Acute Hypoxia: an Overview

Open Access Austin Journal of Emergency and Critical Care Medicine Review Article Acute Hypoxia: An Overview Weledji EP1*, Ngomba MN2 and Njie KT3 1Department of Surgery, Faculty of Health Sciences, Abstract University of Buea, Cameroon Hypoxia kills and kills quickly. In the healthy individual total oxygen delivery 2Intensive Care Unit, Regional Hospital limbe, S.W. far exceeds total oxygen consumption. An acute hypoxic patient would therefore Region, Cameroon be critically ill and need oxygen. The assessment of the suitability of a patient to 3Infectious Disease & Community Health, Director, withstand surgery and anaesthesia is closely related to their ability to increase Regional Hospital Limbe, S.W. Region, Cameroon oxygen delivery to vital tissues. Optimizing these factors would maximize the *Corresponding author: Weledji EP, Pemset House, patient’s reserves. Oxygen is only one aspect of treatment and the underlying Lumpsum Qrts, Limbe, S.W. Region, Cameroon cause of the respiratory failure must be treated. This article reviewed acute hypoxia, respiratory failure and the indications for oxygen therapy. Received: February 15, 2020; Accepted: April 13, 2020; Published: April 20, 2020 Keywords: Hypoxia; Acute; Respiratory failure; Monitoring; Oxygen therapy Introduction Type I or acute hypoxaemic respiratory failure occurs in diseases which damage lung tissue with hypoxaemia due to R to L shunting Respiratory failure occurs when pulmonary gas exchange or ventilation/perfusion mismatch. The PaO2 is low but PaCO2 is sufficiently impaired to cause hypoxaemia with or without is normal or low i.e. there is some compensation by spontaneous hypercarbia. In practical terms, respiratory failure is present when hyperventilation. Hyperventilation results from a decreased arterial the partial pressure of arterial oxygen (PaO ) is <8KPa (60mmHg) i.e. 2 PO but the response is non linear. There is little effect until arterial an arterial saturation of oxygen (Sa0 ) of <90% or the partial pressure 2 2 PO is reduced to about 7kPa (52mmHg) and maximal response is of arterial carbon dioxide (PaCO ) is >7kPa (55mmHg). The normal 2 2 at 4kPa (30mmHg). The common causes are pneumonia, pulmonary PaO is 75-100mmHg (10-13.3KPa). It is the oxygen content of the 2 oedema, acute respiratory distress syndrome (ARDS) and the arterial blood that matters and this is determined by the percentage chronic situation such as pulmonary fibrosing alveolitis. Type II or saturation of haemoglobin (Hb) with oxygen. The measure of the ventilatory failure occurs when alveolar ventilation is insufficient oxygen saturation of Hb-oxyhaemoglobin gives an estimate +/-2% of to excrete the volume of carbon dioxide (CO ) being produced by the percentage of oxygen carrying sites which are occupied in Hb. The 2 tissue metabolism. Thus, CO2 retention is the hallmark. Inadequate maximum number of sites which could be occupied is obviously 100%. ventilation is due to reduced ventilatory effort, inability to overcome Thus the normal oxygen saturation for arterial blood (SaO2) is 95- an increased resistance to ventilation and failure to compensate for an 100% (Figure 1). In health SaO2 is normally near maximal i.e. around increase in dead space and/or CO2 production, or a combination of 97%. Oxygen supplementation should ideally be given when oxygen these. The most common cause is chronic bronchitis. Others include saturation is <95%. Pulmonary distribution of blood flow is improved chest wall deformities, respiratory muscle weakness e.g. Guillaine- by hypoxia as a result of hypoxic pulmonary vasoconstriction to favour Barre syndrome, and depression of the respiratory centre by sedative ventilation-perfusion matching. The persistence of chronic alveolar morphine-type drugs [5,6]. hypoxia and hypercapnoea leads to constriction of the pulmonary arterioles and subsequent pulmonary arterial hypertension [1,2]. It Monitoring is also important to appreciate that the lungs normally never empty Clinical assessment completely. At the end of each breath an ‘average’ man with a total The clinical signs of respiratory distress would include the lung capacity (TLC) of 6 litres will still have around 2.5 litres of use of accessory muscles of respiration, tachypnoea, tachycardia, gas in the lungs (functional residual capacity, FRC) and even if he sweating, pulsus paradoxus, inability to speak from exhaustion and expires as much as he can the expiratory reserve volume, (ERV) he signs of carbon dioxide retention. With regard to tachypnoea, the will only reduce this to 1.2 litres (residual volume, RV) (Figure 2) [3]. minute ventilation rises initially in acute respiratory failure and falls In addition, the signs of hypoxia are generally non-specific and often precipitously only at a late stage when the patient is exhausted. In difficult to assess. The primary aim of the management of respiratory Guillain- Barre syndrome the vital capacity decreases as the respiratory failure is to improve the PaO by continuous controlled oxygen 2 muscle weakness increases. Tachycardia is the most sensitive clinical therapy. This nearly always leads to a rise in the PaCO . A small 2 indication of increasing respiratory difficulty. The signs of CO increase in PaCO can be tolerated but not if the pH falls dramatically 2 2 retention include a bounding pulse, peripheral vasodilatation and or below 7.25. Under such circumstances, increased ventilation must later a coarse flapping tremor of the outstretched hands. More severe be achieved either by the use of a respiratory stimulant or artificial hypercapnoea leads to confusion, progressive drowsiness and coma ventilation [4]. However, oxygen is only one aspect of treatment and with papilloedema [7]. the underlying cause of the respiratory failure must be treated. Blood gas analysis Types and Causes of Respiratory Failure Blood gas analysis from an arterial puncture provides the modern There are two types of respiratory failure (type 1 and type II). automated objective assessment of respiratory function and a guide Austin J Emergency & Crit Care Med - Volume 6 Issue 1 - 2020 Citation: Weledji EP, Ngomba MN and Njie KT. Acute Hypoxia: An Overview. Austin J Emergency & Crit Care ISSN : 2380-0879 | www.austinpublishinggroup.com Med. 2020; 6(1): 1069. Weledji et al. © All rights are reserved Weledji EP Austin Publishing Group Table 1: Normal values for measurements obtained when blood gas analysis is performed. H+ 35-45 nmol.l-1 (pH 7.35-7.45) PO2 10-13.3 kPa (75-100 mmHg) PCO2 4.8-6.1 kPa (36-46 mmHg) -1 Plasma HCO3 22-26 mmol.l O2 saturation 95-100% as in a cold patient, abnormal haemoglobins e.g. carboxyhaemoglobin in smokers [11,12], jaundice and excessive ambient light. Factors Influencing Oxygen Delivery It is important to note that the arterial partial pressure of oxygen Figure 1: Oxygen haemoglobin dissociation curve. Keep the saturation is also influenced by oxygen delivery. According to Fick’s principle, >94%! the total oxygen delivery (oxygen flux) to the tissues depends upon the cardiac output (CO) and the amount of oxygen contained in that to oxygen therapy. First, it demonstrates the disturbances of acid- blood, i.e. Oxygen delivery (DO2) = CO x (constant x [Hb] x SaO2] + base balance such as respiratory acidosis caused by retention of dissolved oxygen. With the normal CO of 5l/min, constant 1.34ml/g, carbon dioxide (type II respiratory failure), respiratory alkalosis seen [Hb} 150g/l, SaO 0.97 the oxygen delivery is 1000ml/min at rest. with hypoxaemic (type 1) respiratory failure and in those living at 2 The resting oxygen consumption (VO2) is about 250mls/min, and high altitudes, metabolic acidosis and metabolic alkalosis. Second, it thus there is a four- fold reserve of oxygen utilization [13]. Hypoxia demonstrates the alterations in oxygenation (the percentage saturation presents a serious threat to the body as it only occurs when this four- of haemoglobin with oxygen) which is important with regard to the fold reserve has been utilized. Oxygen delivery is increased during oxygen haemoglobin dissociation curve (Figure 1). According to the exercise by increase in the cardiac output which could reach 30l/ sigmoid shape of the oxygen haemoglobin dissociation curve, if the min via increase in heart rate and stroke volume by the sympathetic percentage oxygen saturation is >90% oxygenation can be considered nervous system [14]. The sympathetic system is concerned in many adequate i.e. PaO is within the normal (75-100mmHg). After this a 2 of the responses to hypoxia, particularly the increase in organ small fall in PaO will cause significant reduction in oxygen content. 2 perfusion [15]. The immediate response is reflex and is initiated by A SaO of 60% is associated with a PaO of only about 35mmHg. No 2 2 chemoreceptor stimulation occurring before there is any measurable oxygen is available below SaO of 30% (Figure 1). It is important to 2 increase in circulating catecholamines [16]. Cardiac output is note that PaO complements SaO and is a better indicator of the 2 2 increased by hypoxia, together with the regional blood flow to almost pressure driving oxygen into the tissues. Normal PaO depends on the 2 every major organ, particularly the brain [17]. Reduction of cerebral fraction of inspired oxygen (FiO ) As the FiO increases towards 1, so 2 2 and probably myocardial vascular resistance is not dependent on should the PaO increases-a PaO of 100mmHg (13.3kPa) indicates 2 2 the autonomic system but on local microcirculatory responses good oxygenating ability for an individual breathing air (FiO 0.21) 2 via paracrine-dependent vasorelaxation by endothelium- derived but not for a patient on high flow oxygen. Errors in blood gas analysis production of the gaseous mediator nitric oxide (NO) [18,19]. With can result from malfunction of the analyser or incorrect sampling the exception of pulmonary vessels which cause vasoconstriction and techniques [8]. Table 1 illustrates the normal values when blood gas shunting for V/Q match [2], hypoxia causes vasodilatation of blood analysis is used.

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