypovolaemic shock and find the SPO₂ reading has normalised.

Improving outcomes

Although the accuracy of pulse oximetry readings are based on a large number of assumptions, it is still a valuable substitute for the measurement of PaO₂ in clinically stable patients.

Understanding the above concepts will allow you to derive a lot more information when used in the context of your patient’s oxyhaemoglobin dissociation curve and their clinical status.

This will help improve patient outcomes, while early recognition of changes will allow prompt intervention and management of a patient’s disease.
February 27, 2023
Oxyhaemoglobin dissociation curve, pt 1: oxygen saturation
With the widespread availability of pulse oximetry and its relative ease of use, it’s easy to become complacent and overly reliant on its values.

One must remember pulse oximetry is only an indirect measurement of the arterial haemoglobin saturation, so these values are based on a series of assumptions on the other important factors that determine oxygenation.

To understand the overall oxygen status of a patient, you will need to assess the pulse oximetry values in the context of the oxyhaemoglobin dissociation curve (OHDC).

SaO₂ and PaO₂

The OHDC depicts the relationship between oxygen saturation of haemoglobin (SaO₂) and the partial pressure of arterial oxygen (PaO₂). However, before discussing the interpretation of the OHDC graph, we must understand the difference between SaO₂ and PaO₂:
- SaO₂ refers to the percentage of haemoglobin molecule within red blood cells that is bound to oxygen. Each haemoglobin molecule is made up of four strands of amino acids, each of which are able to bind to one molecule of oxygen. When these binding sites are fully saturated (bound), this is reflected as 100% oxygen saturation.
- PaO₂ is the partial pressure of oxygen dissolved in blood (expressed in mmHg) – or, more simply put, the measurement of oxygen content in arterial blood. The higher the PaO₂, the more readily oxygen binds to haemoglobin.
Influences

Pulse oximetry for a patient with hypoxaemia.

Aside from SaO₂ and PaO₂, other important elements exist that determine the effectiveness of oxygenation of tissue, both in terms of oxygen delivery and oxygen dissociation (offloading from haemoglobin).

The efficiency of oxygen transport is influenced by:
- the number of haemoglobin molecules available for oxygen uptake
- a sufficient blood volume
- a competent circulatory system (cardiac output and blood pressure)
Oxygen dissociation from haemoglobin molecules to the target tissue is directly determined by the tissue demand for oxygen.

It should be noted none of these factors are assessed by the pulse oximeter, thus highlighting the limited value of pulse oximetry in the absence of the knowledge of these other factors and the context of the OHDC.

So, the first key point is the number displayed on the pulse oximeter should not be interpreted in isolation.

The goal of the next couple of posts is to help you understand the limitations of pulse oximeters, what the OHDC means, what factors can affect this curve, and how to interpret pulse oximetry values in the context of the curve to get a more accurate picture of the patient’s overall oxygenation status.
February 20, 2023
Blood gas analysis, pt 1: why everyone needs to know about it

For those of you who have received referral histories from emergency or specialists hospitals, blood gas analysis is probably no stranger to you. For those who have never heard of them before, fear not – you are in for a treat.

In my emergency hospital, the blood gas analyser is arguably one of the most frequently used bench top lab machines, second only to centrifuge, and for good reasons…

Acid-base disturbances are common in critically ill and emergency patients, and it can help determine the severity of their condition and sometimes provide the answer. Tracking changes in blood gas parameters can provide information about the patient’s response to your interventions.

Blood gas analysis can help assess the severity of a patient’s condition and help guide your diagnostic plan.

The information gained from pulse oximetry is very limited in patients with severe respiratory compromise, and the only way to accurately assess their oxygenation and/or ventilation status is by looking at their blood gas status.

So what does the blood gas analysis actually measure?

Most blood gas panels assess the pH of the blood, partial pressure of oxygen (PO₂) and partial pressure of carbon dioxide (PCO₂). From these, the machine is able to derive the percentage of haemoglobin saturated with oxygen (SO₂), bicarbonate (HCO₃^–) concentration and base excess of the extracellular fluid (BEecf).

In most machines, they are also able to measure other parameters, such as electrolytes (Na, K, Ca, Cl), glucose and lactate.

While arterial blood gas samples are required for determining the ability of the body to oxygenate the haemoglobin, venous samples are suitable for determining the ventilation status, assessing acid base balance, electrolytes, glucose and lactate levels.

So how can this help as a point-of-care test?

As mentioned previously, blood gas analysis can help assess the severity of a patient’s condition and help guide your diagnostic plan. It can also provide a diagnosis (such as diabetic ketoacidosis, typical hypoadrenocorticism and high gastrointestinal obstructions).

The changes in these parameters over time can be essential in managing critical patients in the emergency setting; it will help guide you in developing an appropriate IV fluid therapy regime and fluid choice, address the patient’s oxygenation and/or ventilation needs, correct any electrolyte and glucose abnormalities, and – although fallen out of favour – the administration of sodium-bicarbonate therapy.

In upcoming blogs, I will teach you how to interpret the blood gas results. At the end of this, I hope everyone will incorporate blood gas analysis as their standard point-of-care test for the better assessment and management of patients.

If given the choice between a biochemistry and a blood gas panel in a critical patient, I would hands down select blood gas every time.

August 9, 2022
Lipaemia – the bane of biochemistry
Last week we covered haemolysed samples – this week we’re looking at lipaemic samples.

Lipaemic samples are caused by an excess of lipoproteins in the blood, creating a milky/turbid appearance that interferes with multiple biochemical tests and can even cause haemolysis of red blood cells.

A severely lipaemic sample (red arrow). IMAGE: eClinPath.com (CC BY-NC-SA 4.0).

Lipaemia can follow recent ingestion of a meal – especially one high in fat. Although not pathognomonic for any diseases, its presence can help increase the suspicion of certain diseases, including:
- pancreatitis
- diabetes mellitus
- hypothyroidism
- hyperadrenocorticism
- primary hyperlipidaemia (in some specific breeds, such as the miniature schnauzer)
It warrants further investigation in patients that have been ill and inappetent.

Irksome interpretations

Lipaemia can dramatically impact laboratory testing and is often troublesome in critically ill patients, making interpretation of biochemistry particularly difficult, if not impossible.

Lipaemia can affect different analysers in different ways, most commonly causing:
- Falsely increased calcium, phosphorus, bilirubin, glucose and total protein (via refractometer) and some liver parameters such as alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, haemoglobin concentration, and mean corpuscular haemoglobin concentration.
- Falsely decreased sodium, potassium, chloride, albumin and bicarbonate.
Tube tips

Assessment of a centrifuged haematocrit tube before running a biochemistry panel can help reduce wasted biochemistry consumables.

If the sample is lipaemic in the haematocrit tube then maybe try some of the following tips.
- If blood tests are planned in advance, try fasting the patient beforehand for 12 to 24 hours.
- Repeat sampling a couple of hours later may yield a less lipaemic sample.
- Collecting and centrifuging a larger amount of blood (3ml to 5ml, for example) can sometimes yield enough clear sample between the lipid layer and red blood cells.
- Refrigeration of the sample can help the separation.
- Extract lipids using polar solvents, such as polyethylene glycol.
- Centrifugation at higher than normal speeds (if possible) can also assist in clearing the layer.
February 28, 2022
PCV/total solids interpretation: serum colour
When interpreting the often misinterpreted and underused PCV and total solids test, it is important to take note of the serum colour as this may give clues into the diagnosis.

Normal serum colour (left) compared to a patient with immune-mediated haemolytic anaemia. The serum is haemolysed and anaemia is present.

The most common abnormalities seen in clinic are icteric, haemolysed and lipaemic serum.

Clear serum can also be of importance – especially when you interpret it with blood counts and urine colour.

Haemolysis

The most common abnormality of serum colour changes is haemolysis. In my experience, the most common cause is suboptimal collection technique. To confirm this, simply collect another sample and repeat.

If it is repeatable, and concurrent anaemia or pigmenturia is present, it warrants further investigation.

Intravascular haemolysis can be caused by:
- immune-mediated haemolytic anaemia
- blood transfusion reactions
- infectious diseases such as Mycoplasma haemofelis, Babesia canis, Ehrlichia canis, FeLV and others
- Heinz bodies from the ingestion of heavy metal, onions or paracetamol
- hypophosphataemia
- macroangiopathic disease (neoplasia, for example)
- envenomation – typically, snake bites
Testing issues

Haemolysis can also affect other laboratory testing. It can lead to an artefactual increase in glucose, phosphorus, bilirubin, total protein, fructosamine and triglycerides, and a decrease in sodium (pseudohyponatraemia), cholesterol, calcium, potassium and albumin.

Extravascular haemolysis often does not cause haemolysed serum as it is generally slower and the body is able to clear the haemoglobin before it can lead to discolouration of the serum.
February 21, 2022
Pulse oximetry is great, but know its limitations
Pulse oximetry is a very useful diagnostic and monitoring tool that has become commonplace in veterinary clinics. It measures the percentage of haemoglobin saturated with oxygen, and is an indirect measure of arterial oxygen levels.

Dog with pulse oximetry.

However, here are several important points to help you understand the limitations of pulse oximetry.

Causes for false readings

Falsely low readings:
- motion artefact
- peripheral vasoconstriction/low tissue perfusion from hypothermia or shock
- pigmentation of mucous membranes
- thick hair coat
Falsely high readings:
- haemoglobin abnormalities (carboxyhaemoglobin and methaemoglobin, for example)
False sense of security

Pulse oximetry can give us a false sense of security. We hold on to the adage “95% and above means everything is going along swimmingly”, but that couldn’t be further from the truth:

Pulse oximetry can give us a false sense of security.
- It does not detect hypoventilation or apnoea: it can take several minutes for apnoea to result in hypoxaemia that is detected on pulse oximetry; therefore, it cannot be used as a sole measure of respiratory adequacy. This is best measured by capnography.
- A common misconception is the oxygen saturation will drop with patients with anaemia. This is incorrect. The haemoglobin present in the decreased number of red blood cells will still be saturated to normal levels. However, this cannot be interpreted as the patient having adequate oxygen delivery to its tissues.
- One last point: due to the oxyhaemoglobin dissociation curve, any drop below 94-95% is significant and warrants investigation. At 95% SpO₂ the partial pressure of oxygen in the arterial blood is 80mmHg (normal), but at 90% SpO₂ the partial pressure is 60mmHg (severe hypoxaemia) – for only a small percentage decrease, there is an exponential reduction in arterial oxygen content. This is even more important when patients are receiving oxygen therapy as the patient’s SpO₂ should be 99-100% normally. So when a patient has an SpO₂ of 95%, but is on high rates of oxygen, then significant respiratory compromise/disease must be present for an SpO₂ of 95% or lower to occur.
August 23, 2021