ACUTE MEDICAL EMERGENCIES - PART 8 potx

transferring team can help move the patient from the ambulance trolley to the receiving unit’s bed. A copy of the transfer sheet should then be handed over with one copy retained by the transfer team who can then retrieve all their equipment and personnel for the return journey. The data collection sheets should be subjected to regular audit by a designated con- sultant in each hospital. This will ensure transfers are appropriate and to the correct standards. Problems can also be addressed and corrected. SUMMARY The safe transfer and retrieval of a patient requires a systematic approach. By following the ACCEPT method, important activities can be done at the appropriate time. TIME OUT 21.1 A 27 year old mechanic presented with an occipital headache. A clinical diagnosis of subarachnoid haemorrhage is confirmed by CT scan and lumber puncture. The local neurosurgical centre is 30 miles away by road. The patient’s vital signs are: A – patent (FiO 2 0·85) B – rate 14 per minute C – sinus tachycardia 110 per minute BP 120/70 (IV access secured) D – GCS 15/15; PERLA, no lateralising signs glucose 7·0 mmol/l Write down an outline of how you, as the doctor in charge, would arrange this patient’s transfer to the neurosurgical centre. TRANSPORTATION OF THE SERIOUSLY ILL PATIENT 319 21-AcuteMed-21-cpp 28/9/2000 4:45 pm Page 319 This Page Intentionally Left Blank PART VI INTERPRETATION OF EMERGENCY INVESTIGATIONS 22-AcuteMed-22-cpp 28/9/2000 4:47 pm Page 321 This Page Intentionally Left Blank CHAPTER 22 Acid–base balance and blood gas analysis OBJECTIVES After reading this chapter you will be able to: ● describe the meanings of the common terms used in acid–base balance ● describe how the body removes carbon dioxide and acid ● explain the cause of an increased anion gap ● understand the system for interpreting a blood gas result. TERMINOLOGY It is important to understand the meaning of the terms commonly used when discussing acid–base balance. Acids and bases Originally the word “acid” was used to describe the sour taste of unripe fruit. Subsequently many different meanings have led to considerable confusion and mis- understanding. This was not resolved until 1923 when the following definition was proposed. A strong acid is a substance that will readily provide many hydrogen ions and conversely, a weak acid provides only a few. In the body we are mainly dealing with weak acids such as carbonic acid and lactic acid. The opposite of an acid is a base and this is defined as any substance that “accepts” hydrogen ions. One of the commonest bases found in the body is bicarbonate (HCO 3 – ). The pH scale, acidosis/acidaemia, alkalosis and alkalaemia The concentration of hydrogen ions in solution is usually very small, even with strong acids. This is particularly true when dealing with acids found in the body where the hydrogen ion concentrations are in the order of 40 nanomoles/litre (nmol/l). An acid is any substance which is capable of providing hydrogen ions (H + ) 323 Reading: 30 minutes 22-AcuteMed-22-cpp 28/9/2000 4:47 pm Page 323 To place this low concentration in perspective compare it with the concentration of other commonly measured electrolytes. For example, the plasma sodium is around 135 mmol/l, i.e. 3 million times greater! Dealing with such very small numbers is obviously difficult and so in 1909 the pH scale was developed. This scale has the advantage of being able to express any hydrogen ion concentration as a number between 1 and 14 inclusively. The pH of a normal arter- ial blood sample lies between 7·36 and 7·44 and is equivalent to a hydrogen ion concen- tration of 44–36 nanomoles/litre respectively. It is important to realise that when using the pH scale, the numerical value increases as the concentration of hydrogen ions decreases (Figure 22.1).This is a consequence of the mathematical process that was used to develop the scale. Therefore an arterial blood pH below 7·36 indicates that the concentration of hydrogen ions has increased from normal. This is referred to as an acidaemia. Conversely, a pH above 7·44 would result from a reduction in the concentration of hydrogen ions. This condition is referred to as an alkalaemia. Figure 22.1 The hydrogen ion scale Another important consequence of the derivation of the pH scale is that small changes in pH mean relatively large changes in hydrogen ion concentration ; for example, a fall in the pH from 7·40 to 7·10 means the hydrogen ion concentration has risen from 40 to 80 nanomoles/litre, i.e. it has doubled. Summary ● Hydrogen ions are only present in the body in very low concentrations. ● As the hydrogen ion concentration increases the pH falls. pH [H + ] n.mol/l Acidaemia Alkalaemia 7.00 7.10 7.20 7.30 7.40 7.50 7.60 100 80 60 40 20 N N 1 nanomole = 1 billionth of a mole ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH 324 22-AcuteMed-22-cpp 28/9/2000 4:47 pm Page 324 ● As the hydrogen ion concentration falls the pH rises. ● An acidaemia occurs when the pH falls below 7·36 and an alkalaemia occurs when it rises above 7·44. Buffers Many of the complex chemical reactions occurring at a cellular level are controlled by special proteins called enzymes. These substances can only function effectively at very narrow ranges of pH (7·36–7·44). However, during normal activity the body produces massive amounts of hydrogen ions which if left unchecked would lead to significant falls in pH. Clearly a system is required to prevent these hydrogen ions causing large changes in pH before they are eliminated from the body. This is achieved by the “buffers”. They “take up” the free hydrogen ions in the cells and in the blood stream, thereby preventing a change in pH. There are a variety of buffers in the body. The main intracellular ones are proteins, phosphate, and haemoglobin. Extracellularly there are also plasma proteins and bicar- bonate. Proteins “soak up” the hydrogen ions like a sponge and transport them to their place of elimination from the body, mainly the kidneys. In contrast, bicarbonate reacts with hydrogen ions to produce water and carbon dioxide. H + + HCO 3 – ⇔ H 2 O + CO 2 The carbon dioxide is subsequently removed by the lungs. With these common terms defined, let us now consider why people can become acidaemic and how this can be corrected by the body. ACID PRODUCTION AND ITS REMOVAL All of us, whether we are healthy or ill, produce large amounts of water, acid, and carbon dioxide each day. A healthy adult will normally produce 15 000 000 nanomoles of hydro- gen ions each day as waste products generated when food is metabolised to release energy. This process occurs at a cellular level where these products initially accumulate. If this was left unchecked irreparable cellular damage would result. The first acute compensatory mechanism is the intracellular buffering system. As described previously, this provides the cell with a temporary way of minimising the fluc- tuations in acidity. Subsequently, these waste products (i.e. carbon dioxide and hydrogen ions) are excreted into the blood stream where they are taken up by the extracellular buffers (Figure 22.2). However, this is only a temporary solution because there is only a limited amount of buffer. If this was the sum total of the body’s defence to acids and carbon dioxide then the buffers would soon be exhausted, thereby allowing the products of metabolism to accumulate in the blood stream. A system is, therefore, needed to remove these harmful substances from the body so that they do not reach toxic levels and, at the same time, regenerate the buffers. Fortunately the body can eliminate these waste products removed by the lungs and the kidneys. Let us look at each of these in turn. Key point Small changes in the pH scale represent large changes in the concentration of hydrogen ions ACID–BASE BALANCE AND BLOOD GAS ANALYSIS 325 22-AcuteMed-22-cpp 28/9/2000 4:47 pm Page 325 Figure 22.2 Removal of waste products from cells Carbon dioxide removal (the respiratory component) Carbon dioxide (CO 2 ) released from cells is transported in the blood to the lungs and, after diffusing into the alveoli, it is ultimately removed from the body during expiration (Figure 22.3). Figure 22.3 Removal of carbon dioxide by the lungs If carbon dioxide is produced faster than it can be eliminated or there is a blockage to its removal, then it will accumulate in the blood stream. Here it reacts with water in the plasma to produce hydrogen ions (H + ) and bicarbonate (HCO 3 – ): H + H + H 2 O CO 2 CO 2 H 2 O CO 2 Lungs Cell Capillary H + H + H 2 O CO 2 CO 2 H 2 O ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH 326 22-AcuteMed-22-cpp 28/9/2000 4:47 pm Page 326 CO 2 + H 2 O ⇔ H + + HCO 3 – The greater the amount of carbon dioxide, the more hydrogen ions are produced. If this increase in plasma concentration of hydrogen ions causes the pH to fall below 7·36 then an acidaemia has been produced. As the cause of the acidaemia in this case is a problem in the respiratory system, it is known as a respiratory acidosis. If a sample of arterial blood was taken immediately this occurred then the result given in Table 22.1 would be obtained. Table 22.1 Effect of a respiratory acidosis on blood gas analysis As a by-product of the reaction between carbon dioxide and water, the bicarbonate concentration also increases by the same amount as the hydrogen ions. However, this increase is usually in the order of several nanomoles. As the normal concentration is 21–27 mmoles (i.e. 21–27 thousand nanomoles) the net increase in bicarbonate is very small. Consequently these changes in concentration are enough to change the pH scale but are not large enough to alter significantly the plasma bicarbonate concentration. In a normal person at rest, the respiratory component will excrete at least 12 000 000 nanomoles of hydrogen ions per day. It is therefore easy to see that there can be a rapid onset of acidosis during episodes of hypoventilation. Acid removal (the metabolic component) As has already been described, acids are continuously produced as a result of cellular metabolism.The amount produced from normal metabolism is approximately 3 000 000 nanomoles/day. This acid load is soaked up by buffers in the blood stream so that they can be transported safely for elimination (Figure 22.4). One of the buffers is bicarbonate. This is generated by the kidneys and released into the blood stream where it reacts with free hydrogen (Figure 22.5). In certain circumstances, so much acid is produced by the cells that it exceeds the capacity of the protein buffers and bicarbonate. If this results in an accumulation of free hydrogen ions in the plasma so that the pH falls below 7·36 then an acidaemia has been produced. As this is a result of a defect in the metabolic system, it is termed a metabolic acidosis. If a sample of arterial blood was taken when this occurred then the result given in Table 22.2 would be obtained. Table 22.2 Effect of a metabolic acidosis Normal values Effect of a metabolic acidosis pH 7·36–7·44 ↓ ↓ PaCO 2 4·8–5·3 kPa 4·8–5·3 kPa 36–40 mm Hg 36–40 mm Hg HCO 3 – 21–27 mmol/l ↓ Normal values Effect of a respiratory acidosis pH 7·36–7·44 ↓ ↓ PaCO 2 4·8–5·3 kPa ↑ ↑ ↑ 36–40 mm Hg HCO 3 – 21–27 mmol/l ↑ ACID–BASE BALANCE AND BLOOD GAS ANALYSIS 327 22-AcuteMed-22-cpp 28/9/2000 4:47 pm Page 327 Figure 22.4 Removal of hydrogen ions by the kidney Figure 22.5 Release of bicarbonate into the blood H + H + H + H 2 O CO 2 CO 2 H 2 O HCO 3 H + H + H + H 2 O CO 2 CO 2 H 2 O Kidney ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH 328 22-AcuteMed-22-cpp 28/9/2000 4:47 pm Page 328 [...]... compensation 339 ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH Using the arithmetic model: if the patient had a metabolic acidaemia, a 1 mmol/l fall in actual HCO3 produces a 1·0–1·3 mm Hg fall in PaCO2 Therefore: a 18 5 mmol/l fall in HCO3 would produce a 18 5–24·1 mm Hg fall in PaCO2 Therefore using the midpoints of the normal ranges, this patient’s PaCO2 should be between: (40 – 18 5) and (40 –... metabolic acidaemia with a wide anion gap 341 ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH SUMMARY The body’s system for removing the carbon dioxide and acid produced by metabolism has both a respiratory and metabolic component These are linked by the effects of carbonic acid which enables one component to compensate for a defect in the other In acute medical emergencies one or both of these systems... then it is likely that the patient has more than one acid–base disturbance 337 ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH Expected changes * Acute respiratory acidaemia: a 1·0 mm Hg rise in PaCO2 produces a 0·1 mmol/l rise in HCO3 Chronic respiratory acidaemia: a 1·0 mm Hg rise in PaCO2 produces a 0·5 mmol/l rise in HCO3 Acute respiratory alkalaemia: a 1·0 mm Hg fall in PaCO2 produces a 0·2 mmol/l... an arterial PaCO2 of 23·7 kPa ( 180 mm Hg) in a patient breathing 50% oxygen indicates that there is a defect in the take-up of oxygen ● Using kPa An inspired oxygen of 50% will have a partial pressure of approximately 50 kPa.This would mean the expected PaCO2 would be at least 50 – 10 = 40 kPa ● Using mm Hg An inspired oxygen of 50% will have a partial pressure of 380 mm Hg (i.e half the normal atmospheric... does not fully compensate in the acute situation Therefore even after several hours, respiratory compensation will only be partial and the patient will still be slightly acidaemic (Table 22.4) Table 22.4 Slight acidaemia Normal values pH PaCO2 HCO3– Effect of a metabolic acidosis Effect of respiratory compensation 7·36–7·44 4 8 5·3 kPa 36–40 mm Hg 21–27 mmol/l ↓↓ 4 8 5·3 kPa 36–40 mm Hg ↑ ↓ ↓ ↓ It... respiratory acidosis by increasing the production of bicarbonate by the kidneys 331 ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH Increase in expired CO2 CO2 Equation pushed to the right + due to increased H production + + HCO– 3 H2CO3 H H2O + CO 2 Increased production of CO2 Increased H+ production H H + H2O CO2 + Figure 22 .8 Respiratory compensation for a metabolic acidosis (i.e the respiratory system... metabolic acidaemia (Table 22.5) Table 22.5 Combined respiratory and metabolic acidaemia Normal values pH PaCO2 HCO3– Effect of respiratory and metabolic acidosis 7·36–7·44 4 8 5·3 kPa 36–40 mm Hg 21–27 mmol/l ↓↓↓↓ ↑↑ ↓↓ 333 ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH CENTRAL VENOUS AND ARTERIAL BLOOD SAMPLES So far we have concentrated on arterial blood analysis This is blood that has had the benefit... Comparison of the composition of arterial and central venous blood Arterial blood pH PCO2 HCO3– PO2 on air Central venous blood 7·36–7·44 4 8 5·3 kPa 36–40 mm Hg 21–27 mmol/l Over 10·6 kPa Over 80 mm Hg 7·31–7·40 5·5–6 8 kPa 41–51 mm Hg 25–29 mmol/l 5·1–5·6 kPa 38 42 mm Hg Compare these results with those following a cardiorespiratory arrest In the absence of cardiopulmonary resuscitation no blood will... 347 ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH Leads I, II, III, and AVR, AVL, and AVF look at the heart in the vertical plane (Figure 23.5) Figure 23.5 The standard lead view of the heart Leads V1–6 view the heart in the horizontal plane such that V1 and V2 look at the right ventricle,V3 and V4 the interventricular septum, and V5 and V6 mainly the left ventricle (Figure 23.6) Figure 23.6 3 48. .. can help the body respond to an excess of either carbon dioxide or acid CO2 H+ HCO3 H+ H2CO3 H2O CO2 H2O CO2 H+ Figure 22.6 Carbonic acid–bicarbonate buffers: acid production and its removal 329 ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH Example 1 In a patient with inadequate alveolar ventilation, e.g chronic bronchitis, carbon dioxide accumulates As we have seen this will tend to cause a respiratory . SERIOUSLY ILL PATIENT 319 21-AcuteMed-21-cpp 28/ 9/2000 4:45 pm Page 319 This Page Intentionally Left Blank PART VI INTERPRETATION OF EMERGENCY INVESTIGATIONS 22-AcuteMed-22-cpp 28/ 9/2000 4:47 pm Page. blood H + H + H + H 2 O CO 2 CO 2 H 2 O HCO 3 H + H + H + H 2 O CO 2 CO 2 H 2 O Kidney ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH 3 28 22-AcuteMed-22-cpp 28/ 9/2000 4:47 pm Page 3 28 The bicarbonate level has fallen as a consequence. urine ACUTE MEDICAL EMERGENCIES: THE PRACTICAL APPROACH 330 22-AcuteMed-22-cpp 28/ 9/2000 4:47 pm Page 330 It is important to realise that in the acute situation the body does not fully compen- sate .