A medical director who oversees paramedic services says paramedics must "assume the worst, and assume that until proven otherwise" when responding to a call, like the one where Yosif Al-Hasnawi was shot and killed on Dec. 2, 2017.

"What I say to trainees is you start with saying to yourself, 'what could kill this patient?'" said Dr. Richard Verbeek, medical director for Toronto paramedics at the Sunnybrook Centre for Prehospital Medicine.

He testified on Monday at the trial of two former Hamilton paramedics — Christopher Marchant, 32, and Steven Snively, 55 — who are charged with failing to provide the necessaries of life for the teenager. They have pleaded not guilty in the landmark trial where paramedics are criminally charged based on the care they give a patient.

The paramedics and others on the scene thought that the teenager was shot with a BB or pellet gun, the court has heard, and didn't leave for the hospital for 23 minutes. Al-Hasnawi was actually shot with a .22 caliber handgun, and died about one hour later.

The standard of care that Ontario paramedics must follow for all 911 patients is designed to avoid assumption, said Verbeek. The section for treating penetrating abdomen wounds — Al-Hasnawi was shot in the abdomen — doesn't give different treatments depending on the weapon specifically because some assumptions are inaccurate.

"The reason we don't do that is because of the unreliability of that kind of information," said Verbeek, who was the medical editor of the Basic Life Support Patient Care Standards that was in place when Al-Hasnawi died.

There's special caution in the standard, he said, for chest and abdominal penetrating injuries. You can't see to the bottom of these types of wounds, he said. That means you don't know how deep the injury is, the direction it goes, or what organs were hit.

Depending on all these factors, he said, "you can have very significant injuries that are inapparent when you just do an external exam."

When Al-Hasnawi was shot, the hollow-point bullet perforated the right iliac artery and vein. Last week, Dr. Elena Bulakhtina, who performed the autopsy, told the courtroom that Al-Hasnawi had two litres of blood in his abdomen when he died.

When Justice Harrison Arrell asked if Al-Hasnawi's chances of living were "50 per cent chance at best," she said yes.

Verbeek said the standards also provide an objective way to assess the quality of care provided to a patient. The standard has been a central focus in the trial, with Hal Klassen — a now-retired Hamilton Paramedic Service deputy chief who testified for more than four days — walking the courtroom through the document of a nearly 300 pages.

An overarching principle, Verbeek said, is that paramedics stay "objective, non-judgmental and thorough in what [they] do."

Verbeek said during his testimony that it would have been "extraordinarily difficult" for the paramedics to recognize the injury on the night Al-Hasnawi was shot. They arrived after 9 p.m. in the winter, he said, meaning it would be dark and hard to see.

When shown a picture of Al-Hasnawi's torso, he called the wound "deceiving." Verbeek noted he wasn't a ballistics expert, but said that the hole was larger than what he would expect, having seen BB gun and pellet gun injuries. The skin also stayed separated, which in his experience, doesn't happen in those kinds of wounds.

Verbeek was retained by Niagara Regional Police Service in 2018 to write a report without using witness statements or seeing the video of the scene.

He continued with the Crown and wrote a follow up the next year, which was based on witness statements from civilians, other first responders, and the testimony of physicians. Before testifying Monday, he watched videos from the scene and hospital, as well as reviewed evidence from the trial.

As part of his analysis, he examined waveforms that showed Al-Hasnawi's heart rate and blood oxygen saturation — the percentage of hemoglobin carrying oxygen molecules in the bloodstream — during the ambulance ride to St. Joseph's Hospital, where he was pronounced dead at 9:58 p.m.

Paramedics are trained to recognize normal waveforms, he said, which look very different from each other depending on what's being measured.

Verbeek said he wasn't able to find any blood oxygen saturation waveforms that appeared normal in Al-Hasnawi's tracings from that night.

There were abnormalities throughout, sometimes with no readings at all. Verbeek said this could be due to patient movement, a tremor, or not enough blood flow to the finger where the monitor is clipped. The monitor could also be improperly placed and become disconnected.

When Crown attorney Linda Shin asked if wrist restraints could affect the reading, Verbeek replied that if they were tight, they could possibly restrict blood flow. Al-Hasnawi was restrained in the ambulance, the court has heard.

The values with the heart rate waveforms also weren't accurate and there were abnormalities, Verbeek said, so he calculated his own by looking at the tracings.

Al-Hasnawi's heart rate dropped from a high that reached 150, he said, down to 33 in less than 10 minutes.

Verbeek said that a such a high heart rate in someone who was otherwise healthy with a suspected penetrating wound would be considered as extreme tachycardia, and compatible with "severe shock" or a hemorrhage.

From around 9:33 p.m. onwards, the abnormalities stopped, which meant whatever was causing them — be it movement or a loose cable — stopped too.

Verbeek said a button on the machine that takes blood pressure was pressed, but there was no reading. He isn't able to say whether the cuff was in place.

A 12 lead electrocardiogram was also taken, which records information from different sides of the heart. Verbeek said this process is normally followed for someone suspected of having a heart attack.

Jeffrey Manishen of Hamilton is representing Marchant, and Michael DelGobbo of St. Catharines is representing Snively.

The Crown attorneys are Shin and Scott Patterson.

The superior court trial is expected to take five weeks, and Arrell will render a verdict. The trial began on Nov. 24 at John Sopinka Courthouse in Hamilton. It will break for the holidays and continue in 2021.

Among others, dispatchers, bystanders and first responders, including firefighters, police officers, and a paramedic supervisor, have been called as witnesses for the Crown. The defence will start calling witnesses in the new year.

The person who shot Al-Hasnawi, Dale King, was acquitted last year of second-degree murder. That case is being appealed.