Code to run pulse sensor on Adafruit Flora

I have a Pulse Sensor Amped from pulsesensor.com and an Adafruit Flora board.

Adafruit Flora is based on ATmega32u4.

I followed the instructions from here to setup the pulse sensor.

However, the sensor outputs results without even touching it.
It show various values at a 1-2 seconds interval, usually between 130-180 BPM.

I don’t know what I’m doing wrong, or what should I do to fix it.

The code is available on GitHub.

These are the code lines which are meaningful to me:
Interrupt setup:

void interruptSetup(){     
  TCCR1A = 0x00;
  TCCR1B = 0x0C; 
  OCR1A = 0x7C; 
  TIMSK1 = 0x02; 

  sei();
}

Option to use Timer1:

ISR(TIMER1_COMPA_vect)

Important: Flora pinout schema, because digital pins are associated with analog pins too.

PulseSensorAmped_Arduino_1dot4 code:

Set pulse pin (D12 on Flora is A11):

int pulsePin = 11;

Setting for pulse sensor to work on 3.3v:

analogReference(EXTERNAL);

Any ideas?