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Acid-Base Balance CLINICAL systems of life Homeostasis part 2: acid-base balance Authors Brendan Docherty, msc, PGCe, RN, is fiG 1. CaRBoN Co2 Co2+h2o patient access manager, executive director’s unit, t uDioxiDe DissoCiatioN Prince of Wales Hospital, Sydney, Australia; Colette t Carbonic foudy, GradDip, RN, is clinical care coordinator, anhydrase intensive care unit, St George Private Hospital, t Sydney, Australia. h Co Plasma 2 3 t Carbon dioxide – - transport in the t hCo3 hCo3 This article, the second in this series on homeostasis + t h t blood and and its role in maintaining stable bodily conditions, C1- haemoglobin discusses acid-base balance and explores blood gas t + buffering analysis, related conditions and nursing care. H +HbDhhb Blood acid-base balance is under homeostatic control through the nervous and endocrine systems, which maintain normal levels of components in the blood. This ensures that cellular processes are optimised and not life-threatening (Tortora and Grabowski, 2002). The normal components in the blood are shown in Table 1. The pH of blood reflects the number of hydrogen Blood ions in it – either too few or too many results in brain cellular dysfunction and inadequate oxygen barrier transportation around the main body organs. This in turn leads to tissue hypoxia and results in tissue death if it is not reversed (Tortora and Grabowski, 2002; Adam and Osborne, 1997). Although carbon dioxide (CO2) does not contain hydrogen ions it rapidly reacts with water to form carbonic acid (H2CO3), which further dissociates into hydrogen and – bicarbonate ions (HCO3 ). This reaction is shown as – + CO2 + H20 ⇔ H2CO3 ⇔ HCO3 + H (Proehl, 1999; Adam and Osborne, 1997) (Fig 1). Chemoreceptors in medulla-sensitive to H+ Potential hydrogen concentration (pH) Blood pH is monitored to ensure that when it fiG 2. CoNtRolliNG To respiratory centres deviates from the norm clinical care is adapted to the aCiD-Base BalaNCue resolve the underlying issue (Docherty, 2002b). A pH either less than 7.0 or above 7.8 is incompatible with cellular metabolism and is therefore life-threatening (Proehl, 1999). Low (acidic) pH is often considered to be acute, while more serious high (alkalotic) pH is to 2–3 days to correct (Resuscitation Council (UK), often due to chronic medical conditions such as 2000; Proehl, 1999). In respiratory dysfunction CO2 chronic renal failure and is less easy to resolve in the levels deviate from the norm. They are usually raised short term (Pruitt and Jacobs, 2004; Woodrow, 2004). as a result of inadequate breathing, which leads to excess CO2 combining with water to form carbonic Homeostatic control acid. This in turn lowers the blood pH, resulting in There are two main components of acid-base blood acidosis (Docherty, 2002b). homeostatic control: respiratory and metabolic. Both The respiratory system alters CO2 levels to work to correct imbalance. As a general rule, a counteract changes in blood pH. In low pH (acidosis) respiratory abnormality will correct itself within the medulla oblongata increases the respiration rate Johnny Zygo Johnny hours, whereas a metabolic abnormality may take up and depth to correct the excessive CO2 level in the 24 NT 11 April 2006 Vol 102 No 15 www.nursingtimes.net keywoRDs n Homeostatic control n Acid-base balance n Acidosis n Alkalosis RefeRences taBle 1. NoRmal values of BlooD aCiD-Base BalaNCe (1kPa = 7.5mmhg) Adam, S., Osborne, S. (1997). item NoRmal value Notes Critical Care Nursing: science and practice. Oxford: Oxford University pH – potential hydrogen 7.35 to 7.45 <7.35=acidosis Press. concentration >7.45=alkalosis Docherty, B. (2002a) Cardiorespiratory physical SaO2 – saturation of arterial 10 to 14 kPa or Lower values are hypoxic. Ensure oxygen 75 to 105 mmHg haemoglobin is normalised assessment for the acutely ill: part 1. British Journal of Nursing; 11: 11, 750–758. SaCO2 – saturation of arterial 4 to 6kPa or Higher values are acidotic, carbon dioxide 30 to 45mmHg lower values are alkalotic Docherty, B. (2002b) Cardiorespiratory physical BE – base excess -2 to +2 Negative values are acidotic, assessment for the acutely ill: part positive values are alkalotic 2. British Journal of Nursing; 11: 12, 800–807. Bicarbonate 22 to 26mmol/L Lower values are acidotic, higher values are alkalotic Proehl, J.A. (1999) Emergency Nursing Procedures. Philadelphia, (Pruitt and Jacobs, 2004; Woodrow, 2004; Docherty, 2002b) PA: W.B. Saunders. Pruitt, W.C., Jacobs M. (2004) blood (Tortora and Grabowski, 2002). In some Other factors Interpreting arterial blood gases: patients, especially those with underlying chronic In acid-base balance other factors are involved in the easy as ABC. Nursing; 34: 8, 50–53. airway conditions or those who are neurologically homeostatic control of the vital components. These unstable, this change in respiratory function to correct have their own homeostatic mechanisms, which Resuscitation Council (UK) (2000) Advanced Life Support. London: the acid-base imbalance (Fig 2) can make them tire should be considered when looking at the patient RCUK. easily and respiratory support may be required, such holistically. They include: as continuous positive airway pressure or mechanical 1. Haemoglobin – The amount of haemoglobin in the Smith, G. (2000) Acute Life- ventilation (Docherty, 2002b). However, patients with blood affects its oxygen-carrying capacity. When this threatening Events Recognition chronic airways disease are able to live normally with is identified, for example, through cyanosis, action and Treatment Manual. a high CO2. In fact they require it as a stimulus to should be taken to restore the correct level, because Portsmouth: Open Learning, University of Portsmouth. breathe. In these patients restoration of CO2 should the body’s own homeostatic mechanisms take several be back to their usual level and not the normal days to work. This may be too long in the acute values listed in Table 1. setting (Proehl, 1999). Tortora, G.J., Grabowski, S.R. In high pH (alkalosis) the opposite will happen to Haemoglobin also acts as a buffer to hydrogen (2002) Principles of Anatomy and Physiology. Chichester: John Wiley assist in retaining CO in the blood – resulting in more ions in red blood cells, so in acidosis in low 2 & Sons. hydrogen ions to counteract the alkaline effect. haemoglobin states the cells are less able to buffer In metabolic imbalance anaerobic processes the acidic effect as efficiently (Woodrow, 2004). Woodrow, P. (2004) Arterial blood replace the normal oxidative metabolism, leading to 2. TemperaTure – Temperature will impact on the gas analysis. Nursing Standard; 18: the formation of lactic acid in the blood (Tortora and amount of oxygen dissociation from oxyhaemoglobin 21, 45–55. Grabowski, 2002). This lowers the pH, resulting in molecules in the circulating blood, and so this should blood acidosis. The metabolic (renal) system will also be considered and corrected where possible control bicarbonate levels to alter pH. (Adam and Osborne, 1997). In low pH (acidosis) the base excess (alkali level) 3. blood pressure – The cardiovascular system will be negative indicating metabolic acidosis and the should be sufficiently strong to circulate adequate acids must be neutralised (possibly by diluting with volumes of blood around the body and if there is additional fluid management), excreted by the heart failure (for example low cardiac output) this will kidney or metabolised by increasing bicarbonate impact on the delivery of oxygen to tissues and the levels (Woodrow, 2004; Adam and Osborne, 1997). clearance of waste products such as carbon dioxide In high pH (alkalosis) the opposite will happen, (Proehl, 1999; Smith, 2000). resulting in less bicarbonate production or reduced Where possible, support should be given if the body’s renal clearance. Bicarbonate is an effective buffer but own mechanisms are unable to cope with the demand. should only be used in documented severe acidosis This might be in the form of fluid management (for This article has been double-blind peer-reviewed. (pH<7.1) as it releases more CO2 when it is example blood cells or crystalloid), drug therapy (for metabolised. It therefore improves the acidosis example inotropic support) or mechanical support (for For related articles on this subject initially but then worsens it if respiratory example a pacemaker or intra-aortic balloon pump) and links to relevant websites see compensations are not effective (RCUK, 2000). (Docherty, 2002a; Smith, 2000). n www.nursingtimes.net NT 11 April 2006 Vol 102 No 15 www.nursingtimes.net 25.
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