Blog Archives

Pitfalls of pulse oximetry

4/30/2024

🖥️ Watch teaching video

Pitfalls of pulse oximetry

In this lecture, we will discuss the pitfalls of using pulse oximeter in assessing patients with dyspnea.

Pulse oximetry is a non-invasive method of measuring the oxygen saturation of hemoglobin in arterial blood. It has become a ubiquitous tool in clinical practice, providing a continuous estimate of a patient's oxygenation status. However, there are several important limitations and potential pitfalls associated with the use of pulse oximetry, particularly in the assessment of patients presenting with dyspnea or respiratory distress.

The fundamental principle behind pulse oximetry involves the different absorption spectra of oxyhemoglobin and deoxyhemoglobin for red and infrared light. The ratio of these absorption values is used to calculate the oxygen saturation (SpO2). This calculation assumes that only two species of hemoglobin are present: oxyhemoglobin and deoxyhemoglobin. However, in certain clinical situations, other hemoglobin species may be present, leading to inaccurate SpO2 readings.

One such situation is carbon monoxide poisoning. Carboxyhemoglobin, formed by the binding of carbon monoxide to hemoglobin, has a similar absorption spectrum to oxyhemoglobin. Since oximeters misinterpret carboxyhemoglobin as 100% oxyhemoglobin, SpO2 will trend toward 100% in patients with significant carbon monoxide poisoning, potentially masking the severity of their condition. This can lead to a false sense of reassurance and delayed treatment.

Another limitation of pulse oximetry is its inability to detect hypoxemia in the setting of abnormal hemoglobin species, such as methemoglobin and sulfhemoglobin. Methemoglobinemia can occur due to certain drugs, toxins, or inherited disorders, while sulfhemoglobinemia is typically associated with sulfur compound exposure. Sulfhemoglobinemia can mimic methemoglobinemia clinically and by SpO2 measurements. Since oximeters misinterpret methemoglobinemia as 85% oxyhemoglobin, SpO2 will trend toward 85% in patients with significant methemoglobinemia. Thus, SpO2 can either overestimate or underestimate the oxygen saturation of patients with methemoglobinemia or sulfhemoglobinemia.

Furthermore, pulse oximetry does not consider hemoglobin level, and has limitations in accurately measuring oxygen saturation at extremes of hemoglobin concentration. For non-hypoxic cases of severe anemia with hemoglobin levels below 5 g/dL, pulse oximeters may overestimate the oxygen saturation. Conversely, in patients with polycythemia or high hemoglobin levels, pulse oximeters may underestimate the oxygen saturation.

It is also important to note that pulse oximetry measures the functional saturation of hemoglobin, which may not accurately reflect the overall oxygen content of the blood, particularly in cases of dyshemoglobinemias or shifts in the oxygen-hemoglobin dissociation curve. Pulse oximetry does not consider A-a gradient, supplemental O2 can mask severe pulmonary process. In such situations, arterial blood gas analysis provides a more comprehensive assessment of oxygenation status and acid-base balance. Fluctuations in PaO2 can lead to rapid, unpredictable changes in SpO2 that may be missed by intermittent monitoring, thus, continuous SpO2 monitoring is suggested for dyspneic patients.

In addition to these limitations, pulse oximetry can be affected by factors such as poor perfusion, motion artifacts, and the presence of certain dyes or pigments. Conditions such as hypothermia, hypotension, or peripheral vascular disease can lead to reduced peripheral perfusion, resulting in unreliable SpO2 readings. Similarly, excessive patient movement can introduce motion artifacts, compromising the accuracy of the measurements. Another pitfall is that pulse oximetry measures peripheral oxygen saturation, which can lag behind changes in central arterial oxygenation. This delay may mask acute deterioration in some patients.

When assessing patients with dyspnea or respiratory distress, it is crucial to interpret pulse oximetry readings in the context of the patient's clinical presentation, physical examination findings, and other diagnostic modalities. Relying solely on pulse oximetry can potentially lead to missed diagnoses or inappropriate management decisions.

Arterial Blood Gas (ABG) provides a more comprehensive assessment of oxygenation and acid-base balance. ABG allows the calculation of A-a gradient which helps clinicians narrow down the differential diagnosis for hypoxemia. One should consider ABG analysis when appropriate.

In summary, while pulse oximetry is a valuable tool in clinical practice, healthcare professionals must be aware of its limitations, particularly in the assessment of patients with dyspnea or respiratory distress. Potential pitfalls include interference from abnormal hemoglobin species, inaccuracies at extremes of hemoglobin concentration, the inability to detect hypoxemia in certain conditions, and the effects of poor perfusion or motion artifacts. Integrating pulse oximetry findings with a comprehensive clinical evaluation, including arterial blood gas analysis when appropriate, is essential for accurate diagnosis and appropriate management of respiratory disorders.

Take Home Message

Pulse oximetry does not consider hemoglobin level, SpO2 may be overestimated if Hb is below 5 g/dL.
Pulse oximetry does not consider A-a gradient, supplemental O2 can mask severe pulmonary process.
SpO2 will trend toward 100% in patients with significant CO poisoning.
SpO2 will trend toward 85% in patients with significant methemoglobinemia.
Peripheral SpO2 can lag behind changes in central arterial oxygenation.

0 Comments

Back pain Emergencies

4/27/2024

0 Comments

🖥️ Watch teaching video

Back pain Emergencies

Back pain is a common complaint, with 90% of cases being benign. However, it is crucial to identify the 10% of cases that are dangerous, as they can be easily missed.
This lecture will discuss the five back pain emergencies, their causes, diagnosis, and management.
The five back pain emergencies include: Aortic syndrome or retroperitoneal bleed, spinal epidural abscess, cauda equina syndrome, spinal metastases, and spinal fractures.

Aortic syndrome or retroperitoneal bleeding

Aortic syndrome and retroperitoneal bleeding are life-threatening conditions that require immediate attention.
Patients may present with sudden, severe back pain, along with other symptoms like abdominal pain, hypotension, or syncope.
Point-of-care ultrasound is a valuable diagnostic tool for quickly identifying abdominal aortic aneurysms.
Diagnosis of retroperitoneal bleeding may require a CT scan.

Spinal epidural abscess (SEA)

Spinal epidural abscess can cause severe back pain and neurologic deficits.
The classic triad of fever, back pain, and neurologic deficit is present in only 15% of cases, and these infections are often missed on the first ED visit.
Elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are useful markers for the diagnosis of spinal epidural abscess (SEA), and MRI is the imaging modality of choice.

Cauda equina syndrome

Cauda equina syndrome is a surgical emergency, usually caused by lumbar disc herniation (HIVD).
The symptoms include bilateral sciatica, urinary retention, saddle anesthesia, reduced sphincter tone, and lower limb weakness.
The straight leg raise test (SLRT) and slump test can help diagnose sciatica, but they are not specific for cauda equina syndrome. MRI is the imaging modality of choice.

Straight Leg Raise Test (SLRT)

The straight leg raise test is used to assess for lower back and leg conditions like nerve root irritation, disc herniation, and disc degeneration.
It involves slowly raising the patient's leg while keeping the knee extended, and pain below the knee that occurs between 30 to 70 degrees of flexion may indicate lumbar disc herniation.
The test is performed with the patient lying flat, and the examiner gently raises the patient's leg by flexing the hip with the knee extended.
A positive test is indicated by pain along the lower limb.

The slump test

A positive slump test indicates the presence of a herniated lumbar disc.
The slump test is performed as follows:
1. The patient sits with their hands behind their back to achieve a neutral spine position.
2. The patient then slumps forward at the thoracic and lumbar spine.
3. If this does not cause pain, the patient is asked to flex their neck by placing their chin on their chest.
4. The patient is then asked to extend one knee as much as possible. If this causes pain, the patient extends their neck back to neutral.
5. If the patient is still unable to extend the knee due to pain, the test is considered positive for nerve root irritation or HIVD.
6. The patient may also be asked to actively dorsiflex the ankle, which can further reproduce symptoms if the test is positive.

Spinal Metastases

Cancer is a common cause of back pain.
Spinal metastases can cause neurologic deficits and require urgent management.
Hypercalcemia is a common complication of cancer and can cause general weakness, constipation, and confusion

Spinal Fractures

Spinal fractures that require urgent management include burst fractures, Chance fractures, and spondylolysis.
Burst fracture is a result of compression injury which causes vertebral fragments to be retropulsed into the spinal canal.
Chance fracture is a horizontal fracture through the posterior elements due to flexion-distraction injury caused by a seatbelt.
Spondylolysis is a stress fracture of pars interarticularis, and is commonly seen in young athletes.

Red Flags for Back Pain

The following red flags should raise suspicion for a serious underlying condition:
Age below 18 or above 60.
Immunodeficiency.
Previous spinal interventions.
Recent infections.
Cancer history.
Trauma history.
Acute urine retention (AUR) or saddle anesthesia.
Coagulopathy.
Pain not resolved by analgesia.

Important Physical Findings of patients with back pain

Physical findings of back pain can vary depending on the underlying cause.
For retroperitoneal bleeding, one may observe bruising in the groin, umbilicus (Cullen's sign), or flank (Grey-Turner's sign).
An abdominal aortic aneurysm may be palpable as a pulsatile abdominal mass.
In cases of spinal epidural abscess, symptoms may include fever and percussion tenderness over the spinous processes.
Herniated intervertebral disc or cauda equina syndrome can present with acute urinary retention, saddle anesthesia, decreased anal tone, gait disturbance, paraparesis, and positive straight leg raise test and slump test.
Spinal metastases may exhibit physical findings that mimic those of HIVD.
Lastly, spinal fractures can also have percussion tenderness over the spinous processes.

Diagnostic Tests for Back Pain

Point-of-care ultrasound (POCUS) is utilized to assess for conditions like abdominal aortic aneurysm (AAA) and aortic dissection, particularly focusing on the presence of an intimal flap.
CT or MRI scans are commonly employed to investigate spinal epidural abscess, osteomyelitis, spinal metastases, and retroperitoneal bleeding, providing detailed anatomical information.
In cases where infection is suspected, such as SEA and osteomyelitis, ESR and CRP levels are measured.
A calcium profile may be ordered to evaluate for hypercalcemia secondary to spinal metastases.
X-rays play a crucial role in detecting spinal metastases, fractures, osteoblastic or osteolytic lesions, and pedicle erosion, providing valuable insights into the structural integrity of the spine.

Management of Back Pain Emergencies

The management of back pain emergencies depends on the underlying condition.
Aortic syndrome and retroperitoneal bleeding require immediate surgical intervention.
SEA and osteomyelitis require antibiotics and surgical intervention.
Cauda equina syndrome requires urgent surgical intervention.
Spinal metastases require urgent radiation therapy and medical management.
Spinal fractures require immobilization and surgical intervention.

Take Home Message

Back pain is a common complaint, and it is crucial to identify the 10% of cases that are dangerous.
The five back pain emergencies are vascular aortic syndrome, infection, disk herniation, cancer, and fracture.
Red flags, physical exam maneuvers, and diagnostic tests can help diagnose serious underlying conditions.

The management of back pain emergencies depends on the underlying condition and requires urgent intervention.

0 Comments

EM Literature #1

4/27/2024

0 Comments

🖥️ Watch teaching video

EM Literature #1
- MgS04 for rapid A-fib
- NaHC03 for DKA
- Norepinephrine for post-ROSC
- Antihistamines for vertigo
- Contrast nephropathy
- NTG for RV MI
- Less fluid for pancreatitis
- High-dose NTG for SCAPE

0 Comments

VBG As a Replacement for ABG

4/22/2024

0 Comments

📺 Watch teaching video

Venous Blood Gas as a Replacement for Arterial Blood Gas in Emergency Settings

In emergency medical settings, the assessment of blood gases is crucial for managing patients with acute respiratory or metabolic conditions.
Traditionally, arterial blood gas (ABG) analysis has been the standard for evaluating oxygenation, ventilation, and acid-base status.
However, the use of venous blood gas (VBG) as an alternative to ABG is gaining recognition for its ease of collection, reduced patient discomfort, and numerous benefits.
This lecture explores the comparison between VBG and ABG, highlighting the advantages of VBG as a replacement for ABG in emergency settings.

Comparison between VBG and ABG

ABG has long been considered the gold standard for assessing blood gases due to its accuracy in measuring various parameters.
While ABG provides precise data on pH, pCO2, and HCO3 levels, it is invasive and can be very painful for patients.
On the other hand, VBG, obtained from peripheral veins, offers a less invasive and more comfortable alternative.
Recent studies have shown that VBG can provide comparable results to ABG in assessing pH, HCO3, pCO2, lactate, electrolytes, and base excess in many clinical scenarios.
However, VBG may not be as accurate as ABG in severe shock or cardiac arrest situations.

Benefits and Advantages of VBG

The use of VBG in emergency settings offers several benefits and advantages over ABG. VBG is easier to obtain, less painful for patients, and carries a lower risk of complications such as vascular or nerve injury.
It provides valuable information on acid-base status, electrolyte levels, and lactate, making it a useful tool for screening and monitoring critically ill patients. In hypotensive patients,
VBG has shown a strong correlation and agreement with ABG results, making it a viable alternative for assessing blood gases in this population.
Additionally, VBG analysis is a non-invasive procedure, increasing its accessibility in the emergency department.

Clinical Applications of VBG

VBG is a reliable tool for assessing respiratory and metabolic conditions in the emergency department. It can correctly diagnose respiratory acidosis, metabolic acidosis, diabetic ketoacidosis, and renal failure, making it a useful alternative to ABG analysis.
In patients with diabetic ketoacidosis, venous pH and PCO2 levels reasonably correlate with ABG values.
In COPD patients, there is no significant difference between arterial and venous CO2 tensions, and the two are closely correlated.
The venous pH is generally 0.03 lower than the arterial pH, and venous PCO2 is about 6 mmHg higher than arterial PCO2, but the correlation between venous and arterial PCO2 is not consistent, particularly in severe shock or hypotension

Limitations of VBG

While VBG is a valuable alternative to ABG in many cases, it has limitations that should be considered.
In conditions of severe hemodynamic compromise, such as profound shock and cardiac arrest, VBG analysis of lactate and pH may be less accurate compared to ABG. VBG also correlates less accurately with ABG in measuring PCO2 in patients with severe shock and hypercapnia.
Lastly, VBG analyses are not able to assess oxygenation. Pulse oximetry is a non-invasive alternative for arterial blood gas sampling.

Take Home Message

In conclusion, VBG analysis presents a promising alternative to ABG analysis in emergency settings.
However, ABG may still be necessary to accurately determine PCO2 in severe shock and hypercapnia.

Pulse oximetry is a non-invasive alternative for ABG to assess oxygenation.

0 Comments

ACLS and BLS Lecture Notes

4/18/2024

0 Comments

🖥️ Watch teaching video

ACLS and BLS Lecture Notes：

In this video, we will delve into the protocols for responding to cardiac arrest, both non-shockable and shockable rhythms, as well as symptomatic bradycardia. By following the American Heart Association's algorithms, you will gain a clear understanding of the essential steps to take in these critical situations.

Time is critical in cardiac emergencies; act promptly to save lives.
Ensure the safety of the scene and prevent injury.
Focus on early CPR and quick defibrillation in BLS.
Oropharyngeal airways should not be used with conscious patients.
Properly position the mask during CPR.
IV or IO routes are preferred for drug delivery; ET route is discouraged.
Minimize interruptions during CPR after delivering a shock.
Targeted temperature management is used after the return of spontaneous Cirrculation.
Confirm asystole twice in two separate leads.
Immediate defibrillate ventricular fibrillation and pulseless ventricular tachycardia.
Treat reversible causes of cardiac arrest including the 5 H and 5 T.
Capnography is a valuable tool in resuscitation. Aim for EtCO2 of at least 10 mm Hg and ideally above 20 mm Hg.

In conclusion, Immediate recognition and intervention are crucial in cardiac arrest or symptomatic bradycardia. The sooner treatment is administered, the higher the chances of positive outcomes. By following these ACLS guidelines, you will be well-equipped to confidently respond to cardiac emergencies and improve patient outcomes.

0 Comments

<<Previous

Pitfalls of pulse oximetry

Back pain Emergencies

EM Literature #1

VBG As a Replacement for ABG

ACLS and BLS Lecture Notes

AuthorTwitter: @jackcfchong[email protected]

Archives

Author
Twitter: @jackcfchong
[email protected]