High frequency ventilation

Currently HFOV is used as salvage therapy for patients failing conventional mechanical ventilation. It should restricted to centres with training and experience in this mode of ventilation. HFOV provides tidal volume below the anatomic dead space at frequencies greater than 60 breaths per second.

Benefits include reduce barotrauma, improve V/Q matching and less respiratory compromise. Complications include dessication and inspissation of mucus, airway damage due to high gas velocities, air trapping and high shear forces at interfaces between areas of the lung at different impedences.

Gas exxchange during HFOV:

• 
  
  direct bulk flow


• 
  
  longitudinal (taylor) dispersion


• 
  
  pendeluft


• 
  
  asymmetric velocity profile


• 
  
  cardiogenic mixing


• 
  
  molecular diffusion

1. Two observational studies, adults who failed to respond to conventional ventilation were managed with HFOV at 5 Hz. Improvement in oxygenation were noted within 8 hours in both studies. (Fort CCM 1997, Mehta CCM 2001).

2. HFOV for ARDS in adults: a RCT. Derdak AJRCCM 2002.

Multicentre Oscillatory Ventilation for ARDS Trial (MOAT)

148 adults were randomised to HFOV or conventional ventilation. The HFOV group had significant improvement in oxygenation within 16 hours,compared to conventional ventilation. However the improvement did not persist and oxygenation was the same in both groups by 24 hours. The survival trend favour HFOV over CV but is underpowered (would need n=199 to evaluate mortality)

The difference in survival rate did not reach significance at 30 or 90 days.

3. High frequency oscillatory ventilation in adults: the Toronto experience. Mehta Chest 2004 Canadian experience, Retrospective chart review of 156 patients treated with HFOV. The authors suggested that HFOV might be an effective rescue therapy for patients with severe oxygenation failure. Because mortality was associated with a greater number of days receiving conventional ventilation prior to HFOV, the authors also suggested that earlier institution of HFOV could be beneficial.

4. High Frequency Oscillatory Ventilation Compared to Conventional Ventilation in ARDS: a RCT. Bollen et al. Crit Care 2005

ICU in London, Cardiff, Paris, Mainz. n = 61

Study stopped prematurely because of la ow inclusion rate and the completion of similar MOAT trial.

No difference in 30 day mortality. Post hoc analysis- better treatment effect of HFOV in patients with higer baseline oxygen index (OI).

Critics: Small number of patients, lack of explicit ventilation protocol and underpowered to show differences in efficacy or safety.

Conclusions:

1. HFOV as safe and efficacious as lung protective controlled ventilation in RCTs.

2. There is a trend of improved mortality with HFOV but this needs to be repeated in a fully powered, properly controlled RCT.

3. HVOV may be more effective in patients with high baseline OI, but this should be studied directly.

Thrombocytopenia in critically ill patients

 

 

 

 

Definition:

1. Plt count less than 150, 000/micL, generally not significant until less than 100,000.
2. Relative  thrombocytopenia - acute drop from a higher platelet count may be pathologic.

Pathophysiology:

1. Decreased production
 a) Isolated thrombocytopenia
 b) Multiple cytopenias

2. Increased destruction and consumption

 a) Immune or alloimmune: primary (idiopathic) or secondary -associated with other autoimmune disease (e.g. SLE), associated with malignancy, maybe complication of infection and drug associated thrombocytopenia

b) Nonimmune -isolated or combined platelet consumption

3. Increased sequestration in enlarge spleen

a) portal hypertension
b) myloproliferative disease
c) lymphoma
d) storage and infiltrative deseases of spleen
e) chronic hemolysis
f) granulomatoses -eg tuberculosis, sarcoidosis

4. Dilutional - effect of massive transfusion and fluid resuscitation

5. States with multiple causes of thrombocytopenia:

a) cirrhosis with portal hypertension
b) hepatitis
c) HIV
d) other viral illnesses
e) patients with multiple medical problems on multiple drugs

Diagnosis

1. FBP with peripheral smear: -Review of rbc abnormalities e.g. schistocytes, teardrops, nucleated RBC. Rule out pseudothrombocytopenia due to platelet clumping.
2. Coagulation testing: Identify associated coagulation abnormalities
3. Additional blood tests, if required:  

• viral titres and antibodies
• autoimmune disorders (e.g. collagen vascular disease)
• other disorders

     4. Radiologic

• abdominal US -evaluation of spleen size
• CT scan -evaluation for lymphoproliferative disease

    5. Indication for bone marrow examination

• unclear pathology
• multiple cytopenias
• suspected infiltrative process

Therapy

1. Transfusion therapy indications:

• bleeding or necessary invasive procedures
• prophylactic - for very severe (less than 10,000) thrombocytopenia
• other blood components as indicated to correct coagulation abnormalities

2. Platelet transfusion relatively contraindicated:

• TTP unless bleeding present
• ITP unless bleeding present
• HIT without bleeding - unknown

3. Primary thrombocytopenia - depends on specific disorder

4. Secondary thrombocytopenia -direct therapy at underlying causes

Propofol Infusion Syndrome

Propofol is an intravenous anesthetic that is commonly used for sedation in ICU. the elimination of propofol is not impaired by hepatic or renal dysfunction. Propofol has a large volume of distribution and is highly protein bound. During administration serum triglycerides should be monitored in all patients who receive propofol in doses > 50 mcg/kg/min for more than 2 days. If hypertriglyceride is detected, alternative should be considered. Unusual and potentially serious complications are associated with continous infusion of propofol for longer tnan 24 to 48 hours. These include progressive hyperglyceridaemia, pancreatitis, increased carbon dioxide production and excessive caloric load (the emulsion contains approcimately 1.1 kcal/ml, most of which derived from lipids).

Propofol Infusion Syndrome (PRIS): PRIS is a rare complication of propofol for longer than 24 to 48 hours. It is associated with high doses ( more than 4 mg/kg/hour of mone than 67 mcg/kg/min) and prolonged use (more than 48 hr). Characteristic of the syndrome include acute refractory bradycardia, severe metabolic acidosis, CVS collapse, rhabdomyolysis, hyperlipidaemia, renal failure and hepatomegaly. Mortality is high, more than 60% in a study. Risk factors:

• large doses and duration more than 48hrs


• younger age


• acute neurological injury


• low carbohydrate intake


• catecholamine and corticosteroid infusion

Clinical and lab findings:

• unexplained lactic acidosis


• increasing inotropic support


• Brugada like ECG abnormalities


• Green urine


• CVS collapse


• rhabdomyolysis, high CK, hyperkalaemia


• arrythmia/ heart block


• renal failure

Management:

1. high index of suspicion


2. discontinue infusion immediately


3. monitor for early warning signs: lactate, CK, urine myoglobin, ECG


4. standard cardiorespiratory support


5. consider pacing for bradycardia ( often resistant to high dose of CA )


6. adequate carbohydrate intake


7. ECMO: 2 case reports


8. haemoperfusion and haemodialysis are used with uproven benefit


9. carnitine supplementation: theoretical benefit