Using millis to measure intervals to trigger a digital output

Projects Programming
4

My code will be used to measure the pulse of the user. If their pulse is below a certain threshold, the user will be shocked by a dog collar until they get their heart rate back up. Currently the code just repeatedly turns pin 9 on and off and does not stop.

Also thanks for the tips!

int PulseSensorPurplePin = 0;
int ledPin = 13;
int SHOCK =  9;

int Signal;
int Threshold = 550; 
unsigned long previousMillis = 0;
unsigned long startMillis;
const unsigned long interval = 500;

// The SetUp Function:
void setup() {
  pinMode(SHOCK,OUTPUT);
  pinMode(ledPin, OUTPUT);
   Serial.begin(9600);
   
}

void loop() {

  Signal = analogRead(PulseSensorPurplePin); 
                                         
  unsigned long currentMillis = millis();
   Serial.println(Signal);

   
   if(Signal > Threshold){  
     digitalWrite(ledPin,HIGH);       
   } else {
     digitalWrite(ledPin,LOW);
   }

   if(currentMillis - previousMillis > interval){       
     digitalWrite(SHOCK,HIGH);       
   } else {
     digitalWrite(SHOCK,LOW);
   } 
delay(10);
  }