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. 

Introduction

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.

Conclusions

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.

References

  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 31, 2020

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