Monitoring cardiorespiratory system

• Oesophageal Stethoscope


  1. Indication: listening to the heart (and breath) sounds in the anaesthetised animal

  2. Inserted via the oesophagus to level of the heart base

  3. Palpate apex beat and measure the stethoscope against the animals' lateral neck and thorax

  4. Listen as you pass the stethoscope into the oesophagus
• Electrocardiogram (ECG)


  1. Electrical activity of the heart

  2. Rate and rhythm can be determined – note that sometimes the ECG on monitoring equipment will “double count” by counting T waves as well as QRS complexes

  3. Used to detect arrhythmias and conduction defects

  4. May be used to assess chamber size

  5. Small animals only (not horses)

  6. Depends on the correct positioning of the animal
• Clinical Indications for Use of ECG


  • Arrhythmia detected on auscultation
  • Investigation of syncope
  • Investigation of suspected cardiovascular disease
  • Monitoring for arrhythmias in sick animals eg in ICU post road traffic accident (RTA)
  • Monitoring during anaesthesia
• ECG Attachment – Limb Electrodes
• ECG – Oesophageal Probes
• Normal Small Animal ECG
• Normal Equine ECG
• ECG with Electrical Interference
• Normal Equine ECG is the same as the pulse rate
• Reason for negative deflection of the QRS in horses
• Respiratory Sinus Arrhythmia


  Heart rhythm varies depending on breathing – heart rate increases during inspiration and decreases during expiration due to changes in vagal tone
• Second Degree AV Block

  1. P waves without subsequent QRST complexes

  2. Common in animals with high vagal tone at rest (e.g. horses) or after some sedatives (α2- adrenoreceptor agonists)
• Ventricular Premature Complex


  QRS complex appears “wide and bizarre” and appears before the next anticipated beat
• Ventricular Tachycardia


  Dog – heart rate 375 beats per minute
• Ventricular Fibrillation


  There’ll be more on ECG interpretation in your clinical cardiology lectures
• Blood Pressure is commonly measured in animals with the aim of keeping mean arterial pressure, MAP, greater than 60mmHg
• Direct measurement of perfusion is not possible in clinical cases
• Factors linking cardiac output and blood pressure

  Equations
• Indications for Measuring Blood Pressure

  • During anaesthesia
  • As part of the assessment of cardiovascular function in “sick”, potentially hypovolaemic, animals
  • As part of clinical assessment of cats older than 7 years of age
  • Cats with conditions that either predispose to hypertension (chronic kidney disease, hyperthyroidism) or may be secondary to hypertension (sudden onset blindness, ocular or neuro conditions)
• Information in this paper will be useful in the context of future clinical medicine and surgery lectures and clinical rotations
• Blood Pressure Measurement Methods

  • Direct (invasive)
  • Doppler (non-invasive)
  • Oscillometric (non-invasive)
• Focus on non-invasive methods as they are most commonly used and can be used easily in conscious animals
• Risks associated with arterial catheterisation
• Direct Arterial Blood Pressure Measurement

  1. Accurate and real time

  2. Involves arterial catheterisation and connection to a transducer for measurement

  3. Not routinely used in small animal practice due to risks involved

  4. Routinely used in equine anaesthesia when an inhalational agent is administered, even in healthy horses
• Non-invasive Blood Pressure Measurement

  1. Doppler and oscillometric techniques

  2. Both use cuffs

  3. Align mark on cuff with the artery

  4. Cuff width = 40% limb circumference

  5. Too big – value lower than real value

  6. Too small – value higher than real value
• Doppler


  1. Doppler probe emits and receives ultrasonic pulses to detect blood flow

  2. Cuff attached to sphygmomanometer

  3. Cuff proximal to Doppler probe (tail, metacarpal and metatarsal arteries are suitable sites
• Main methods used in small animal practice

  • Doppler
  • Oscillometric
  • Pulse Oximetry
  • Capnography
• Doppler


  1. Doppler probe emits and receives ultrasonic pulses to detect blood flow

  2. Cuff attached to sphygmomanometer

  3. Cuff proximal to Doppler probe (tail, metacarpal and metatarsal arteries are suitable sites

  4. Cuff inflated until no sounds heard

  5. Deflated – point at which sounds return = systolic pressure

  6. Repeated and average taken
• Doppler may underestimate systolic pressure (SAP) – should be above 100mmHg
• Hypertension is when SAP > 160mmHg
• Measurement may be difficult in animals with chondrodystrophoid conformation or if they are moving
• May be difficult to establish accurate pressure at which pulse sounds return in animals with severe bradycardia or arrhythmias
• Measurement should be repeated and the mean used as an estimate of SAP. Conscious cats – 5-7 repetitions recommended, anaesthetised animals min of 3 repetitions
• Oscillometric


  1. Cuff positioned over the median artery in thoracic limb. Alternative positions include over the metatarsal artery in the pelvic limb or the artery on the ventral surface of the Tail (if the animal has one)

  2. Cuff automatically inflated and deflated by microprocessor
• How to assess whether these values are believable or not in Oscillometric method
• Readings for systolic, mean and diastolic pressures
• Mean pressure reading usually the most accurate of the values
• Compare the heart rate from the machine to a manually measured heart rate – if there’s a large discrepancy the blood pressure reading accuracy should be viewed cautiously
• Intermittent readings – usually every 3-5 minutes
• May fail to give accurate readings in animals with cardiac arrhythmias, severe hypo or hypertension