Acid-Base Physiology

Case 15 : An old man with abdominal pain and shock

updated 30Aug2015

Clinical Details

An 85 year old man was admitted with severe abdominal pain and shock. The abdominal pain had started about 1500hrs and quickly became quite severe. There was no radiation to the back. The patient was known to have an abdominal aortic aneurysm (AAA). On arrival at hospital, the patient was shocked with peripheral circulatory failure and hypotension (BP 70-80 systolic). His abdomen was guarded and quite tender. He was distressed but able to talk and could understand instructions. Past history was of hypertension (on metoprolol and prazosin) and angina (on Isordil). Prior to this event, the patient was mobile and independent.

A ruptured AAA was diagnosed clinically and he was transferred to theatre for emergency laparotomy. On arrival in theatre, BP was 120 systolic. The patient was talking but distressed by pain with rapid respirations at a rate of 30/min. It was noted that neck veins were very distended. An external jugular triple lumen central line and a brachial arterial line were placed before the surgical team had arrived in theatre. CVP was +40 mmHg. The blood gases were collected from an arterial line during preoxygenation with 100% oxygen at 1738 hrs (i.e. about 4.5 hours after onset of symptoms).

Investigations: Biochemistry at 1520hrs was Na+ 138, K+ 4.9, Cl- 107, Bicarbonate 20, Glucose 11.2, Urea 12.8, creatinine 0.188, lactate 8.3 (all results in mmol/l). Haemoglobin 133 G/l.

Arterial Blood Gases

pH 7.35

pCO2 24 mmHg

pO2 182 mmHg

HCO3 13.8 mmol/l

Assessment

Initial clinical assessment

The clinical expectation was an acute metabolic acidosis (lactic acidosis) due to peripheral circulatory failure, and respiratory alkalosis due to pain-induced hyperventilation.

Respiratory compensation for the metabolic acidosis would be at an early stage only as such compensation (i.e. hyperventilation) takes 12 to 24 hours to reach maximum. The clinical details given do not give sufficient detail about previous medical conditions (eg history of chronic airways disease) or any medication history - such details are important.

Acid-base diagnosis

  1. pH The pH is almost normal. This means either a mild acidosis, or compensating acid-base disorders (i.e. a mixed disorder with an acidosis and an alkalosis)
  2. Pattern: The low pCO2 & low bicarbonate suggest either a metabolic acidosis OR a respiratory alkalosis. The option of no acid-base disorder is rejected on these results leaving us with the option of balancing disorders. In this case, this means a metabolic acidosis & a respiratory alkalosis. A bicarbonate level of 13.8 by itself indicates the presence of a metabolic acidosis as it is below the limit of compensation (18 mmol/l) for an acute respiratory alkalosis. The limit of compensation for a chronic respiratory alkalosis is lower (12-15) but there is no clinical evidence for such a pre-existing disorder in this patient. (Clinical details of past history are absent so this should perhaps not be totally excluded)
  3. Clues: The anion gap is 11. The chloride is slightly elevated. The urea and creatinine are elevated but are not high enough to support an acute renal failure as a cause for acidosis. The glucose is elevated but there are no urine test results given so we don't know whether ketonuria is present, but ketoacidosis as the cause is not supported by the history. The lactate level was quite elevated (at 1738hrs) and this was collected about 2 hours after the biochemistry which showed a normal anion gap. The bicarbonate on the initial biochemistry was 20 but had decreased to 13.8 by the time of the gas collection. This indicates a rapid development of the disorder.
  4. Compensation:As we know a metabolic acidosis is present, we use rule 5 to check the amount of compensation. Expected pCO2 at maximal compensation = (1.5 x 12.8 + 8) = 27.2mmHg. The actual value of 24 is slightly lower than this and we might conclude that compensation was within the limits for maximal compensation. However, these gases were collected at 4.5 hours after onset of symptoms and this is insufficient time for maximal compensation to be reached. At this time a lesser amount of compensation would be expected, not a bit more as here. So an actual pCO2 must definitely indicate the presence of a second acid-base disorder (as we have suspected initially based on the near return of the pH to the reference range).
  5. Formulation: Metabolic acidosis (due to lactic acidosis) and a respiratory alkalosis (due to hyperventilation in response to severe abdominal pain). These are of similar (but opposite) magnitute so the pH is almost within the reference range.
  6. Confirmation:A repeat set of electrolyte results with the gases would be expected to show an increase in the anion gap. A urinalysis result should be obtained from all patients.

(Modern blood gases machines provide just blood gas results, but also other results (electrolytes, lactate, haemoglobin). This was not the situation when this patient presented.)

Clinical Diagnosis

Acute mesenteric occlusion causing extensive bowel infarction, shock and lactic acidosis. The ischaemic bowel contributes to increased lactate production. The mesenteric occlusion and the hypotension results in failure of hepatic clearance of the lactate from the circulation.

Comments

Intraoperative findings were almost total infarction of the small bowel and part of the stomach due to acute mesenteric vascular obstruction. The AAA was unruptured. The surgical assessment was that the situation was inoperable and not survivable. The patient arrested and died on the table at 1805hrs.

A significant shunt is indicated by a pO2 of only 182mmHg despite 100% O2. On Nunn's Isoshunt Diagram, this would be a shunt of about 25%.

Clinical shock always suggests lactic acidosis, and there is a correlation between blood lactate and mortality. Expected mortality for a lactate of 8mmol/l due to shock is 90% (e.g. see fig 5 in Mizock and Falk). This relationship depends on context; some conditions (e.g. prolonged fitting) can result in high but transient lactate levels which do not carry the same bad prognosis as in shocked or critically ill patients. The high lactate is due to a combination of regional hyperproduction from the ischaemic bowel, and by peripheral tissues due to the shock, and decreased uptake by the liver. Severely ischaemic gut alone can produce large quantities of lactate.

Continued marked hyperventilation is required following intubation to keep the pCO2 low and prevent marked deterioration in pH. End-tidal pCO2 would be an unreliable guide to arterial pCO2 in this patient because significant alveolar dead space would cause a large difference between them. Serial blood gases will be required.

LEARNING POINTS
1. The pH can be within the reference range if both acidosis and alkalosis of similar severity are present.

2. Significant lactic acidosis due to shock is associated with a high mortality

References

  1. Mizock and Falk Lactic acidosis in critical illness Crit Care Med 1992 Jan;20(1):80-93.