17 year old female with acute exacerbation of asthma…

Respiratory alkalaemia – pH7.55 pCO2 17mmHg

Compensation – Expected HCO3 – 2 for 10 rule

24-{2x(40-measured pCO2/10)} = 19.4 Actual HCO3 14mmol/l

AG= Na – (Cl+HCO3)= 17

Delta gap = Change in AG/Change in HCO3 =0.5

A-a gradient:

PAO2 =(713×0.4) – (PCO2x1.25)=263

PAO2- PaO2 = 188mmHg (expected for age=8)

Description: There is a respiratory alkalaemia, that is not completely compensated for with an expected HCO3 of 19.4mmol/l. There is a raised AG, actual HCO3 of 14mmol/l and a base deficit of 6, and therefore a likely underlying HAGMA. The delta gap of 0.5 points to a coexisting NAGMA. There is a markedly raised A-a gradient. The lactate is mildly raised to 2.5, and the electrolytes are essentially normal, with a low normal potassium.

Interpretation: In this clinical context the patient has a respiratory alkalaemia secondary to acute exacerbation of her asthma and increased respiratory rate. The HAGMA in this case is likely secondary to a lactic acidosis due to multiple doses of salbutamol as well as underlying hypoxaemia. The cause of the NAGMA is uncertain (recall USEDCARP). Normal saline administration might be the cause in this case.

The markedly raised A-a gradient is secondary to V/Q mismatch and increased dead space in the lung. The A-a gradient is higher the more severe the exacerbation of the asthma is. Other causes need to be sort and treated ie pneumonthorax, underlying pneumonia or ARDS.

The potassium is likely lower than 3.5. Recall serum K decreases by 0.3mEq/L for every 0.1U increase in pH above normal. This will need to be kept in mind as more salbutamol will cause a further decrease in the potassium.

Risk Factors

Based on the patients history – previous ICU admission, increased use of salbutamol, current use of steroids.

Other risk factors for increased mortality and morbidity in asthma include:

-history of previous intubation

-hospital visits for asthma in the last month/>3 visits in the last year

-undelying health problems eg CCF

-underlying psychiatric problems

-illicit drug use

-patients poor understanding of asthma and its severity

Ventilation in asthmatic

The ventilation settings are set to avoid hyperinflation, breath stacking and barotrauma.

TV 6ml/kg

PEEP 3-5

FIO2 1.0 titrate to sats

RR 6-10 breaths/min – allow permissive hypercapnoea

I:E >1:4

Pmax >30mmHg

For homework – what to do if the asthmatic ventilated patient crashes??