D9 Physiological Telemetry

Topic

An implantable cardiac recorder—an electrocardiographic device weighing between one and five grams—allows for the estimation of a bird's metabolic expenditure during free flight under natural conditions, free from the constraints of wind tunnels, which require measuring metabolic power directly via respirometry. Unlike open-flow respirometry, which can only be performed while a bird flies inside a wind tunnel, the implantable cardiac recorder accompanies the bird during actual flight in its own environment.

The physical principle behind this method involves a three-step chain: a bird's heart rate serves as an indirect indicator of its cardiac output (the volume of blood the heart pumps per unit of time); this cardiac output, in turn, determines the rate at which oxygen reaches the pectoral muscle; and this rate of oxygen delivery to the pectoral muscle ultimately determines the metabolic power the bird can sustain at any given moment during flight.

The relationship between heart rate and metabolic power is non-linear and varies between species, so it cannot be assumed beforehand; prior to deploying an implantable cardiac recorder on a free-ranging bird, this species-specific relationship must be established in a wind tunnel by simultaneously measuring heart rate and actual metabolic power using open-flow respirometry.

Once this relationship has been calibrated in the wind tunnel, heart rate measurements taken in the field allow for the estimation of the bird's metabolic power with sufficient precision to study the various modes of flight employed during actual, prolonged flights—something neither open-flow respirometry nor any other method can achieve outside a wind tunnel. This same calibrated heart rate also makes it possible to detect when a migratory bird has begun burning protein: when the bird resorts to protein catabolism because its fat reserves are depleted, its heart rate rises disproportionately relative to the expected metabolic power output. This occurs because burning protein places a greater strain on the cardiovascular system than burning fat to deliver the same amount of oxygen to the pectoral muscle.

The same cardiac signal also reveals a distinct neuromuscular disturbance: when a neurotoxin alters the autonomic control the nervous system exerts over the heart, the variability in the timing between successive beats changes, regardless of the effort the bird is exerting at the time. This altered beat-to-beat variability thus serves as an indicator of neuromuscular disturbance that depends not on whether the average heart rate rises or falls, but on the regularity or irregularity of the heartbeat rhythm.

Finally, this same implantable cardiac monitor links the bird's cardiovascular system to the limit of muscular power it can sustain during flight, and to how that limit drops when the bird faces extreme environmental conditions: the circulatory strain imposed on the heart by severe heat or extreme altitude—hindering oxygen delivery to the pectoral muscle—is directly reflected in the heart rate measured by the device, thereby bridging the gap between estimating metabolic power in the field and the physiological ceiling the bird can achieve.

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