Overview of Anesthesia Monitoring of Mixed Venous Saturation
Oxygen saturation tells us how much of the hemoglobin, a protein in our red blood cells, has oxygen attached to it. On the other hand, mixed venous oxygen saturation, often shown as SvO2, is about the amount of oxygen in the blood returning to the heart, after it has delivered oxygen to the body tissues that needed it.
In medical practices, doctors often measure venous oxygen saturation when they’re taking care of patients just before, during, or after surgery, during critical care, or when dealing with cases of shock. While there’s been differing views on how much of a role this measurement plays in early treatments, it’s been widely accepted that abnormal readings usually mean the patient’s risk of mortality is higher.
So, by tracking mixed venous oxygen, doctors can do a better job of predicting patient outcomes, determining the prognosis, and spotting low oxygen levels in the tissues of very ill patients. This is just one of many factors that should be looked at when checking to make sure body tissues are getting enough blood.
Anatomy and Physiology of Anesthesia Monitoring of Mixed Venous Saturation
Mixed venous oxygen saturation relates to the combination of oxygen levels in the blood coming from different parts of the body. Blood from the upper parts of the body and head, including your arms, is sent back to the heart through a large vein called the superior vena cava. Meanwhile, another large vein, the inferior vena cava, sends blood from the lower parts of your body, your kidneys, and digestive organs back to the heart. Both of these blood flows then mix together in the right side of your heart.
The mixed blood enters your lungs through an artery called the pulmonary artery, which helps reoxygenate the blood. Oxygen-rich blood returns to the left part of your heart through a large blood vessel known as a pulmonary vein. Once in the left part of the heart, this oxygen-rich blood is then pushed out to your body through a large blood vessel called the aorta, which plays a key role in delivering oxygen to the body that later becomes a part of the oxygen exchange.
It’s essential to understand that the amount of oxygen extracted from the blood can be different in the upper and lower parts of your body. This variation depends on many factors, such as differences in metabolism, blood flow, and how much oxygen different body parts use. For instance, it’s estimated that the heart uses more oxygen from the blood than the kidneys do.
When the body tissues need more oxygen, there are several ways the body can increase the delivery of oxygen. These include boosting the heart’s output, changing how blood is distributed in the body, and increasing the density of working capillaries, which are tiny blood vessels.
To measure the levels of oxygen in different parts of the body, medical professionals usually use a specific tool called a pulmonary artery catheter. This catheter is inserted via a central venous catheter and placed in the pulmonary artery. Healthcare professionals use the changes in the pressure of various parts of the heart as the catheter moves through them to help position the catheter correctly.
Why do People Need Anesthesia Monitoring of Mixed Venous Saturation
Keeping a close watch on the level of oxygen in the blood mixed from all the veins, also known as mixed venous oxygen saturation (SvO2), can be helpful in certain medical situations. These situations might include while undergoing surgery, during critical care, managing heart failure, or sepsis (a serious body-wide infection). It becomes very significant to understand the changes observed in the percentage of oxygen in the blood in these situations. Just like any other monitoring procedure, it’s important to combine this oxygen saturation data with other details related to blood circulation. Also, doctors need to match this data with the patient’s overall health condition.
If there’s an imbalance between the oxygen demand and supply in the body such that the tissues don’t get enough oxygen, the body will try to pull more oxygen from the blood. This will result in a drop in SvO2. If the demand for oxygen suddenly increases due to conditions like fever, shivering, pain, or stress, and the body is unable to compensate with enough oxygen supply, the SvO2 will decrease. Similarly, lower SvO2 could be a response to decreased oxygen supply due to anemia (low red blood cells), hypoxemia (low oxygen in the blood), or reduced cardiac output, which could occur during a heart attack.
On the other hand, if SvO2 increases because the body doesn’t need as much oxygen, this might indicate that medications for pain relief or sedation are being used, mechanical breathing assistance is being provided, or the patient is being intentionally cooled down (hypothermia).
If the supply of oxygen increases but SvO2 remains high (over 90%), it means the tissues are unable to use the available oxygen, which isn’t a good sign. It could indicate that there’s likely an issue with the oxygen being used at the cellular level, resulting in increased lactic acid and cell death. This situation could occur if the cells are damaged, or there’s poisoning preventing cells from using oxygen. Other causes may include septic shock or arteriovenous shunting, a condition where the blood flow is diverted, causing too much blood to flow to certain body regions. Similar trends can also show in cases of reduced workload on the heart or improvement in cardiac functioning.
When a Person Should Avoid Anesthesia Monitoring of Mixed Venous Saturation
The use of central venous cannulation, a procedure where a tube is inserted into a large vein, is sometimes avoided because it’s quite invasive. The doctor always has to consider if the benefits outweigh the risks for each patient. Some reasons a patient can’t have this procedure include existing infection where they would insert the tube, issues with blood that might cause too much bleeding, conditions stopping the patient from lying in a certain position (known as the Trendelenburg position), having high pressure inside the skull, or the patient simply refusing the procedure.
Additionally, pulmonary artery catheterization, which involves inserting a tube into the artery of the lung, may not be right for certain individuals. These include situations where there isn’t a knowledgeable healthcare worker available to insert and look after the tube – as it could cause complications, patients who are allergic to the substance heparin, or those allergic to latex (in this case, it’s recommended to use latex-free catheters if possible). If the patient has growths or moving blood clots in their right heart, the tube could cause these to move to other parts of the body which could be harmful.
Lastly, people with a certain type of heart defect called ‘left bundle branch block’ should avoid this procedure. This is because inserting the catheter could further damage the heart’s electrical system and cause it to stop working properly.
Equipment used for Anesthesia Monitoring of Mixed Venous Saturation
To measure the oxygen levels in mixed venous blood (blood where oxygen poor and oxygen-rich blood have mixed together), doctors use a special tool called a pulmonary artery catheter. This is a thin, flexible tube that is guided into your pulmonary artery, a large blood vessel in your lungs. The procedure normally involves using images from an echocardiogram (a type of ultrasound scan) or real-time x-ray (fluoroscopy) for guidance. This device works by using the principle of reflection spectrophotometry, a physical process that involves the measurement of reflected light. It includes a LED photodetector that connects to a module on the catheter which shines a light to analyze the amount of oxygen in red blood cells in the bloodstream.
Once a vein in the central part of the body has been identified, it is ready for cannulation, which is the process of placing a tube in it. Generally, doctors use ultrasound as a guide when inserting these tubes, known as central venous catheters. There are several steps to this process, including using a finder needle and a thin plastic tube (angiocath) along with a guide wire. If a pulmonary artery catheter is being used, an introducer (a larger tube) is also used to make the path for it. This catheter has multiple functions, aside from measuring mixed venous oxygen levels, it can check heart output, body temperature and pressure in the pulmonary artery. It can also be used to deliver medications or fluids into the body.
The pulmonary artery catheter who is implemented into the body is composed of several important parts. The blue part, known as the proximal injectate port, rests in the right atrium of your heart and allows for the insertion of fluids or to monitor central venous pressures. Another port, the proximal infusion port, is also in the right atrium, and it can be used to inject fluids and medications. The yellow part is the PA port, which measures PA pressures and levels of mixed venous oxygen. The part of the catheter that stays outside your body has multiple different valves for various purposes, and is kept sterile with a cover.
During and after the placement of the catheter, sterility is of the utmost importance to avoid any infections entering the bloodstream from the central line (CLABSI). Each time the ports are accessed, they should be cleaned with an antiseptic to decrease the risk of contamination.
After the initial placement of the catheter, it’s important for doctors to note how deep the catheter is and to confirm the catheter’s position by examining pressure waveforms and x-ray. Highly trained healthcare professionals continuously monitor the patient to avoid complications. Another thing to consider when measuring SvO2 is the signal quality index. The quality of the signal affects the accuracy of the measurement, with 1 being normal signal, and 4 being a signal so poor it can’t be used. If the signal is problematic, the catheter position may need to be checked and adjusted or recalibrated to measure the SvO2 accurately.
Who is needed to perform Anesthesia Monitoring of Mixed Venous Saturation?
When a special tube is inserted into your chest or a major vein (a procedure called pulmonary artery or central venous catheterization), the procedure should be performed by well-trained medical staff. They need to know how to do the procedure correctly to avoid complications and to recognize early signs of trouble that could occur after starting the procedure. Other medical professionals should also be ready to understand and interpret the results from the procedure, as it monitors changes in your body’s circulation.
If this procedure’s results aren’t understood or addressed quickly, it can limit the effectiveness of treatment and, in some circumstances, potentially harm the patient. Using an ultrasound to guide the insertion of central venous catheters (tubes inserted into a large vein) can help ensure the tube is put in the right place. Interpreting the pressure changes shown on the monitor and checking the position of the catheter on a chest X-ray can also help to avoid incorrectly placing the catheter.
Preparing for Anesthesia Monitoring of Mixed Venous Saturation
Before a doctor places a tube in one of your major veins, called central venous access through catheterization, it’s really important for them to follow clean procedures. This could include using a substance called chlorhexidine to kill any germs and covering you with sterile drapes. This helps to prevent any infections from occurring through the catheter. Your position during the procedure also matters because it can make it easier for doctors to insert the catheter.
One such position is called the Trendelenburg position. In this position, you lie flat on your back with your legs elevated higher than your head. This makes your veins swell due to gravity, which provides doctors with a larger target for placing the catheter. The swollen veins can also help to prevent a potentially dangerous complication where air gets into your veins.
Your doctor might use ultrasound to guide them and make the procedure safer. Before the tube or catheter is inserted, doctors might use gadgets called pressure transducers to monitor your blood pressure. These devices are connected to a monitor which converts the measurements into readings that are helpful for placement of the catheter and to check your health afterwards.
How is Anesthesia Monitoring of Mixed Venous Saturation performed
The amount of oxygen in your blood can be measured in two main places, the pulmonary artery (a major blood vessel in the lungs) or the superior vena cava (a large vein carrying blood into the heart). This can be done using special tubes called a pulmonary artery catheter (PAC) and central venous catheter (CVC).
The CVC is less expensive and simpler to use, but it measures a value called central venous oxygen saturation (ScvO2). Because PAC and CVC take measurements from different areas, they can show different levels of oxygen in your blood.
Remember, the CVC takes a sample from the superior vena cava. This mainly shows how much oxygen is being taken from the blood in your head and upper body, but it doesn’t pay attention to the lower body and abdominal organs. On the other hand, the blood flow from these lower areas of the body usually goes to places such as the kidneys, liver circulation, and portal circulation, where less oxygen is taken out of the blood.
We know from other studies that the oxygen saturation level for blood returning from the superior vena cava is usually between 70 to 75%, and from the inferior vena cava (another large vein carrying blood into your heart from the lower half of your body) it’s usually around 75 to 80%. Therefore, the mixed venous oxygen saturation, a combination of oxygen levels from multiple veins, would fall between these ranges. In adults, when the mixed venous oxygen comes back to the heart, it still has about 60 to 80% oxygen saturation.
When a PAC is used to draw blood for testing, the process must be slow so that oxygen-rich blood isn’t accidentally sucked up. This would incorrectly show higher levels of oxygen saturation. The CVC has a lower risk of problems and allows for constant monitoring which can be valuable when hoping to examine the oxygen levels more often. The tip of the catheter measures the amount of oxygen in the blood using a special technology called reflection spectrophotometry.
In general, the central venous oxygen saturation (ScvO2) tends to follow mixed venous oxygen saturation (SvO2) trends, but ScvO2 usually measures about 2 to 3% lower than SvO2. However, this relationship can change under different circumstances. For example, in certain types of shock when the body is not getting enough blood flow, the circulatory system can change to ensure blood flow to the brain, yet reduce blood flow to the intestines and kidneys. In these situations, ScvO2 may temporarily be almost 20% higher than SvO2. Other conditions, like septic shock where blood flow and oxygen levels are affected, may result in ScvO2 being almost 8% higher than SvO2.
Possible Complications of Anesthesia Monitoring of Mixed Venous Saturation
Using catheters, especially those associated with pulmonary artery procedures, comes with potential problems. Pulmonary artery catheters (tiny tubes inserted into the pulmonary artery to measure blood pressure) can cause complications when they’re inserted, manipulated, or maintained. One common issue when a catheter is first placed inside a vein in the neck, known as an internal jugular catheter, is thrombosis (formation of a blood clot) with a risk of 7.6%. Other risks that are less common include accidentally pricking an artery, forming a hematoma (a collection of clotted blood outside of a blood vessel), creating a connection between an artery and vein (AV fistula), injury to the thoracic duct (part of the lymphatic system), introducing an air bubble into the bloodstream (air embolization), and collapsing lung (pneumothorax).
Moreover, when a PAC is manipulated and maintained, it can cause brief heart rhythm problems, happening with a frequency of 12.5 to 70%. The most frequently observed forms of heart rhythm issues are PVCs (premature ventricular contractions) and VTs (ventricular tachycardia). A catheter-associated infection is also an ongoing concern, with a PAC-associated bacterial infection occurring in 1.3 to 2.3% of cases. Some very rare cases can experience pulmonary artery rupture, chamber rupture, misplacement in the coronary sinus, lung tissue death (pulmonary infarction), and PAC knotting (when the catheter ties up in a knot).
What Else Should I Know About Anesthesia Monitoring of Mixed Venous Saturation?
Mixed venous measurement is used to determine the balance between the body’s oxygen supply and usage. The factors that can influence its measurement include:
- Cardiac output – this basically means how well your heart is pumping blood. It can change due to heart issues, infections, dehydration, etc.
- Hemoglobin – this is a protein in your blood that carries oxygen. Low levels may be due to bleeding or shock.
- Oxygenation – the amount of oxygen in your body. This can be affected by how much oxygen you’re breathing in, lung disease, etc.
- Oxygen consumption – the rate at which your body uses up oxygen. This may increase due to infections, burns, fever, stress, heavy breathing, etc.
There are two types of measurements: mixed venous oxygen saturation (SvO2) and central venous oxygen saturation (ScvO2). The benefits of these have been studied, but results have been mixed.
In sepsis (a severe infection), levels of SvO2 and ScvO2 that are lower than usual indicate a poor outlook. Some studies suggest that keeping ScvO2 levels above 70% could reduce the risk of death. Therefore, it can be used as a guide in sepsis treatment. These levels should ideally be reached within the first 6 hours. However, this is not a standard treatment guideline yet.
Other studies contradicted these findings. For example, the use of ScvO2 in early goal-directed therapy (EGDT) in septic shock didn’t show any benefits in some studies. Also, normal or high ScvO2 levels don’t rule out tissue oxygen shortage, as the body may not be able to use the available oxygen effectively. This hints that abnormally low or high ScvO2 levels are linked to higher death rates in septic shock patients.
In the surgical period, low ScvO2 levels can also be linked with higher health complications. However, again, using goal-directed therapy using ScvO2 levels has shown mixed results. Some studies found benefits, while others didn’t.
In cases of heart failure, ongoing monitoring of ScvO2 levels can help guide the treatment. A recent study has shown that low ScvO2 levels indicate poor outcomes and can predict serious heart-related adverse events.
To summarise, SvO2 and ScvO2 measurements can be useful in understanding your body’s oxygen levels and guiding treatment in certain conditions, but their effectiveness is still being researched.
