capnometry versus pulse oximetry during procedural sedation

During emergency department procedural sedation, some clinicians (myself included) advocate non-invasive capnography for the early detection of apnoea. Others argue against routine administration of oxygen so that if desaturation occurs it provides an earlier more correctable warning of respiratory depression than if it occurs on supplemental oxygen. A Canadian study using prospective data from research on propofol with either ketamine or fentanyl compared changes in capnography with desaturation in sedated patients breathing only room air. Desaturation detectable by pulse oximeter occurred before overt changes in capnometry were identified.

It’s hard to ascertain the relevance of this finding. The authors wisely state ‘these findings should not be extrapolated to patients administered supplemental oxygen where it is possible capnometry may be helpful’. Since I use capnography in the hope that it will assist in the earlier detection of ketamine-associated laryngospasm in children, I’m not going to discard it in favour of waiting for the saturation to fall. Perhaps we just need to be clear that capnography may be more useful at detecting apnoea than hypoventilation.

A comparative evaluation of capnometry versus pulse oximetry during procedural sedation and analgesia on room air
CJEM. 2010 Sep;12(5):397-404

Echo in life support feasible

Emergency physicians in Cambridge and Ipswich in the UK prospectively documented their echo use in cardiac arrest. Images were easily acquired, were quickly obtained, and influenced management. In keeping with previous studies, absence of ventricular wall motion predicted lack of return of spontaneous circulation, with a negative predictive value of 97%.

An evaluation of echo in life support (ELS): is it feasible? What does it add?
Emerg Med J. 2010 Oct 4. [Epub ahead of print]

RCT of 7.5% saline in head injury

Over a thousand patients in North America with blunt traumatic head injury and coma who did not have hypovolaemic shock were randomised to different fluids pre-hospital. 250 ml Hypertonic (7.5%) saline was compared with normal (0.9%) saline and hypertonic saline dextran (7.5% saline/6% dextran 70). There was no difference in 6-month neurologic outcome or survival.

Out-of-Hospital Hypertonic Resuscitation Following Severe Traumatic Brain Injury
JAMA. 2010;304(13):1455-1464.

McConnell’s sign revisited

In acute pulmonary embolism, a well-recognised pattern of right ventricular wall motion reported by McConnell is characterised by normal RV apex (RVa) contractility with akinesia of the RV free wall. A study using an echo techique called longitudinal velocity vector imaging (VVI) was conducted to describe RVa mechanics in relation to the rest of the RV in patients with a proven acute PE (aPE) and to compare these results to healthy volunteers and to patients with known chronic pulmonary hypertension (cPH). There were no significant differences in segmental strain patterns between the aPE and cPH groups. The authors suggest that McConnell’s sign is probably a visual illusion; preserved RVa contractility might be due to tethering of the RVa to a hyperdynamic left ventricle in the presence of an acutely dilated RV and this is the most likely explanation of the regional pattern of RV dysfunction seen in aPE patients.

Video describing McConnell’s sign from YouTube:

Right Ventricular Apical Contractility in Acute Pulmonary Embolism: The McConnell Sign Revisited
Echocardiography. 2010 Jul;27(6):614-20

Two smaller lines may be quicker

Using Poiseuille’s law and standardized gauge sizes, an 18-gauge (g) intravenous catheter (IV) should be 2.5 times faster than a 20-g IV, but this is not borne out by observation, in vitro testing, and manufacturer’s data. A nice simple study on normal volunteers compared simultaneous flow rates between a single 18G iv in one arm with two 20G ivs in the other arm. The two smaller ones provided significantly faster flow than the single larger one, although flow rates were slower than manufacturer’s estimates. This is in keeping with this other study on cannula flow rates.

Are 2 smaller intravenous catheters as good as 1 larger intravenous catheter?
Am J Emerg Med. 2010 Jul;28(6):724-7

Scoop minimises cervical movement

A cadaveric study using a 3-dimensional electromagnetic tracking device to asses cervical motion compared the application of a scoop stretcher with two other manual transfer techniques, including log rolling onto an extrication (spine) board. The scoop method restricted cervical spine movement more than log rolling, although this was not statistically significant.

The authors conclude: the effectiveness of the scoop stretcher to limit spinal motion in the destabilized spine is comparable or better than manual techniques currently being used by primary responders.
Are scoop stretchers suitable for use on spine-injured patients?
Am J Emerg Med. 2010 Sep;28(7):751-6

Can we tell if AF is new?

One of the dilemmas in selecting appropriate therapy for atrial fibrillation in the emergency situation is determining whether the AF is of acute onset or not. AF causes release of natriuretic peptide from the heart, so measuring these peptides may give a clue to the recency of onset if the kinetics are known. This of course can only apply to those patients without heart failure, who have another cause for elevated natriuretic peptide levels.

A study of N-terminal pro-BNP levels in patients with acute onset AF, and without clinical or radiological evidence of heart failure, showed the pattern of rise and fall. The key finding in this study is the rapid rise of plasma NT-proBNP levels to peak followed by a rapid decline, probably due to depletion of the granules in atrial myocytes in which pro-BNP is stored.

The authors describe the following implication of the study:
According to our observations, a rising trend is markedly indicative of the fact that AF onset did not happen more than 24–48 h before presentation. As a consequence, obtaining two to three plasma NT-proBNP levels within 24 h of presentation in patients with AF without heart failure who cannot satisfactorily pinpoint the time of onset may assist in determining whether the onset of the arrhythmia was recent. Such information is pertinent to decisions concerning anticoagulation and cardioversion.

Short-term fluctuations of plasma NT-proBNP levels in patients with new-onset atrial fibrillation: a way to assess time of onset?
Heart. 2010 Jul;96(13):1033-6

Fancy new ACS tests

It’s hard to keep track of all new proposed biomarkers that may be useful in the work up of acute coronary syndrome (ACS) patients. We’re all used to troponin now – so we really want to know how novel tests measure up against the existing standard, in particular for the timeframe in which troponin is less helpful, namely the first six hours.
A systematic review examine the evidence for the following biomarkers in the early assessment (ie, within 6 h of symptom onset) of suspected ACS:

  • CK-MB: CK-MB and 2 h ΔCK-MB have potential in diagnosing AMI in the first hours after symptom onset. Whether CK-MB has value in the early diagnostic assessment of ACS in addition to clinical symptoms, ECG or other markers has rarely been studied.
  • myoglobin: myoglobin might be of value in early ruling out of AMI and ACS in suspected patients because of the relatively high NPV; its PPV, however, is low. However, it is not yet known whether myoglobin has diagnostic value in addition to symptoms, signs and other diagnostic tests (eg, ECG), because of the lack of multivariable analysis
  • heart-type fatty acid binding protein (H-FABP): H-FABP seems to have some potential as an early diagnostic marker of AMI or ACS, but its value in addition to clinical features and other markers has not been studied
  • ischemia modified albumin (IMA): IMA could be a potential marker for early ruling out of ACS in chest pain patients because of its relatively high NPV, especially combined with cTn and ECG. However, its PPV is low. Importantly; IMA seems to add relevant diagnostic information to more readily available diagnostic parameters. However, problems with the stability of IMA and its lack of cardiospecificity have been reported
  • pregnancy-associated plasma protein A: contradictory results indicate that the diagnostic value of PAPP-A in patients suspected of having ACS has not been evaluated properly
  • glycogen phosphorylase isoenzyme BB: GPBB might be a marker for myocardial ischaemia and myocardial necrosis, although the available research is limited and does not assess the added value of the marker.
  • myeloid-related protein 8/14: more research is needed to evaluate the (added) diagnostic value of MRP 8/14 in patients suspected of having ACS.

The authors also point out the introduction of high-sensitivity troponin assays may further improve diagnosis of ACS. They refer to a study in patients with suspected AMI presenting at the emergency department, which showed that a sensitive cTnI assay had a higher NPV than a standard cTnT assay and comparable PPV for detecting AMI within 6 h of symptom onset. The PPV was 79.3% and 80.7%, respectively; the NPVs were 95.3% and 88%. The Full Text of this high-sensitivity troponin paper is here

The review makes the following conclusions:

  • current guidelines advocate the use of cardiac troponin or CK-MB when cTn is not available, and myoglobin in the first 6 h in addition to cTn
  • IMA and H-FABP seem to be promising diagnostic biomarkers in the early diagnostic assessment of patients suspected of having ACS
  • There is an urgent need for adequately designed studies of (novel) ACS markers and their combinations against contemporary troponin assays

Novel biochemical markers in suspected acute coronary syndrome: systematic review and critical appraisal
Heart. 2010 Jul;96(13):1001-1

Pre-hospital cooling post arrest

An Australian randomised controlled trial assessed the effect of pre-hospital cooling (using 2 litres ice cold Hartmann’s) of post-cardiac arrest patients on functional status at hospital discharge. The intervention group were marginally cooler on arrival but did not have improved outcomes.

The authors conclude: In adults who have been resuscitated from out-of-hospital cardiac arrest with an initial cardiac rhythm of ventricular fibrillation, paramedic cooling with a rapid infusion of large-volume, ice-cold intravenous fluid decreased core temperature at hospital arrival but was not shown to improve outcome at hospital discharge compared with cooling commenced in the hospital.

Induction of Therapeutic Hypothermia by Paramedics After Resuscitation From Out-of-Hospital Ventricular Fibrillation Cardiac Arrest
Circulation. 2010 Aug 17;122(7):737-42 Free Full Text

One issue from this study was that relatively short urban pre-hospital transport times meant some patients did not get the full two litres, and some had already received room temperature fluids during the cardiac arrest resuscitation. The authors suggest further study should involved initiating cooling during the arrest. In fact a European study has done just that, using a device call a RhinoChill (a portable transnasal cooling device) to lower temperature during arrest in a randomised controlled trial. This trial showed pre-hospital intra-arrest transnasal cooling is safe and feasible and is associated with a significant improvement in the time intervals required to cool patients.

Intra-arrest transnasal evaporative cooling: a randomized, prehospital, multicenter study (PRINCE: Pre-ROSC IntraNasal Cooling Effectiveness)
Circulation. 2010 Aug 17;122(7):729-36