Oxygenation and Oximetry

Oxygenation and Oximetry

Authors: Job Rodríguez Guillén, Chief of Emergency Department. Hospital H+ Querétaro, México. Regina Pineda Leyte Internal Medic, Anahuac Querétaro University, Mexico. 


One of the main goals of mechanical ventilation is oxygenation. Both hypoxemia and hyperoxemia must be avoided and the objectives must be individualized according to the clinical situation and comorbidities of each patient. Oxygenation monitoring is possible at the bedside by physical examination (late clinical signs), pulse oximetry (non-invasive continuous monitoring), and arterial blood gas analysis (gold standard for arterial oxygenation analysis).

Determinants of oxygenation

The main determinant of oxygenation is the mean airway pressure (Paw) and the inspired fraction of oxygen (FiO2). Paw is the average pressure to which the lung is exposed during inspiration and expiration mechanical ventilation (Figure 1). Paw improves oxygenation by allowing the redistribution of oxygen from highly compliant alveoli to less compliant alveoli.(1,2)
Oxygenation and Oximetry - figure 1
Figure 1: Mean airway pressure (Paw) is the integral (area under the curve) of pressure and time. PIP: peak inspiratory pressure; PEEP: positive pressure at the end of expiration; Ti: inspiratory time; Te: expiratory time.
According to the determinants of Paw and the relationship between them, there are five different ways to increase it (Figure 2)
Oxygenation and Oximetry - figure 2
Figure 2: Maneuvers to increase the mean airway pressure (Paw). PEEP: positive pressure at the end of expiration. Only maneuvers 3 and 4 are used in clinical practice to increase Paw and improve oxygenation.

The second determinant of oxygenation is Inspired Oxygen Fraction (FiO2). The use of supplemental oxygen at the hospital level is a common practice and a critical element of intensive care in patients with mechanical ventilation for the management of hypoxemia. However, in recent years it has been shown that higher oxygenation is not the goal. (3) In the same way that hypoxemia should be avoided, hyperoxia should be prevented. (4)

Although the FiO2 can be adjusted in ranges of 21% and up to 100% the lowest value required must be set (preferably <60%) to reach the desired oxygen saturation (SO2) target.

Oxygenation monitoring

Pulse oximetry allows non-invasive monitoring of oxygenation (SpO2), it is simple and reliable in many areas of clinical practice. SpO2 has a confidence rate of 95% ± 4%, so readings ranging between 70% and 100% are considered reliable.(5) In patients with mechanical ventilation, the objective is to identify hypoxemia.

It is important to remember that oximeters do not measure arterial oxygen pressure (PaO2), for this reason, they cannot directly diagnose hypoxemia or hyperoxemia (PaO2 <60 mmHg and PaO2> 120 mmHg respectively).(6)  What they do is “estimate” hypoxemia when SpO2 falls <90%, which would correspond to a PaO2 <60 mmHg according to the oxyhemoglobin dissociation curve (Table 1). (7)  However, it must be taken into account that changes in temperature and pH cause changes in this relationship. As the pH increases (alkalosis) or the temperature decreases (hypothermia), the shift of the curve is to the left since hemoglobin binds more strongly with oxygen, delaying its release to the tissues. Acidosis and fever shift the curve to the right as the hemoglobin molecule decreases its affinity for oxygen, facilitating the release of oxygen to the tissues.

Oxygenation and Oximetry - Table 1
Table 1: Estimation of the oxygenation state according to SpO2. SpO2: oxygen saturation by pulse oximetry; PaO2: arterial oxygen pressure.

SpO2 values <70% are not reliable. If necessary, the oxygenation assessment should be supplemented by arterial gas analysis. The arterial oxygen saturation (SaO2) is the oxygen saturation obtained by this test.

Oxygenation Goals

According to the oxyhemoglobin dissociation curve, the goal of oxygen titration is to achieve a PaO2 in the range of 60-65 mmHg or an SpO2 of approximately 90-92%. However, the objectives must be individualized and the current recommendations for oxygen therapy in critically ill patients. (8) are as follows (Table 2).

Table 2: Recommendations for oxygenation by SpO2. O2: oxygen; SpO2: oxygen saturation by pulse oximetry; AMI: Acute myocardial infarction; EVC: Cerebral vascular event; VM: mechanical ventilation; SIRA: Acute respiratory distress syndrome. Some exceptions apply like carbon monoxide poisoning.

It has been suggested that critically ill patients can tolerate lower levels of PaO2 (“permissive hypoxemia”) (9-10), however, studies are limited to make a recommendation to routine clinical practice.


Oxygenation goals should be established once the requirement for mechanical ventilation is indicated according to the clinical condition of each patient and monitoring that these objectives are met. Pulse oximetry allows continuous, non-invasive monitoring at the bedside. It should be remembered that hyperoxemia, as well as hypoxemia, should be avoided.


  1. Marini JJ,Ravenscraft SA. Mean airway pressure: physiologic determinants and clinical importance–Part 1: Physiologic determinants and measurements. Crit Care Med. 1992 Oct;20(10):1461-72.
  2. Marini JJ,Ravenscraft SA. Mean airway pressure: physiologic determinants and clinical importance–Part 2: Clinical implications. Crit Care Med. 1992 Nov;20(11):1604-16.
  3. Girardis M, Busani S, Damiani E, et al. Effect of conservative vs conventional oxygen therapy on mortality among patients in an intensive care unit: the Oxygen-ICU Randomized Clinical Trial. JAMA. 2016;316(15):1583-1589.
  4. Bitterman, Haim. “Bench-to-bedside review: oxygen as a drug.” Critical Care1 (2009): 205.
  5. Chan MM,Chan MM, Chan ED. What is the effect of fingernail polish on pulse oximetry?. Chest. 2003 Jun;123(6):2163-4.
  6. Wandrup JH. Quantifying pulmonary oxygen transfer deficits in critically ill patients. Acta Anaesthesiol Scand Suppl 1995;107:37–44
  7. Allen J. Photoplethysmography and its application in clinical physiological measurement. Physiol Meas 2007;28:R1–39
  8. Siemieniuk Reed A C, Chu Derek K, Kim Lisa Ha-Yeon, Güell-Rous Maria-Rosa, Alhazzani Waleed, Soccal Paola M et al. Oxygen therapy for acutely ill medical patients: a clinical practice guideline BMJ 2018;  363 :k4169
  9. Gilbert-Kawai ET, Mitchell K, Martin D, Carlisle J, Grocott MP. Permissive hypoxaemia versus normoxaemia for mechanically ventilated critically ill patients. Cochrane Database Syst Rev 2014;5:CD009931.
  10. Capellier G, Panwar R. Is it time for permissive hypoxaemia in the intensive care unit? Crit Care Resusc 2011;13:139–141.
Cite this article as: Job Guillen, Mexico, "Oxygenation and Oximetry," in International Emergency Medicine Education Project, October 5, 2020, https://iem-student.org/2020/10/05/oxygenation-and-oximetry/, date accessed: October 23, 2020

Clinical examination of the hemodynamically unstable patient

Clinical examination of the hemodynamically unstable patient

Authors: Job Rodríguez Guillén. Chief of Emergency Department. Hospital H+ Querétaro. México and Paola Rivero Castañeda. Medical Intern, Anahuac Querétaro University, Mexico. 


Clinical examination accounts as a fundamental part in the management of most critical scenarios. Although there are few publications and it remains controversial, its value considered as limited by 50% of medical practicioners (1). None of the well-known semiology books include any section about the physical examination in the critically ill patient (2). Nonetheless, an adequate clinical evaluation at the patient’s bedside may save lives in the context of a serious situation.

Clinical Examination Objectives

The main objectives are identifying and discerning from types of shock, emphasizing in the identification of life-threatening conditions, clinical signs of organic hypoperfusion, as well as to evaluate treatment response regarding therapies employed, and risk stratifying.

Identify hemodynamic instability

  • Life-threatening conditions (Tension pneumothorax, Cardiac tamponade, Pulmonary thromboembolism, Active hemorrhage, etc.)
  • Organ hypoperfusion
    (Altered mental state, decreased uresis, mottled skin, prolonged CFT, etc.)

Evaluate treatment response

  • Vital signs and normalization of the clinical state
    (Mental state improvement, diminished skin mottling, improved uresis, normalization of prolonged capillary filling time, etc.)

Risk stratifying

  • Scale and prognostic scores calculation. Prognostic scores use a combination of clinical and/or laboratoy variables (SOFA: Squential Organ Failure Assessment; APACHE: Acute Physiology and Chronic Health Evaluation; SAPS: Simplified Acute Physiology Score; MPM: Mortality Probability Models, etc.)

Clinical Exam Systematization

The clinician must be able to do a quick and efficient clinical examination to recognize different states of shock as early as possible, or even situations that may compromise organic perfusion. At a given time, it’s suggested to check out the clinical history, re-interrogate the patient and his/her family members, as well as patient’s family/regular physician (or even look for their previous medical notes), in order to help clinical integration, and so for decision making.

Systematization of the evaluating process, based on the previously proposed objectives, can be identified with the following mnemonic: PROA.

PROA - Summary

P - Probabilistic thinking

  • Think about any probability.
  • Look for intentionally.
  • Analyze clinical context and individualize.

R - Risk of dying

Identify life-threatening causes: Cardiac tamponade, Tensionpneumothorax, Pulmonary thromboembolism, Active hemorrhage, etc.

O - Organic hypoperfusion

Cutaneous perfusion signs: examine mottled skin and capillary filling time.

A - Approach of the clinical examination

Clinical exam by regions. Some components may not be relevant for all patients, even requiring other physical maneuvers. Even though laboratory and imaging are not part of the clinical exam, their interpretation must be integrated with the examination findings.

Probabilistic Thinking

Medicine is a science of uncertainty and an art of probability.

— William Osler

Clinical decision making in the emergency department begins with the estimation of the probability of a determined patient to have or do not have specific conditions (Bayesian reasoning or pretest probability).

Example; the probability of septic shock in a young patient after having a car crash is very low compared to the high probability of presenting with hemorrhagic or obstructive shock.

Proposed decisions related to initial probabilistic thinking vary in clinical relevance depending on the patient’s condition. It should always be re-evaluated through available additional data (posttest probability) (Figure 1).

Relationship between probability thresholds and decision‐making zones
Figure 1: Relationship between probability thresholds and decision‐making zones (3).

Risk of Dying

Shock is a momentary pause in the act of death.

— John Collins Warren

Currently, there are four types of shock, all with a common pathophysiological pathway: acute circulatory insufficiency associated with cell oxygen utilization dysfunction (altered-balance between oxygen input and consumption: DO2/VO2 dysfunction), a central situation that takes part in the development of multiorgan dysfunction (4-5).

Initial physical examination should be directed to the identification of immediate life-threating pathologies such as obstructive shock (Tension pneumothorax, cardiac tamponade, pulmonary thromboembolism), hemorrhagic shock etc.

These pathologies require immediate action. Otherwise, early multi-organ dysfunction and death may occur. The Point of Care Ultrasound (PoCUS), is a fundamental tool used for the evaluation of patients with hemodynamic instability of unknown origin.

Organ Hypoperfusion

When assessing the damage an earthquake or fire has caused inside a building, one looks through the windows. Using this analogy, it would be useful to be able to see inside the body to view the damage caused by the shock process.

— Jean-Louis Vincent

The initial approach to clinical examination begins with the skin. It is essential to remember that microcirculation cannot be globally defined through its dependency with macrocirculation, autoregulation mechanisms and organ interactions. Moreover, the availability of devices to evaluate it remains limited. Therefore, the evaluation is done from clinical, biochemical and hemodynamic data integration (6) (Figure 2)

Figure 2: three windows of shock

The correct way of measuring capillary filling time

Approach of The Clinical Examination

Clinical exam is not an art, is an essential ability.

— Leonel Martínez-Ramírez

During the initial evaluation, multiple situations can affect the accomplishment of a detailed physical examination. Therefore, it is recommended to follow a structured exploration method, looking at every main organ system and region. Documenting its results would allow avoiding the inclusion of essential data, and would permit to identify tendencies or any change in the patient’s clinical status.

Clinical examination approach in the critically-ill patient.

7Clinical examination approach emphasized in the critically-ill patient. This examination is realized based on every region in the body. Some components may not be relevant for all patients, or even some other maneuvers shall be executed in the physical examination. The verification list should be modified to be adapted to each patient’s circumstances. Laboratory and other studies analysis does not conform part of the clinical examination, although, their interpretation should be added to exploration findings (7).

  • General appearance

    Introduce yourself to the patient. Evaluate general appearance, physical state, complexity or the presence of particular face patterns, etc.

  • Head

    Inspect pupils' symmetry and reactiveness to light. Look for facial asymmetry and signs of bleeding in nostrils and oropharynx. Inspect lips, mouth and tongue, searching for lesions or signs of ulceration.

  • Neck

    Evaluate neck symmetry, venous distension and tracheal positioning. Palpate searching for adenopathies, subcutaneous emphysema, etc.

  • Thorax

    Expose the thorax, inspect the use of accessory respiratory muscles, diaphragmatic movement, and type of respiration. Also, look for ecchymosis or hematomas. Palpate searching for subcutaneous emphysema or bone crepitations. Auscultate respiratory sounds bilaterally, as well as heart sounds, noting the physiological splitting of the second heart sound, murmurs, friction and gallop rhythm or third heart sound.

  • Upper extremities

    Evaluate upper extremities symmetry. Inspect all arterial and venous line catheters. Evaluate for presence of mottled skin, peripheral pulses and perfusion through capillary filling time.

  • Abdomen

    Take into consideration the diaphragmatic movement during ventilation. Evaluate distension and tympanic sounds during the percussion of the abdomen. Palpate for any rigidity or involuntary guarding. Evaluate abnormal growth of spleen and liver, palpable masses, murmurs or other intestinal sounds.

  • Lower extremities

    Evaluate all sites of vascular accesses and palpate pulses. Evaluate mottled skin, peripheral perfusion and edema.

  • Central Nervous System and Mental State

    Evaluate if the patient is able to follow orders and if his/her four extremities can move equally. Evaluate plantar response as well as withdrawal to pain stimuli. Check pupils and facial symmetry if they were not previously evaluated.

  • Devices and Incisions

    Every possible surgical site should be evaluated, as well as the entrance of every device, including endotracheal tubes, vascular accesses, thoracic tubes, enteral probes and urinary catheters. It should be taken into consideration the characteristics and quantity of urine in the Foley bag.

  • Monitors and waveforms

    The mode, pressures, ventilation per minute and waveforms, hemodynamic monitor (venous pressure, arterial pressure), telemetry and vital signs, as well as any other type of bedside monitor, should be inspected in order to detect any qualitative or quantitative alteration/abnormality.

  • Posterior region

    Exam executed when the patient is in a prone position. Inspect looking for lesions or penetrating wounds. Pressure ulcer appearance should be evaluated.

  • Environment

    Family’s or visitors' moods should be taken into consideration. Light quality, ambient temperature, etc. should be evaluated.


Clinical integration of initial clinical history and the physical examination should be added to the biochemical complementation as well as advanced hemodynamic monitoring parameters, when these are available. Even so, if clinical examination answers raised questions during the initial evaluating process, the clinician must act according to physiological principles. There is no ideal hemodynamic monitoring, meaning that all parameters have to be individualized for each patient and his/her clinical context. Therefore, clinical examination systematization results are an excellent aid for the clinician regarding his/her clinical practice.  

References and Further Reading

  1. Vazquez R, Vazquez Guillamet C, Adeel Rishi M, Florindez J, Dhawan PS, Allen SE, Manthous CA, Lighthall G.  Physical examination in the intensive care unit: opinions of physicians at three teaching hospitals. Southwest J Pulm Crit Care. 2015;10(1):34-43. DOI: http://dx.doi.org/10.13175/swjpcc165-14
  2. Cook CJ, Smith GB. Do textbooks of clinical examination contain information regarding the assessment of critically ill patients?Resuscitation. 2004;60:129–136.
  3. Zehtabchi S, Kline J.A. The Art and Science of Probabilistic Decision‐making in Emergency Medicine. Academic Emergency Medicine, 17:521-523. DOI: http://doi.org/10.1111/j.1553-2712.2010.00739.x
  4. Weil MH, Shubin H. Proposed reclassification of shock states with special reference to distributive defects. Adv Exp Med Biol.1971 Oct;23(0):13-23.
  5. Ince C. The microcirculation is the motor of sepsis. Crit Care. 2005;9 Suppl 4:S13-9. DOI: 1186/cc3753
  6. Vincent JL, Ince C, Bakker J. Clinical review: Circulatory shock–an update: a tribute to Professor Max Harry Weil.Crit Care. 2012 Nov 20;16(6):239. DOI: 10.1186/cc11510.
  7. Metkus TS, Kim BS. Bedside Diagnosis in the Intensive Care Unit. Is Looking Overlooked?. Ann Am Thorac Soc.2015 Oct;12(10):1447-50. DOI: 10.1513/AnnalsATS.201505-271OI.
Cite this article as: Job Guillen, Mexico, "Clinical examination of the hemodynamically unstable patient," in International Emergency Medicine Education Project, December 6, 2019, https://iem-student.org/2019/12/06/clinical-examination-of-the-hemodynamically-unstable-patient/, date accessed: October 23, 2020

Goals in Mechanical Ventilation: Concepts for the Students

Goals in Mechanical Ventilation: Concepts for the Students
Authors: Dr. Job Heriberto Rodríguez Guillén (@job_rdz), Dr. Sergio Edgar Zamora Gómez (@ezg_galeno)


Mechanical ventilation (MV) is one of the cornerstones of life support in the emergency department. It provides time for establishing therapeutic management aimed at the triggering cause of injury until the patient improves physiologic balance (1). Therefore, MV can not be a unique and specific treatment for any disease by itself; but it has two general and fundamental goals: to support the injured lung and protect the healthy lung.

Set your goals: Support and Protect


MV supports the respiratory system; meanwhile, the primary disease becomes under control.

Example: A patient with acute respiratory distress syndrome (ARDS) due to pneumonia, where MV provides support to improve gas exchange and reduce work of breathing (WOB) meanwhile antibiotic treatment induces remission of the infectious disease.


MV is aimed to avoid complications not related to the primary disease. The patient-ventilator relationship becomes of benefit for the patient as his respiratory function is in the risk of injury because the primary disease does not allow him to breathe properly or because therapeutic interventions can reduce protective airway reflexes and lead to respiratory complications.

Example: Patients presenting neuromuscular diseases (Guillain-Barre syndrome), diseases affecting bulbar muscles (myasthenic crisis), decreased consciousness (stroke, poisoning) or severe traumatic brain injury, all these without lung injury at first but in high risk of pneumonitis and pneumonia due to aspiration of gastric content.

Goals of Mechanical Ventilation
Mechanical ventilation has two general and fundamental goals: to support the injured lung and protect the healthy lung.

Specific goals of mechanical ventilation

One of the specific objectives of MV is to promote the optimization of arterial blood gases levels and acid-base balance by providing oxygen and eliminating carbon dioxide (ventilation). MV can reduce the work of breathing by taking effort from respiratory muscles and maintaining the long-term respiratory support for patients with chronic diseases.

MV´s circle (2) begins by recognizing the patient´s need for mechanical ventilatory support. Intubation and ventilation decision making is an essential skill for emergency physicians. Consideration of the patient´s needs is the basis of this decision making. The main indications for intubation and mechanical ventilation are (3):

  1. Refractory hypoxemia
  2. Increased respiratory effort
  3. Apnea/hypopnea leading to inadequate ventilation (Hypercapnia)
  4. The inability for airway protection

The goals should be individualized and established according to the clinical situation that led the patient to required ventilatory support. Although standard criteria traditionally have been specified for the onset of MV (3), we must remember that indication for intubation and ventilation is an essential skill for every physician treating critical care patients and the key is just thinking about what the patient needs.

Standard criteria for starting mechanical ventilation
Acute Ventilatory Failure
pCO2 > 50 mmHg + pH < 7.30
Impending Ventilatory Failure
Maintains normal gasometric levels by increasing respiratory effort.
Severe Hypoxemia
pO2 < 60 mmHg + FiO2 > 50%

pCO2 and pO2 values at sea level

In general, we can encompass the specific objectives of MV in three fundamental principles that must be fulfilled in every patient by setting the goals according to the primary disease:

  1. Improve oxygenation (O2) and ventilation (CO2)
  2. Reduce respiratory effort
  3. Minimize ventilator-induced lung injury (VILI)


The goals of MV are established based on the primary disease that led the patient to need MV support, under the concept of protecting and supporting the lungs. Primum non nocere; lung-protective ventilation should be initiated in all patients who need it.

References and Further Reading

  1. Frank Lodeserto MD, “Simplifying Mechanical Ventilation – Part I: Types of Breaths”, REBEL EM blog, March 8, 2018. Available at: https://rebelem.com/simplifying-mechanical-ventilation-part/.
  2. Frank Lodeserto MD, “Simplifying Mechanical Ventilation – Part 2: Goals of Mechanical Ventilation & Factors Controlling Oxygenation and Ventilation”, REBEL EM blog, May 18, 2018. Available at: https://rebelem.com/simplifying-mechanical-ventilation-part-2-goals-of-mechanical-ventilation-factors-controlling-oxygenation-and-ventilation/.
  3. Scott Weingart. EMCrit Lecture – Dominating the Vent: Part I. EMCrit Blog. Published on May 24, 2010. Accessed on August 30th 2019. Available at [https://emcrit.org/emcrit/vent-part-1/ ].
Cite this article as: Job Guillen, Mexico, "Goals in Mechanical Ventilation: Concepts for the Students," in International Emergency Medicine Education Project, September 2, 2019, https://iem-student.org/2019/09/02/goals-in-mechanical-ventilation-concepts-for-the-students/, date accessed: October 23, 2020

Venous blood gas analysis: Less arterial punctures!


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


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


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


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.


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.


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, Mexico, "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: October 23, 2020