Here are some interesting (or boring and routine) blood gases, observed in real clinical scenarios.
The ED staff were struggling to intubate an elderly gentleman with hypoxic respiratory failure. This man is a 75 year old non-smoker who has had a NSTEMI at home after an elective laparoscopic cholecystectomy, and went on to develop acute pulmonary oedema which brought him to hospital. Background is that of rheumatoid arthritis with decreased jaw opening, and C-spine fusion surgery which significantly limits neck extension. The first attempt at intubation did not go as planned; direct laryngoscopy proved fruitless and though the curved-blade videolaryngoscope revealed a nice-looking Grade 1 view, the bougie ended up poking uselessly into every recess other than the larynx, and the whole thing was abandoned after the saturation probe fell off.
This 79 year old lady presented to emergency with abdominal pain, nausea and vomiting. On examination, she appeared shocked, with cool mottled extremities and a weak thready pulse. The heart rate was 150 (in AF, but apparently that is long-standing) and the non-invasive blood pressure cuff generated improbable numbers (Pfft, 52/35mmHg? Really?).
This 65 year old gentleman presented to ED with fever and productive cough. For several days he has been too short of breath to eat or drink, and appeared severely dehydrated at triage. The chest Xray revealed an extensive right lower lobe pneumonia. BP on presentation was 78/40, and MAP remained below 60 in spite of large volumes of crystalloid. The ICU team were contacted after the seventh litre of fluid had not only failed to produce a sustained hemodynamic effect, but also catalysed an episode of acute pulmonary oedema. This ABG was taken while the patient was still conscious, being pre-oxygenated for intubation with a Laerdel bag-mask device.
This blood gas gets waved around a lot, particularly at unexpecting medical students and other impressionable individuals. As an isolated result, this one is an abomination. Clinical scenarios with varying levels of improbability are offered to the interpreters. "This is an ABG collected during a cardiac arrest"; "This is a sample of peripheral venous blood from an unwell demented patient"; "This sample suffered prolonged transport time on the way to the laboratory". My favourite has been "This patient presented to emergency complaining of generalised unwellness, dry skin, hoarse speech, ataxia and compulsive brain-eating behaviour".
A patient recovering from a laparotomy was referred to ICU from recovery. She is a 72 year old lady with a background of CCF, who underwent a laparotomy and adhesiolysis in the context of a subacute small bowel obstruction.
After being discovered at his rural property in an unconscious state, this elderly gentleman was intubated and brought to the ICU. Airway ulceration was noted by the rescuers. Apparently he intentionally drank an unknown amount of some unknown smelly liquid. Some sort of canister of chemicals was also discovered at the scene, but the ambulance officers were frightened by its abominable reek, and decided not to bring it (they had no equipment to handle hazardous chemicals).
You're a part of the advanced life support team, arriving at the sleep lab in the small hours of the morning. Worried staff narrate a story of a morbidly obese sleep apnoea patient who presented for a sleep study and has been observed all night with continuous pulse oximetery. Prolonged apnoeic pauses were witnessed, with desaturation into the 60s. However, each time the patient snored back into life after a few seconds.However, recently the saturation probe had become dislodged, and in spite of fiddling with it the sleep lab staff could not get it to read a "realistic" result. A resident had collected an arterial blood gas, "to make sure". The pulse oximeter continued to offer very depressing numbers. Concerned that they may be real, the staff attempted to rouse the patient - and could not.
The resident returned with this blood gas.
Central and mixed venous blood gases offer us a glimpse of whole-body oxygen extraction.
A mixed venous blood gas is a sample aspirated from the most distal port of the PA catheter, offering a mixture of inferior vena cava blood, superior vena cava blood, and the coronary sinuses. Thus, the result is an average of venous blood. But what if I don't have a PA catheter, you might ask? A central venous gas may be almost as good.
Lets face it, we don't really care about venous oxygenation (or arterial, for that matter). The real issue is how much oxygen is in the cells? How are those cells using it? The intracellular environment is a place of massively heterogenous oxygen demands. Some organelles are involved in enzymatic metabolism of drugs or maintenance of electrolyte gradients - these areas require a higher oxygen tension than, for example, mitochondrial ATP synthesis.