Linda English has told the High Court at Hamilton she and other ambulance officers performed resuscitation on the 8-month-old infant for 26 minutes but there was no trace of a heartbeat.
Isaiah was pronounced dead at 7.21pm on November 2, 2015, almost four hours after he had been discovered limp in a hot car.
The baby's grandmother is on trial for his manslaughter. His mother Lacey Te Whetu and father Shane Neil have previously pleaded guilty to the same charge.
"She was distressed," English said of Te Whetu. "She was crying. She was wailing. She kept saying 'Come back to me my baby. Don't leave me'."
The Crown alleges Parangi is culpable because she also cared for the infant and his two older siblings, who lived with their grandparents and parents at the Rūātoki house, near Whatakāne.
English said she arrived at the house in Ngāhina Road at 6.52pm following some confusion as to its location, and with instruction that a baby was suffering a cardiac arrest.
Te Whetu was at the end of the bed and explained Isaiah had been sick with the flu and she had given him Pamol two hours prior.
When English felt the baby he was warm and there was no lividity - a sign of death where blood pools at the bottom of the body causing bruising.
She said this indicated to her Isaiah had died recently.
"Isaiah was in what we call asystole. He was flat-lining. There was no electrical activity in his heart."
Inside the ambulance the distressed Te Whetu pointed out a large bruise on the left side of Isaiah's abdomen saying: "It's his organs".
English said the bruise was large and purple-ish, almost like a haemorrhage. There was also a recent bruise on Isaiah's elbow from an adult bite.
English believed discolouration in skin on the left side of Isaiah's head was also a faded bruise, but another paramedic did not.
When asked by Crown prosecutor Richard Marchant if Neil was involved English said he was standing outside the ambulance.
"I had spoken to Lacey and explained that things were not looking good for Isaiah. I told the father the same thing outside the ambulance."
English said Neil showed no emotion, did not make eye contact with her and did not respond.
After Isaiah was pronounced dead English wrapped him in a blanket and gave him to Te Whetu.
She stepped outside the ambulance to tell Neil who had disappeared. When he returned he was carrying a nappy and baby's clothing.
English gave Neil the news and asked him to go into the ambulance to comfort Te Whetu. He went into the ambulance and sat silently beside Te Whetu, English said.
"He said nothing. He made no attempt to communicate at all."
"I remember thinking his response was quite unusual for someone who had just lost a child.
"He appeared more anxious than anything. He was looking over his shoulder all the time."
The court heard police and other members of the community were at the scene.
Earlier in the trial defence counsel Susan Gray pointed to asphyxiation or abuse rather than heatstroke as the cause of Isaiah's death.
Isaiah was left in a car for about three hours while Te Whetu, Parangi and Neil smoked synthetic cannabis and fell asleep.
The baby was retrieved at 3.30pm when Te Whetu says she tried to give Isaiah a bottle but he was in a "deep sleep". She was not concerned and put the baby in his cot.
When she woke at 6.30pm she could not rouse the baby and called 111.
