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Arterial Blood Gases

Arterial Blood Gases. Aims and Objectives. AIM Assess degree to which lungs are able to provide adequate oxygen & remove CO2 & degree to which the kidneys are able to reabsorb or excrete HCO3 Objectives To give a basic understanding of normal blood gas parameters

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Arterial Blood Gases

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  1. Arterial Blood Gases

  2. Aims and Objectives • AIM Assess degree to which lungs are able to provide adequate oxygen & remove CO2 & degree to which the kidneys are able to reabsorb or excrete HCO3 • Objectives • To give a basic understanding of normal blood gas parameters • Recognise abnormalities and differences between a metabolic and respiratory problem • Enable staff to analyse them in the critical care setting and identify when to alert medical staff

  3. When and Why Do We Take Arterial Blood Gases? • Arterial Blood Gases (ABG’s) can provide valuable clinical information in critically ill patient (www.resus.org.uk/pages/alsmBGap.pdf) • ABG’s are indicated if the patient has a further deterioration in an already critical condition such as: Decrease in oxygen saturations head or neck trauma, injuries that may affect breathing Fall in conscious level (Glasgow Coma Score) Increased respiratory effort Cyanosis Patients undergoing prolonged anesthesia • It is therefore an essential skill of the practitioner to be able to analyse the results and act on appropriately (www.edu.rcsed.ac.uk/lectures/lt8.htm)

  4. Indications For An ABG! • diagnosis, determine severity of a condition and direct further interventions • Enable both medical staff to evaluate how sufficiently the patient is oxygenating, transporting oxygen within the blood flow and acid base levels (Hess D, 2000) However it must be remembered that ABG analysis is only part of a respiratory assessment and cannot be conclusive of a patient condition

  5. ABG’s can also be indicated to: • Diagnose the different types of and severity of a respiratory failure • i.e. cardiac and respiratory failure • Can be used as a guide for treatment therapy i.e. oxygen administration • Monitoring purposes during surgery (Williams, 1998)

  6. NORMAL ARTERIAL BLOOD GAS VALUES! • In order to make an analysis of an ABG we need to have a knowledge of normal ranges on which to base our analysis! • patient who have a preexisting respiratory complaints may almost certainly have altered blood gas values which will be known as normal for them (Pruitt WC, 2004)

  7. What Is PO2? • Partial pressure of O2 in blood plasma • Partial pressure causes O2 to diffuse from a region of higher pressure to a region of lower pressure, lungs to plasma, plasma to haemoglobin, haemoglobin to tissues • When breathing in normal room air a patient PO2 will not exceed 13 kPa (www.acessmedicine.com) • By giving supplementary O2 the PO2 can exceed normal ranges but the amount of O2carried to the tissues won’t increase, due to O2 being carried in haemoglobin (Outreach/Clinical Education 2001)

  8. PO2, OXYGEN SATURATIONS (SpO2) AND OXYGEN CONTENT OF BLOOD • The maintenance of a normal PO2 and SpO2 can depend on the ability of O2 to diffuse from the lungs into the blood stream • This can be impeded by various conditions including infection and swelling. If this occurs then PO2 and SPO2 will fall • However, underlying respiratory conditions can deteriorate further despite the additional O2 and continual assessment will be required (DuBoseet al. 1997)

  9. GAS EXCHANGE • Getting O2to body tissues requires good gas exchange from lung to blood stream, a good amount of heamoglobin to carry O2 and a good blood pressure to circulate the oxygen laden heamoglobin to the tissues • Controlling respiratory function is also essential is gas exchange • Enzymes within the body work well within a narrow pH range, which is controlled by the body (Normal range 7.35 – 7.45) • Changes in the pH can cause both acidosis and alkalosis (Outreach/Clinical Education 2001)

  10. ACIDOSIS • Acidosis occurs when a pH less than 7.35 • Acidosis is often a result of poor respiratory or renal function • However, it should be remembered that acidosis can also be caused by other body systems, known as non – respiratory or metabolic acidosis • Both the kidneys and the lungs control the acid levels in the body, if respiratory failure occurs the kidneys will compensate and vice versa • Alkalosis may also have respiratory or metabolic causes (www.edu.rcsed.ac.uk/lectures/lt8.htm) • As acidosis becomes more severe respiration will increase with the patient becoming increasingly tired causing a rise in CO2 and worsening of acidosis (www.nlm.nih.gov/medlineplus/ency/article/000092.htm)

  11. Respiratory acidosis-Etiology • Upper airway obstruction • Lower airway obstruction • Cardiogenic or non-cardiogenic pulmonary edema • Pneumonia • Pulmonary emboli • Fat emboli • Central nervous system depression • Neuromascular impairment • Ventilatory restriction

  12. Respiratory Acidosis • As CO2 is dissolved it produces an increase in acidity • When respiratory acidosis occurs the patient ventilatory response is decreased causing carbon dioxide to be retained (in PC02 and in pH) • When this occurs the kidneys excrete hydrogen and retain bicarbonate (FitzSullivanet al. 2005) • Bicarbonate can be measured using ABG analysis and the following may be seen • CO2 to above 7kPa pH 7.35 HCO3 to 32 mmols • Kidneys excrete the hydrogen ions that cause acidity and retain bicarbonate to reduce the acidity (Kellum 2005)

  13. Metabolic Acidosis • Hydrogen ions can cause acidosis pH than 7.35 with a normal CO2 • Loss of bicarbonate in conditions such as diarrhoea and failure to excrete hydrogen due to kidney failure • Drugs can also induce metabolic acidosis as can poor circulation • The respiratory compensation for metabolic acidosis includes a in respiratory rate and depth to blow ‘off’ carbon dioxide therefore reducing the acidity. On the ABG analysis a low CO2 will be seen • CO2 3.5kPa or pH 7.35 HCO3 -18 mmols This is a response by the respiratory system on the increased acidity (Saad EB, 1996)

  14. Metabolic Acidosis Normal AG acidosis • The fall in bicarbonate is matched by a proportional rise in serum chloride (hyperchloremic metabolic acidosis). • Most common causes are gastrointestinal and renal loses of bicarbonate.

  15. Alkalosis • If a patient has an in respiratory rate and depth (hyperventilation) therefore ‘blowing off’ more than normal amounts of carbon dioxide a rise in pH can occur causing a respiratory alkalosis • The ABG analysis may show CO2 3.5kPa ph 7.55 HC03 – 26mmols • As this is a reversible condition the C02 is ‘blown off’ by the patient and make C02 and H20, the concentration of the hydrogen ions, causing pH to rise above normal limits and cause an alkalosis • It is also important to remember that patient can be hyperventilating due to anxiety (Murray J et al. 2000)

  16. Respiratory alkalosis-Etiology • Central nervous system stimulation: Fever, pain, fear, cerebrovascular accident, CNS infection, trauma, tumor. • Hypoxia: High altitude, profound anemia, pulmonary disease. • Stimulation of chest receptors: Pulmonary edema, pulmonary emboli, pneumonia, pneumothorax, pleural effusion. • Drugs or hormones :Salicylates, medroxyprogesterone, catecholamines. • Miscellaneous: Sepsis, pregnancy, liver disease, hyperthyroidism.

  17. Renal Compensation in Alkalosis • As we are aware the lungs and kidneys compensate each other, in this scenario the kidneys try to compensate by excreting excess bicarbonate and retain the hydrogen ions • An ABG analysis will show C02 3.5kPa ph 7.45 HC03 – 16mmols • It must again be remembered when caring for clients that they may be chronically anxious therefore having a respiratory rate (Bennett et al. 1996)

  18. METABOLIC ALKALOSIS • Can be caused by various conditions including vomiting causing excess loss of hydrogen ions and excessive gain of bicarbonate • The pH will be greater than 7.45, the HC03 less than 22mmols creating a metabolic alkalosis, it will appear on an ABG analysis as PC02 4.5 kPa pH 7.55 HC03 – 18mmols • The respiratory system will try and compensate in this situation by slowing down respiration rate and depth, therefore retaining C02 acidity of the blood. When the pH starts to return to within normal range the respiration is compensating for metabolic alkalosis (Markouet al. 2004)

  19. SUMMARY • Is the patient well oxygenated, check resp rate, Sp02 , resp pattern, colour etc. • If these vary from the clients norm inform medical staff and complete blood gas analysis • If pH < 7.35 you have an ACIDOSIS • If pH < 7.35 and PC02 > 6kPa it is RESPIRATORY ACIDOSIS • If pH < 7.35 and PC02 normal or < 4.5kPa it is METABOLIC ACIDOSIS • If pH > 7.45 it is an ALKALOSIS • If pH > 7.45 and PC02 < 4.5kPa it is a RESPIRATORY ALKALOSIS • If pH > 7.45 and PC02 normal or > 6kPa it is a METABOLIC ALKALOSIS • Renal compensation can normal pH in a respiratory disorder and will show an abnormal bicarbonate on ABG Analysis • Respiratory compensation can aid in restoring a normal pH in a metabolic disorder creating an abnormal PC02

  20. References • Bennett J, Claude J And Plum F (1996) Cecil Textbook Of Medicine WB Saunders Co. Philadelphia • DuBose Jr., Thomas D (1997) Acidosis And Alkalosis In Harrisons Principles Of Internal Medicine New York Mc Graw - Hill • Hess D (2000) Detection and monitoring of hypoxemia and oxygen therapy Chest (45) pp. 64 – 83 • FitzSullivan E, Salim A, Demetriades D, Asensio J And Martin MJ (2005) Serum Bicarbonate May Replace The Arterial Base Deficiet In The Trauma Intensive Care Unit American Journal Of Surgery December, 190 (6) pp. 941 - 946 • Kellum JA (2005) Determinants Of Plasma Acid-Base Balance Critical Care Clinical April (2) pp:329 – 346 • Markou NK, Myrianthefs PM And Baltopoulos GJ (2004) Respiratory Failure: An Overview Critical Care Nurse October – December 27 (4) pp: 353 - 379 • Murray J And Nadel J (2000) Textbook Of Respiratory Medicine Third Edition Philadelphia WB Saunders • Outreach/Clinical Education 2001 Leeds Teaching Hospitals Guidelines • Pruitt WC and Jacobs M 2004 Interpreting Arterial Blood Gases: Easy As ABC Nursing Aug; 34 (8) pp: 50 -53 • Saad EB (1996) Oxford Textbook Of Medicine Oxford Medical Publications London • Williams AJ (1998) ABC Of Oxygen. Assessing And Interpreting Arterial Blood Gases And Acid Base Balance British Medical Journal (317) pp. 1212-1216 • www.edu.rcsed.ac.uk/lectures/lt.8.htm • www.nlm.nih.gov/medlineplus/ency/article/000092.htm • www.accessmedicine.com/content.aspx?aID=55971

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