Cranial CT Anatomy: A simple image guide for medical students

cranial ct anatomy

Computed tomography (CT) is the most useful brain imaging tool in emergency medical practice. It is also the first imaging modality in patients presenting to the emergency department with headache, stroke and head trauma.

Many cranial lesions can easily be recognized in CT. One of the key points of diagnosing cranial lesions is knowing the anatomical structures. This gives us the advantage to evaluate CT by combining clinical findings with the image.

We created an image series for the most essential eight anatomical structures.

cranial CT slices

Centrum Semiovale

centrum semiovale

Lateral Ventricles

lateral ventricles

3rd Ventricle, Basal Ganglia, Superior Cerebellar Cistern

3rd Ventricle, Basal Ganglia, Superior Cerebellar Cistern​

3rd Ventricle, Basal Ganglia, Quadrigeminal Plate

3rd Ventricle, Basal Ganglia, Quadrigeminal Plate

Midbrain, Interpeduncular Cistern​

interventricular cistern

Suprasellar Cistern, 4th Ventricle

Suprasellar cistern, 4th ventricle

Sella Turcica

sella turcica

Pons, Medullary Junction

pons medullary junction

Further Reading

Bonus Infographic

Cite this article as: Murat Yazici, "Cranial CT Anatomy: A simple image guide for medical students," in International Emergency Medicine Education Project, September 4, 2019, https://iem-student.org/2019/09/04/cranial-ct-anatomy-a-simple-image-guide-for-medical-students/, date accessed: September 16, 2019

Venous blood gas analysis: Less arterial punctures!

Introduction

Blood gas analysis is probably one of the most used tests for diagnosis and therapeutic guidance in the emergency departments (EDs) and intensive care units (ICUs).

The evaluation of arterial blood gas (ABG) analysis is commonly used to estimate acid-base status, oxygenation and concentration of carbon dioxide (CO2) in critically ill patients. However, arterial blood (AB) may be difficult to obtain due to weak pulses or movement of the patient. Furthermore, because the thick walls and their innervation, it is more painful for the patient.

Therefore, venous blood gas (VBG) analysis is an alternative to estimate pH and other values in a quicker and easier way.

Venous blood gas analysis

Venous blood (VB) can be obtained from different places. You should always consider the location and the sampling method to interpret the results.

Figure 1 - Types of samples and locations for extraction

VBG analysis is an alternative for ABG in situations of low peripheral perfusion such as shock states of any etiology.

VBG has been studied in critically ill patients as an alternative in patients who do not have a central venous catheter (CVC) (Tavakol, 2013; Byrne, 2014). If a tourniquet is used to facilitate venous puncture, it should be released approximately a minute before the extraction in order to avoid changes induced by ischemia. (Cengiz, 2009). However, VB is preferred from a CVC given its higher correlation with AB. The values obtained from a VBG and an ABG are interchangeable in clinical practice, in both central VB (Malinoski, 2005; Walkey, 2010; Mallat, 2015) and peripheral VB (Malatesha, 2007; Chu, 2003; Kelly, 2001), except for the values of oxygen saturation (SaO2) and partial pressure of oxygen (PaO2).

VB Central VB Peripheral

pH

0.03 – 0.05 below arterial values
0.02 – 0.04 below arterial values

PCO2

4 – 5 mmHg above arterial values
3 – 8 mmHg above arterial values

HCO3

Minimal variation
1 – 2 mEq/L above arterial values

PaO2 / SaO2

No correlation
No correlation

Table 1 – Correlation between venous blood gases and arterial blood gases

Mixed VB (obtained from a pulmonary artery catheter) gives similar results to the values obtained from a CVC. (Ladakis, 2001; Tsaousi, 2010). One should be cautious when interpreting VBG, it has to be always correlated to the clinical state of the patient and if it is necessary, it should be confirmed with an ABG.

Central venous gas analysis

Central VBG analysis allows us to assess the metabolic state of a patient with a good correlation with ABG. Even though central VB is not adequate to assess oxygenation efficacy, this can be estimated by pulse oximetry. Likewise, central VBG analysis gives us central venous oxygen saturation (SatvO2), which is a very sensitive marker of the respiratory, hemodynamic and metabolic homeostatic variations. (Gattinoni, 2017).

Any change in the pulmonary, hemodynamic, metabolic or oxygen transport functions will affect SatvO2. In other words, when we assess SatvO2 value, we are analyzing the result of the interaction between all its determinants:

1) Oxygen input (respiratory system)
2) Oxygen transport (hemoglobin)
3) Oxygen availability DO2 (cardiac output)
4) Oxygen consumption VO2 (tissues).

Gasometric assessment of a central VB sample and its relation with the pulse oximetry will provide us with more information than an ABG analysis.

Global tissue perfusion

In recent year it has been shown that the difference between the value of CO2 obtained from mixed venous blood or central venous blood sample and the value of CO2 obtained from an arterial blood sample is correlated with an increased anaerobic cellular metabolism when the result shows values above 6mmHg. This increase in the veno-arterial CO2 difference is given by an increase of hydrogen in plasma coming from the intracellular environment because of anaerobic metabolism; these hydrogen molecules are buffered in plasma and metabolized to CO2. The causes of the increase in the veno-arterial CO2 difference are mainly due to hypoperfusion secondary to the inadequate cardiac output of mitochondrial dysfunction. (Ospina-Tascón, 2016). Likewise, the quotient of the veno-arterial CO2 difference and the arterio-venous O2 difference has been related with higher accuracy of the tissue perfusion status.

Conclusion

During the assessment of critically ill patients, the analysis of blood gases stands up as a fundamental step in the process of attention. A VBG analysis and SpO2 can give us enough information to make decisions even if there is no ABG analysis available, besides being easy to obtain a sample, implies less pain and less punctures in general. An indication of taking an AB sample is to assess tissue perfusion in severely ill patients.

References

Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta analysis. Respirology. 2014 Feb;19(2):168-175. doi: 10.1111/resp.12225. Epub 2014 Jan 3. Review. PubMed PMID: 24383789.

Cengiz M, Ulker P, Meiselman HJ, Baskurt OK. Influence of tourniquet application on venous blood sampling for serum chemistry, hematological parameters, leukocyte activation and erythrocyte mechanical properties. Clin Chem Lab Med. 2009;47(6):769-76. doi: 10.1515/CCLM.2009.157. PubMed PMID: 19426141.

Gattinoni L, Pesenti A, Matthay M. Understanding blood gas analysis. Intensive Care Med. 2018 Jan;44(1):91-93. doi: 10.1007/s00134-017-4824-y. Epub 2017 May 11. PubMed PMID: 28497267.

Ladakis C, Myrianthefs P, Karabinis A, Karatzas G, Dosios T, Fildissis G, Gogas J, Baltopoulos G. Central venous and mixed venous oxygen saturation in critically ill patients. Respiration. 2001;68(3):279-85. PubMed PMID: 11416249.

Malatesha G, Singh NK, Bharija A, Rehani B, Goel A. Comparison of arterial and venous pH, bicarbonate, PCO2 and PO2 in initial emergency department assessment.  Emerg Med J. 2007 Aug;24(8):569-71. PubMed PMID: 17652681; PubMed Central PMCID:  PMC2660085.

Malinoski DJ, Todd SR, Slone S, Mullins RJ, Schreiber MA. Correlation of central venous and arterial blood gas measurements in mechanically ventilated trauma patients. Arch Surg. 2005 Nov;140(11):1122-5. PubMed PMID: 16342377.

Mallat J, Lazkani A, Lemyze M, Pepy F, Meddour M, Gasan G, Temime J, Vangrunderbeeck N, Tronchon L, Thevenin D. Repeatability of blood gas parameters, PCO2 gap, and PCO2 gap to arterial-to-venous oxygen content difference in critically ill adult patients. Medicine (Baltimore). 2015 Jan;94(3):e415. doi: 10.1097/MD.0000000000000415. PubMed PMID: 25621691; PubMed Central PMCID: PMC4602629.

Ospina-Tascón GA, Hernández G, Cecconi M. Understanding the venous-arterial CO(2) to arterial-venous O(2) content difference ratio. Intensive Care Med. 2016  Nov;42(11):1801-1804. Epub 2016 Feb 12. Review. PubMed PMID: 26873834.

Tavakol K, Ghahramanpoori B, Fararouei M. Prediction of Arterial Blood pH and Partial Pressure of Carbon dioxide from Venous Blood Samples in Patients Receiving Mechanical Ventilation. J Med Signals Sens. 2013 Jul;3(3):180-4. PubMed PMID: 24672766; PubMed Central PMCID: PMC3959008.

Walkey AJ, Farber HW, O’Donnell C, Cabral H, Eagan JS, Philippides GJ. The accuracy of the central venous blood gas for acid-base monitoring. J Intensive Care Med. 2010 Mar-Apr;25(2):104-10. doi: 10.1177/0885066609356164. Epub 2009 Dec 16. PubMed PMID: 20018607.

Further Reading

Cite this article as: Job Guillen, "Venous blood gas analysis: Less arterial punctures!," in International Emergency Medicine Education Project, July 5, 2019, https://iem-student.org/2019/07/05/venous-blood-gas-analysis-less-arterial-punctures/, date accessed: September 16, 2019

Interview – Vicky Noble – US training in medical schools

We interviewed with world renowned emergency and critical care US expert “Vicky Noble” about US training in medical schools.

Read US Chapters and Posts

Siedel Test

A 42 years old male, presents to the ED 1 hour after he was hammering a nail onto a wooden shelf, where the nail flew and strike his left open eye. In an attempt to help, his friend immediately removed the nail. After that, he has been having severe sharp pain and blurry vision in his left eye. On examination, the left eye had poor visual acuity, and he could only perceive light and movement. The pupil was fixed, dilated and non-reactive to light. Right eye examination was normal.

819.2 - eye penetran trauma 2 -siedel sign
819.1 - eye penetran trauma 1

How would you approach to this patient?

To learn more about it, read chapters below.

Read "Eye Trauma" Chapter

Read "Red Eye" Chapter

Quick Read

Globe rupture

It is an ophthalmologic emergency, consisting of a full-thickness injury in the cornea or sclera caused by penetrating or blunt trauma. Anterior rupture is usually observed, as this is the region where the sclera is the thinnest. Posterior rupture is rare and difficult to diagnose. It can be diagnosed through indirect findings such as contraction in the anterior chamber and decrease in intraocular pressure (IOP) in the affected eye. If there is a risk of globe rupture, a slit lamp test with 10% fluorescein must be conducted. Normal tissue is dark orange under a blue cobalt filter; a lighter color is observed in the damaged zone due to a lower dye concentration. Ultrasonography (USG) can be useful in making a diagnosis, especially with posterior ruptures. Computed tomography (CT) sensitivity ranges 56–75%. In cases of anterior globe injuries, USG use, and if there is a risk of a foreign metal body, magnetic resonance imaging, are contraindicated. Prompt ophthalmology consultation is required. While in the emergency department, tetanus prophylaxis, analgesics, bed rest, head elevation, and systemic antibiotic therapy are required. The most commonly preferred antibiotics are cefazolin and vancomycin. Age over 60 years; injury sustained by assault, on the street/highway, during a fall, or by gunshot; and posterior injuries are indications of a poor prognosis.

Siedel test

Seidel test is used to detect ocular leaks from the globe following injury. If there is penetration to the eye, aqueous leakage happens. However, the fluid is clear and hard to identify. Therefore, non-invasive test “Siedel” is used for better visualization of this leakage. Fluorescein 10% is applied to the injured eye, and the leakage becomes more prominent.

To learn more about it, read chapters below.

Read "Eye Trauma" Chapter

Read "Red Eye" Chapter

Action Plan

Creating Your Action Plan chapter written by Chew Keng Sheng from Malaysia is just uploaded to the Website!

Diagnostic Testing

Diagnostic Testing In Emergency Medicine chapter written by Yusuf Ali Altunci from Turkey is just uploaded to the Website!

Selected Diagnostic Tests

Selected Diagnostic Laboratory Tests are just uploaded to the website.

Arterial and Venous Blood Gas Analysis

by Kemal Gunaydin Introduction Measurements of PaO2, PaCO2, SaO2, pH, and bicarbonate values are made with arterial blood gas (ABG) analysis in order to determine

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Cerebrospinal fluid analysis

by Arwa Alburaiki and Rouda Salem Alnuaimi Introduction CSF is a colorless fluid that is present within the subarachnoid space, central canal of the spinal cord

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Urine Analysis

by Jan Zajc Indications Urinalysis should be performed to evaluate the following Evaluation of renal & lower urinary tract abnormalities Assessment of some metabolic/endocrine disorders

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Whole blood cell count – CBC

by Kaja Cankar and Gregor Prosen   Introduction The whole blood cell count is one of the most commonly ordered tests in medicine. It is

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Do you need more?

From experts to our students! – eFAST

Unilateral or bilateral?

644 - C-spine dislocation

In case you didn’t encounter another trauma today!

You are working in a rural hospital. A 55-year-old female was brought to the ED by EMS. She was found at home, lying on the ground, in front of the stairs. She is vitally stable but unconscious (GCS: E1, V:2, M:3). You applied trauma surveys. After inline stabilization, you intubated the patient. The facility does not have a CT scan, and you order standard X-ray series for trauma including c-spine.

What are abnormal findings in this x-ray?

Facet dislocation? Unilateral or Bilateral?

iEM Education Project Team uploads many clinical picture and videos to the Flickr and YouTube. These images are free to use in education. You can also support this global EM education initiative by providing your resources. Sharing is caring!

Neck pain after a bar fight

674.1 - C1 fx - 6-7 sublux

In case you didn’t encounter a young trauma patient today!

You are working in a rural hospital. A 34-year-old male trauma patient was brought to the ED by his friends! You applied primary and secondary survey. The patient describes only neck pain. He has a central tenderness at the c-spine area. The facility does not have a CT scan, and you order c-spine X-ray. 

iEM Education Project Team uploads many clinical picture and videos to the Flickr and YouTube. These images are free to use in education. You can also support this global EM education initiative by providing your resources. Sharing is caring!

Clinical Images and Videos on Flickr reached 19.6K views

Dear medical students/interns/PGY1s and educators, we upload regularly clinical, imaging pictures and videos to our Flickr channel. Currently, 652 images and videos are in the channel, and all are free to download and use for education purposes. You can search what you need. We have 447 searchable medical tags so far. You can also view specific albums which consist of organ system or anatomic region images/videos. Please share this FOAMed resource with your peers and colleagues.

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iEM Education Project Team uploads many clinical picture and videos to the Flickr and YouTube. These images are free to use in education. You can also support this global EM education initiative by providing your resources. Sharing is caring!

Chest X-ray Interpretation, No Worries!

336.4 - normal PA chest x-ray - BONY STRUCTURES

How to read a chest x-ray chapter written by Ozlem Koksal from Turkey is just uploaded to the Website! For pathologic images, please visit our Flickr channel – Chest Images and Videos Album.