Complex acid-base problems

Working out the expected compensatory response to an acid base disturbance often reveals a second acid-base problem that was otherwise hidden. I regularly use Winter’s formula when I see a metabolic acidosis, but I have trouble remembering the others, so here they are, from Harwood-Nuss’ Clinical Practice of Emergency Medicine (apologies if you ‘think’ in kilopascals):
Formulas Describing Expected Compensatory Response to Primary Acid–Base Disturbances
Simple Metabolic Acidosis

  • Predicted decreased PCO2 mm Hg = 1.2 × Δ(HCO3-) mEq/L
  • Predicted PCO2 mm Hg = 1.5(HCO3-) mEq/L + 8 ± 2
  • Anticipated PCO2 approximates last two digits of arterial pH

Simple Metabolic Alkalosis

  • Predicated increased Δ PCO2 mm Hg = 0.67 × Δ(HCO3-) mEq/L

Simple Acute Respiratory Acidosis

  • Predicted decreased ΔpH units = 0.8 × Δ PCO2 mm Hg
  • Predicted increased Δ(HCO3-) mEq/L = 0.1 × Δ PCO2 mm Hg

Simple Chronic Respiratory Acidosis

  • Predicted decreased ΔpH units = 0.3 × Δ PCO2 mm Hg
  • Predicted increased Δ(HCO3-) mEq/L = 0.35 × Δ PCO2 mm Hg

Simple Acute Respiratory Alkalosis

  • Predicted increased ΔpH units = 0.8 × Δ PCO2 mm Hg
  • Predicted decreased Δ(HCO3-) mEq/L = 0.2 × Δ PCO2 mm Hg

Simple Chronic Respiratory Alkalosis

  • Predicted increased ΔpH units = 0.17 × Δ PCO2 mm Hg
  • Predicted decreased Δ(HCO3-) mEq/L = 0.5 × Δ PCO2 mm Hg

Femoral SvO2 not so useful

Bloods sampled from both femoral vein and SVC-sited catheters in critically ill patients showed good correlation in lactate levels but the oxygen saturation was not so reliable, with >5% variation in more than 50% and >15% variation in some patients. The authors suggest one reason is that the femoral catheter tip usually sits in the iliac vein and samples blood prior to the mixing of blood returning from intra-abdominal organs. They advise caution in using SfvO2 to guide resuscitation when narrow end points are used, as this may lead to inappropriate vasoactive drug or blood component therapy.

Femoral-Based Central Venous Oxygen Saturation Is Not a Reliable Substitute for Subclavian/Internal Jugular-Based Central Venous Oxygen Saturation in Patients Who Are Critically Ill

Chest. 2010 Jul;138(1):76-83

ALI / ARDS strategies

A CME article in Critical Care Medicine summarises the literature on ARDS (including its limitations) and provides evidence based recommendations on what to do about severe hypoxaemia. They summarise:
For life-threatening hypoxaemia, initial management with a recruitment manoeuvre and/or high PEEP should be undertaken if plateau airway pressures and lack of barotrauma allow. If not, or if these are not effective, then proceed to the prone position or HFOV. If hypoxemia still persists, then consider the administration of inhaled NO. If NO fails, then glucocorticoids can then be administered. For elevated plateau airway pressures when tidal volumes are 4 mL/kg, consider prone positioning or HFOV. For life- threatening respiratory acidosis, consider the use of a buffer or continuous veno-venous hemofiltration. It is most important to assess for objective physiologic improvement in the appropriate time period for each intervention. If no benefit is evident, then the therapy should be discontinued to minimise harm and delay in the initiation of another therapy. If the patient continues to have life-threatening hypoxemia, acidosis, or elevated plateau airway pressures, then consider ECMO or extracorporeal carbon dioxide removal.

Therapeutic strategies for severe acute lung injury
Crit Care Med. 2010 Aug;38(8):1644-50

Brain tumours in kids

When might you suspect a brain tumour in a child who presents with, say, nausea and vomiting, or behavioural disturbance? A guideline has been produced which might prompt one to think of this important but often delayed diagnosis.

The diagnosis of brain tumours in children: a guideline to assist healthcare professionals in the assessment of children who may have a brain tumour Arch Dis Child. 2010 Jul;95(7):534-9

Ultrasound of intracranial haematoma

Using a 2Mhz transducer insonating through the temporal acoustic bone window, Italian physicians detected the expansion of an extradural haematoma. In their discussion they highlight that transcranial sonography of brain parenchyma in adults has been proposed by several authors for the evaluation of the ventricular system, monitoring of midline shift, diagnosis and follow-up of intracranial mass lesions. In one study, of 151 patients, 133 (88%) had a sufficient acoustic bone window. Note that the skull contralateral to the acoustic bone window is visualised.

Arrow indicates EDH; asterisk indicates mesencephalon

Bedside detection of acute epidural hematoma by transcranial sonography in a head-injured patient
Intensive Care Med. 2010 Jun;36(6):1091-2

Guidelines on trauma in pregnancy

Guidelines on trauma in pregnancy have been published by the The Eastern Association for the Surgery of Trauma (EAST):
Level I
There are no level I standards.
Level II

  1. All pregnant women >20-week gestation who suffer trauma should have cardiotocographic monitoring for a minimum of 6 hours. Monitoring should be continued and further evaluation should be carried out if uterine contractions, a nonreassuring fetal heart rate pattern, vaginal bleeding, significant uterine tenderness or irritability, serious maternal injury, or rupture of the amniotic membranes is present.
  2. Kleihauer-Betke analysis should be performed in all pregnant patient > 12 week-gestation.

Level III

  1. The best initial treatment for the fetus is the provision of optimum resuscitation of the mother and the early assessment of the fetus.
  2. All female patients of childbearing age with significant trauma should have a human chorionic gonadotropin (HCG) performed and be shielded for X-rays whenever possible.
  3. Concern about possible effects of high-dose ionizing ra- diation exposure should not prevent medically indicated maternal diagnostic X-ray procedures from being per- formed. During pregnancy, other imaging procedures not associated with ionizing radiation should be considered instead of X-rays when possible.
  4. Exposure <5 rad has not been associated with an increase in fetal anomalies or pregnancy loss and is herein deemed to be safe at any point during the entirety of gestation.
  5. Ultrasonography and magnetic resonance imaging are not associated with known adverse fetal effects. However, until more information is available, magnetic resonance imaging is not recommended for use in the first trimester.
  6. Consultation with a radiologist should be considered for purposes of calculating estimated fetal dose when multiple diagnostic X-rays are performed.
  7. Perimortem cesarean section should be considered in any moribund pregnant woman of ≥24 week gestation.
  8. Delivery in perimortem cesarean sections must occur within 20 minutes of maternal death but should ideally start within 4 minutes of the maternal arrest. Fetal neuro- logic outcome is related to delivery time after maternal death.
  9. Consider keeping the pregnant patient tilted left side down 15 degrees to keep the pregnant uterus off the vena cava and prevent supine hypotension syndrome.
  10. Obstetric consult should be considered in all cases of injury in pregnant patients.

Practice Management Guidelines for the Diagnosis and Management of Injury in the Pregnant Patient: The EAST Practice Management Guidelines Work Group
Journal of Trauma 2010;69(1):211-4
Full text guidelines are available here. They are dated 2005 the recommendations appear to be indistinguishable from those published in the July 2010 issue of Journal of Trauma

New College Ketamine Guideline

The College of Emergency Medicine (UK) has updated its guideline on ketamine sedation in children.
The summary is copied below
Full text is available here
Guideline for ketamine sedation of children in Emergency Departments

  1. Before ketamine is used all other options should be fully considered, including analgesia, reassurance, distraction, entonox, intranasal diamorphine, etc.
  2. The doses advised for analgesic sedation are designed to leave the patient capable of protecting their airway. There is a significant risk of a failure of sedation if the procedure is prolonged, and the clinician must recognise that the option of general anaesthesia may be preferred in these circumstances.
  3. There is no evidence that complications are reduced if the child is fasted, however traditional anaesthetic practice favours a period of fasting prior to any sedative procedure. The fasting state of the child should be considered in relation to the urgency of the procedure, but recent food intake should not be considered as an absolute contraindication to ketamine use.
  4. Ketamine should be only used by clinicians experienced in its use and capable of managing any complications, particularly airway obstruction, apnoea and laryngospasm. The doctor managing the ketamine sedation and airway should be suitably trained and experienced in ketamine use, with a full range of advanced airway skills.
  5. At least three staff are required: a doctor to manage the sedation and airway, a clinician to perform the procedure and an experienced nurse to monitor and support the patient, family and clinical staff. Observations should be regularly taken and recorded.
  6. The child should be managed in a high dependency or resuscitation area with immediate access to full resuscitation facilities. Monitoring should include ECG, blood pressure, respiration and pulse oximetry. Supplemental oxygen should be given and suction must be available.
  7. After the procedure the child should recover in a quiet, observed and monitored area under the continuous observation of a trained member of staff. Recovery should be complete between 60 and 120 minutes, depending on the dose and route used.
  8. There should be a documentation and audit system in place within a system of clinical governance.

Tactical Combat Casualty Care

The brave men and women of the military not only risk their lives for us – they also provide a wealth of trauma experience and publish interesting stuff.
This month’s Journal of Trauma contains a military trauma supplement. One of the articles describes the latest guidelines on Tactical Combat Casualty Care. These include:

  • tourniquet use
  • Quikclot Combat Gauze as the haemostatic agent which has replaced Quikclot powder and HemCon. This preference is based on field experience that powder and granular agents do not work well in wounds in which the bleeding vessel is at the bottom of a narrow wound tract or in windy environments. WoundStat was a backup agent but this has been removed because of concerns over possible embolic and thrombotic complications.
  • longer catheters for decompression of tension pneumothorax (Harcke et al. found a mean chest wall thickness of 5.36 cm in 100 autopsy computed tomography studies of military fatalities. Several of the cases in their autopsy series were noted to have had unsuccessful attempts at needle thoracostomy because the needle/catheter units used for the procedure were too short to reach the pleural space*.)
  • close open chest wounds immediately with an occlusive material, such as Vaseline gauze, plastic wrap, foil, or defibrillator pads
  • a rigid eye shield and antibiotics for penetrating eye injury

Tactical Combat Casualty Care: Update 2009
The Journal of TRAUMA 2010;69(1):S10-13 (no abstract available)
Full text of guidelines in PDF at
*Harcke HT, Pearse LA, Levy AD, Getz JM, Robinson SR. Chest wall thickness in military personnel: implications for needle thoracentesis in tension pneumothorax. Mil Med. 2007;172:1260 –1263